State of Bound Water: Measurement and Significance in Food Processing

Mohammad U.H. Joardder  Monjur Mourshed  Mehedi Hasan Masud

State of Bound Water: Measurement and Significance in Food Processing

State of Bound Water: Measurement and Significance in Food Processing

Mohammad U.H. Joardder • Monjur Mourshed Mehedi Hasan Masud

State of Bound Water: Measurement and Significance in Food Processing

Mohammad U.H. Joardder Department of Mechanical Engineering Rajshahi University of Engineering Rajshahi, Bangladesh

Monjur Mourshed Department of Mechanical Engineering Rajshahi University of Engineering Rajshahi, Bangladesh

Mehedi Hasan Masud Department of Mechanical Engineering Rajshahi University of Engineering Rajshahi, Bangladesh

ISBN 978-3-319-99887-9    ISBN 978-3-319-99888-6 (eBook) https://doi.org/10.1007/978-3-319-99888-6 Library of Congress Control Number: 2018956142 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

The book is dedicated to our family members.

Preface

Water contributes the major portion in the food stuffs and actively attributes to the food quality and preservation mechanism. Water is not uniformly distributed in food materials, whereas the spatial distribution of water in the food matrix determines the quality and processing parameters to extend the shelf life. In addition, the free and bound water contained in the complex food matrix controls the physiochemical characteristics of the food materials. Bound water has some distinct characteristics over free water. These distinct natures of bound water play a significant role in its water measurement and contribute in the food stability. An extensive literature review on the significance of bound water removal reveals that there is no straightforward answer in connection with whether it is necessary to remove bound water or not. Moreover, the nutrition value, structure, pore formation, dehydration techniques, and parameters act as dictating factors during food preservation through bound water removal. Water measurement of any kind is a complex task. Bound water molecules show several exceptional characteristics than their free counterpart. Therefore, tracing the amount of bound water in complex materials such as food is one of the complicated tasks encountered by the research of different fields. Moreover, there are no established techniques for the accurate measurement of bound water content in food materials. Several successful methods of bound water measurement have been discussed extensively along with the challenges encountered during measurement. Like the measurement, removal of bound water is not an easy task. Food preservation through drying or frying mainly involves simultaneous heat and mass transfer. Several internal moisture transfer mechanisms and energy transfer mechanisms can be involved during bound water removal. Selected mass transfer mechanisms along with food process deployed to bound water removal have been discussed at the end of this book. Rajshahi, Bangladesh Rajshahi, Bangladesh  Rajshahi, Bangladesh 

Mohammad U.H. Joardder M. Mourshed M. H. Masud

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Acknowledgments

At the very beginning, we express our utmost gratitude to the Almighty Creator for His gracious help to accomplish this work. We would like to thank our families for all their inspiration and love. Also, we would like to express our heartiest gratitude to many good people who have also supported us in our journey of writing this book. We sincerely acknowledged the numerous scientific discussions and assistance given by our colleagues in RUET and the members of the Energy and Drying Group at Queensland University of Technology, in particular, Dr. Azharul Karim, Md. Imran Hossen Khan, and Mr. Mahbub Rahman. We are truly thankful to Mr. Amit Md. Estiaque Arefin, who willingly carried out the most tedious work of patiently reading and correcting the earlier draft of this book. We would like to thank Springer, the publisher of this book, for giving us the opportunity to share our knowledge and findings with the research community. Special thanks go to Daniel Falatko, who has keenly worked with us from the inception to completion of this project.

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Contents

1 Introduction������������������������������������������������������������������������������������������������   1 References����������������������������������������������������������������������������������������������������   4 2 Water in Foods ������������������������������������������������������������������������������������������   7 2.1 Introduction����������������������������������������������������������������������������������������   7 2.2 Classification of Water in Food Materials������������������������������������������   7 2.3 Water Related Terminologies��������������������������������������������������������������   8 2.3.1 Moisture Content��������������������������������������������������������������������   8 2.3.2 Water Concentration ��������������������������������������������������������������   9 2.3.3 Intermediate Moisture Content ����������������������������������������������   9 2.3.4 Equilibrium Moisture Content������������������������������������������������  10 2.3.5 Critical Moisture Content ������������������������������������������������������  11 2.3.6 Moisture Removal Rate����������������������������������������������������������  12 2.3.7 Moisture Sorption Isotherm (MSI) ����������������������������������������  13 2.3.8 Phases of Water����������������������������������������������������������������������  17 2.3.9 Unfrozen Water ����������������������������������������������������������������������  18 2.3.10 Water Potential������������������������������������������������������������������������  18 2.3.11 Water Activity ������������������������������������������������������������������������  19 2.3.12 Water Mobility������������������������������������������������������������������������  19 2.3.13 Water Retention Capacity ������������������������������������������������������  20 2.4 Types of Water������������������������������������������������������������������������������������  20 2.4.1 Free Water ������������������������������������������������������������������������������  20 2.4.2 Bound Water���������������������������������������������������������������������������  21 2.4.3 Spatial Water Distribution������������������������������������������������������  22 References����������������������������������������������������������������������������������������������������  24 3 Characteristics of Bound Water ��������������������������������������������������������������  29 3.1 Introduction����������������������������������������������������������������������������������������  29 3.1.1 Lower Vapour Pressure ����������������������������������������������������������  30 3.1.2 Higher Binding Energy ����������������������������������������������������������  31 3.1.3 Lower Mobility of Water��������������������������������������������������������  32 3.1.4 Unfreeze-Ability at Low Temperature������������������������������������  32 xi

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Contents

3.1.5 Unavailability as a Solvent�����������������������������������������������������  33 3.1.6 Dielectric Property������������������������������������������������������������������  33 3.1.7 Higher Boiling Point��������������������������������������������������������������  34 3.1.8 Higher Density������������������������������������������������������������������������  34 3.1.9 Slower Diffusion ��������������������������������������������������������������������  34 3.1.10 Specific Heat ��������������������������������������������������������������������������  35 3.1.11 Monolayer Moisture Content (MMC)������������������������������������  35 3.1.12 Water Activity ������������������������������������������������������������������������  36 3.1.13 Glass Transition����������������������������������������������������������������������  40 3.2 Limitations������������������������������������������������������������������������������������������  41 References����������������������������������������������������������������������������������������������������  42 4 Bound Water Measurement Techniques��������������������������������������������������  47 4.1 Introduction����������������������������������������������������������������������������������������  47 4.2 Water Content Measurement��������������������������������������������������������������  49 4.2.1 Distillation������������������������������������������������������������������������������  51 4.2.2 Drying Methods����������������������������������������������������������������������  51 4.2.3 Karl Fischer (KF) Method������������������������������������������������������  52 4.2.4 Spectroscopy ��������������������������������������������������������������������������  52 4.2.5 Inferred������������������������������������������������������������������������������������  52 4.2.6 Microwave������������������������������������������������������������������������������  53 4.3 Bound Water Measurement Techniques����������������������������������������������  53 4.3.1 Differential Scanning Calorimetry������������������������������������������  54 4.3.2 Bound Water Estimation from SEM Image����������������������������  57 4.3.3 Dilatometry ����������������������������������������������������������������������������  62 4.3.4 CT Scan����������������������������������������������������������������������������������  64 4.3.5 Thermogravimetric Analysis (TGA)��������������������������������������  67 4.3.6 Bioelectrical Impedance Analysis (BIA)��������������������������������  69 4.3.7 Nuclear Magnetic Resonance (NMR)������������������������������������  74 References����������������������������������������������������������������������������������������������������  78 5 Challenges in Bound Water Measurement����������������������������������������������  83 5.1 Introduction����������������������������������������������������������������������������������������  83 5.2 General Challenges in Determination of Water in Foods ������������������  84 5.3 Specific Challenges in Water Content Measurement��������������������������  85 5.3.1 Distillation������������������������������������������������������������������������������  85 5.3.2 Drying Method������������������������������������������������������������������������  86 5.3.3 Karl Fischer Method ��������������������������������������������������������������  86 5.3.4 Infrared Spectrometry ������������������������������������������������������������  86 5.3.5 Microwave Absorption������������������������������������������������������������  87 5.4 Challenges Associated in Bound Water Measurement Techniques������������������������������������������������������������������������������������������  87 5.4.1 Differential Scanning Calorimetry (DSC)������������������������������  87 5.4.2 SEM Image Processing ����������������������������������������������������������  88 5.4.3 Dilatometry ����������������������������������������������������������������������������  89 5.4.4 CT Scan����������������������������������������������������������������������������������  89

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5.4.5 Thermogravimetric Analysis (TGA)��������������������������������������  89 5.4.6 Bioelectrical Impedance Analysis������������������������������������������  90 5.4.7 Nuclear Magnetic Resonance (NMR)������������������������������������  90 References����������������������������������������������������������������������������������������������������  91 6 Bound Water Removal Techniques����������������������������������������������������������  93 6.1 Introduction����������������������������������������������������������������������������������������  93 6.2 Mechanisms of Moisture Transfer������������������������������������������������������  93 6.2.1 Diffusion ��������������������������������������������������������������������������������  95 6.2.2 Capillary Flow������������������������������������������������������������������������  97 6.2.3 Evaporation-Condensation������������������������������������������������������  98 6.3 Types of Food Processing to Remove Bound Water��������������������������  98 6.3.1 Frying��������������������������������������������������������������������������������������  98 6.3.2 Drying ������������������������������������������������������������������������������������  99 6.3.3 Hybrid Drying������������������������������������������������������������������������ 106 6.3.4 Pre-treatment�������������������������������������������������������������������������� 110 References���������������������������������������������������������������������������������������������������� 112 7 Significance of Bound Water Measurement�������������������������������������������� 119 7.1 Introduction���������������������������������������������������������������������������������������� 119 7.2 Energy and Time �������������������������������������������������������������������������������� 120 7.3 Quality������������������������������������������������������������������������������������������������ 122 7.3.1 Structure���������������������������������������������������������������������������������� 122 7.3.2 Texture������������������������������������������������������������������������������������ 122 7.3.3 Collapse���������������������������������������������������������������������������������� 123 7.3.4 Chemical Reactions���������������������������������������������������������������� 126 7.3.5 Water and Food Appearance �������������������������������������������������� 128 7.4 Stability ���������������������������������������������������������������������������������������������� 129 7.4.1 Is Bound Water Removal Beneficial? ������������������������������������ 131 References���������������������������������������������������������������������������������������������������� 132 8 Conclusion�������������������������������������������������������������������������������������������������� 137 Index�������������������������������������������������������������������������������������������������������������������� 139

Nomenclature

Symbol Meaning D(m) Dependence of the diffusion coefficient on moisture aw Water activity

Symbol μ

Meaning Chemical potential (J/mol)

Vw

Ψ

m

Mean partial molar volume of water (m3/mol) Moisture in g/g solid at water activity aw Absolute temperature Constant Monolayer moisture content Partial pressures of water above the food Volume of the material Volumetric thermal expansion coefficient as a function of temperature Temperature Weight of the reference material Free water content

P0 ΔH T.W. W

Total or combined water potential (Pa) Vapor pressure of water Heat Total weight of a food material Weight

T K mo P

r CB

Radius of the fresh food material Percent bound water

V αv

t t1 β

T W FW

M

Initial thickness of cell wall Final thickness of the cell wall Shrinkage coefficient of the cell wall thickness Mass of

f

Frequency of the signal

Zc

C

Capacitance of the cell membrane A

Z R

Overall impedance Resistance/impedance/ gas constant (KJ/g.mol.K) Water vapor flux (kg/m2.s)

T2i

Porosity

VIntraCW

nwd ∈

D

∆T M(t)

Level difference in the dilatometer from ∆T Impedance because of the cell membrane capacitance Concentration coefficient of reference material Temperature difference Total amplitude of the entire signal Distribution of relaxation time T2 in (ms) Intracellular water

xv

Nomenclature

xvi Symbol τ P T

nw′ Pw

Meaning Tortuosity Total gas pressure (Pa) Absolute temperature (K)

Symbol VInterCW t K

Liquid water flux (kg/m2.s)

ew

Water vapor pressure (Pa)

M

d Pore diameter (m) Subscript m Matric or capillary potential s Solute or osmotic potential p Pressure potential or turgor pressure g Gravitational potential a Adsorption c Condensation S Solid portion of food material SB Nonfreezing water or strongly bound water M Water calculated from the enthalpy of melting WT Water content in food material sample B Total bound water P1 Water calculated from the enthalpy of crystallization peak I P2 Water calculated from the enthalpy of crystallization peak II wg Water vapor in air

T

Meaning Intercellular water Sampling time in (ms) Overall mass transfer coefficient (m/s), permeability (m/s) Water activity Equilibrium or decimal moisture (dry basis), molecular weight Absolute temperature (K)

1

Water in the solution Water in reference condition Dried food material/initial

2 iw cw e i

Temperature Intracellular water Cell wall water Intercellular fluid Intracellular fluid

inf

Infinite frequency

0

Zero frequency

kw we

Knudsen Effective water

s

Surface

w w

0

Chapter 1

Introduction

Food materials are complex in structural nature as they have porous, heterogeneous, and hygroscopic properties along with comprising up to 80–90% water [1]. These high moisture content foodstuff, in which water interactions with other components, play a decision-making role for material and functional properties [2, 3]. Water is not uniformly distributed throughout food material rather located in different spatial environments of other composition and solid structural matrix. Hence, water is essential to understand the material properties of food material prior to any types of processing where heat and mass transfer are involved. Water content is the main measured properties of food materials. Amount of water in food materials is important in different aspects: (a) Stability: water is essential for growth and propagation of all microorganisms. The rate of microbial growth significantly depends on the quantity and distribution of water. In order to ensure a high microbial stability of food, critical moisture content must be maintained. (b) Quality: All of the food qualities including physical, nutritional, and sensorial substantially affected by water content. (c) Unit operation: Proper insight of water content and its distribution is essential to understand underlying physics during food processing including drying, frying, baking or even packaging. (d) Economical: Water is the cheapest component of the manufactured foods. A slight amount of additional water can make a difference in the economic perspective of food. Proper monitoring must need to implement in stopping the manufacturer for maintaining optimum water inclusion during the process. (e) Physical changes: Water content affects the food material properties by influencing the crystal growth, pore formation, and the rate of change of food structure during drying.

© Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_1

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2

1 Introduction

Food is treated as a material it can be classified as non-hygroscopic or hygroscopic according to its water binding characteristics. In addition, on the basis of containing void spaces in food, it can be classified as porous or non–porous. On the other hand, most food materials show an amorphous nature rather than crystal. Taken all of these into consideration, foods are complex materials in terms of physical characteristics compared to other pure and solid crystal materials. In the following sections, different material related characteristics of plant-based foodstuff have been discussed in details. A material can be said to be a hygroscopic material when it possesses a large amount of water physically bound with solid matrix. Considering this nature, many foods can be considered as hygroscopic material although there are some exceptions. In general, hygroscopic material deforms during the water removal processes due to migration of bound water. Another important feature of this type of food material is that vapour pressure in hygroscopic material differs from the vapour pressure for pure water [4]. This discrepancy in vapour pressure depends on the level of water in hygroscopic materials. In a given conditions, the discrepancy in any such properties differ with the types of materials [5]. This can be related to water activity and water mobility in a certain moisture content of food materials [6]. Hygroscopic materials hold a large amount of bound water in the solid matrix and the retention of these bound water content defines the degree of hygroscopicity as well as the water activity [6]. Hygroscopic materials deform during drying due to the loss of bound water and having a relatively low vapour pressure compared to the pure water [4]. From the ongoing discussion on the characteristics of hygroscopic nature, most of the foods, in particular plant and animal-based, can be classified as hygroscopic materials. On the other hand, non-hygroscopic food material contain free water in lion share. The partial pressure of the water in this type of material is almost equal to the vapour pressure of pure water. In this type of water is not strongly attached with adjacent solid structural matrix when the material is completely saturated [7]. In non-hygroscopic materials, the quantity of physically bound water is insignificant and most of the water can be treated as free water. When materials of this kind experience simultaneous heat and mass transfer during processing, their shrinkage is not generally noticeable. Because of an insignificant amount of bound water, non-­ hygroscopic materials generally do not shrink over the time of drying process. Moisture movement in non-hygroscopic materials does not create any further complications as it is found in hygroscopic materials [7, 8]. Food materials also can be also classified as amorphous and crystalline types according to its atomic level orientation. Natural foods including plant and animal-­ based show amorphous nature while many process foods demonstrate crystalline like properties. Moreover, some food processing such as freezing can cause crystallisation of food materials. One of the advantages of these type of processed crystalline food is the prolonged shelf life. However, this modification negatively affects in the flavour, taste, and colour of food materials [9].

1 Introduction

3

Periodically repeated lattice of atoms or molecules are prevailed in crystal materials. In this type of materials, molecules or atoms are packed tightly [10]. Crystals are found in the stable thermodynamic equilibrium state [11]. Introducing heat in it materials up to certain level shows a melting phenomenon. In addition to this, crystal materials that follow this heat capacity concept imply that temperature increased proportionally in response by heating. Taken these feature of crystalline materials, a very limited types of foods including chocolate, margarine ice cream, butter, and sugar can be categorized as crystalline foods. However, most of the biological materials are not found in crystalline form. Materials having a disordered molecular structure, such as atoms or molecules interconnected in a lattice that is not duplicated periodically in space, are known as amorphous materials. Most of the food materials can be placed in this group of material. In amorphous food materials, individual molecules easily interact with external materials. As a result, components such as water can be easily absorbed into amorphous food materials. The morphological feature of amorphous food differs remarkably than crystalline materials. Amorphous materials may consist of a short-range array and regions of varied densities. Eventually, amorphous solids have higher entropy than crystalline materials, since the microstructure of [12]. Therefore, this heterogeneous nature of plant-based food materials including fruits and vegetables at microstructural level could be one of the causes of differences in the water distribution processed foods of the same materials [13]. For example, in plant based food material, the cell wall and extracellular and intercellular space are the main cellular locations in food tissue that contain this water in varying proportions [13]. Plant-based foodstuff can be generally characterized as an insoluble solution of water, composed of a cellular matrix of mainly carbohydrates along with proteins, hemicelluloses, and peptic molecules. These macromolecules influencing the water vapour pressure in the food and vegetables through the interaction with different cellular contains like sugar, salts, hydro colloids, organic acids by polar binding (for small molecules) and surface interaction or capillary action (for large biopolymers) [14]. The effect of water on physical properties depends on the state of water inside the food materials [14, 15]. The water content is of great importance in determining the physicochemical properties, microbiological stability, technological processes, sensory features and shelf life of foods [16]. Dietary fibre of the food materials as a mixture of polysaccharides and lignin [17], are mainly contributed by the cell walls of vegetables, fruits, cereals, pulses which may further be sub-divided into soluble dietary fibre such as pectin and gum and insoluble dietary fibre including lignin, cellulose and hemicellulose depending on their solubility with regards to the water content [17, 18]. The dietary fibre content originated from the fruits and vegetables contain mainly of soluble fibre with high water content whereas the cereals and pulse derived fibres are considered as insoluble dietary fibre due to their main constituent as insoluble cellulose and hemicellulose polymers and form porous matrix structure [19].

4

1 Introduction

Water holding capacity of any material is intensified by its porous matrix and in this similar way, the polysaccharide chain structure performs as a porous media in the food matrix to hold a large portion of the water content [20]. In brief, the diversification in components and structures of foods results in different distribution of water in these. Presence of water at different state make difference in food stability. Modification in amount and distribution of water in food material through any process causes substantial changes in food quality. Therefore, proper knowledge of the amount of water, state and distribution of water in food materials is very essential. In this book, water in food materials has been discussed extensively in Chap. 2. The third chapter is concerned about the characteristics of bound water. The fourth section presents the significance of the removal of bound water from foods during processing. Chapter 5 describes the techniques available in measuring bound water in food material followed by the chapter pertaining common challenges with different techniques of water measurement. Then the appropriate methods for removing bound water from food materials have been discussed. Finally, the conclusion gives a brief summary and critique of the chapters presented in this book.

References 1. M.I.H. Khan, C. Kumar, M.U.H. Joardder, M.A. Karim, Determination of appropriate effective diffusivity for different food materials. Dry. Technol. 35(3), 335–346 (2017) 2. R. Ergun, R. Lietha, R.W. Hartel, Moisture and shelf life in sugar confections. Crit. Rev. Food Sci. Nutr. 50(2), 162–192 (2010) 3. Y. Pomeranz, Functional Properties of Food Components (Academic, New York, 1991) 4. M. Karel, D.B. Lund, Physical Principles of Food Preservation: Revised and Expanded, vol 129 (CRC Press, Boca Raton, 2003) 5. M. Mathlouthi, Water content, water activity, water structure and the stability of foodstuffs. Food Control 12(7), 409–417 (2001) 6. X.D. Chen, A.S. Mujumdar, Drying Technologies in Food Processing (Wiley, New York, 2009) 7. M.U.H. Joardder, A. Karim, C. Kumar, R.J. Brown, Porosity: Establishing the Relationship between Drying Parameters and Dried Food Quality (Springer, Cham, 2015) 8. C.J. Geankoplis, Transport Processes and Unit Operations, 3rd edn. (Prentice Hall, Englewood Cliffs, 1993) 9. Y.H. Roos, S. Drusch, Phase Transitions in Foods (Academic, Oxford, 2015) 10. B. Bhandari, Food Materials Science and Engineering (Wiley, Chichester, 2012) 11. R.W.  Hartel, Recrystallization Processes, Crystallization in Foods, 1st edn. (Aspen Publications, Gaithersburg, 2001), pp. 284–308 12. L. Yu, Amorphous pharmaceutical solids: preparation, characterization and stabilization. Adv. Drug Deliv. Rev. 48(1), 27–42 (2001) 13. J.M. Aguilera, D.W. Stanley, Microstructural Principles of Food Processing and Engineering (Springer, Heidelberg, 1999) 14. E. Maltini, D. Torreggiani, E. Venir, G. Bertolo, Water activity and the preservation of plant foods. Food Chem. 82, 79–86 (2003) 15. S. Basu, U.S. Shivhare, A.S. Mujumdar, Models for sorption isotherms for foods: a review. Dry. Technol. 24(8), 917–930 (2006)

References

5

16. H.D. Isengard, Rapid water determination in foodstuffs. Trends Food Sci. Technol. 6(5), 155– 162 (1995) 17. J.A. Renteria-Flores, Effects of soluble and insoluble dietary fiber on diet digestibility and sow performance. PhD. Dissertation, University of Minnesota, St. Paul (2003) 18. M.U.H. Joardder, R.J. Brown, C. Kumar, M.A. Karim, Effect of cell wall properties on porosity and shrinkage of dried apple. Int. J. Food Prop. 18(10), 2327–2337 (2015) 19. P. Gupta, K.S. Premavalli, In-vitro studies on functional properties of selected natural dietary fibers. Int. J. Food Prop. 14(2), 397–410 (2011) 20. P. Kethireddipalli, Y. Hung, R.D. Phillips, K.H. McWatters, Evaluating the role of cell wall material and soluble protein in the functionality of cowpea (Vigna unguiculata) pastes. J. Food Sci. 67(1), 53–59 (2002)

Chapter 2

Water in Foods

2.1  Introduction Most of the foods especially fruits and vegetables, meat and fish are highly moisture content, where interactions of water with other components play a decision making role for physical and functional properties [1, 2]. Food stuffs encompasses water, cellular matrix of mainly carbohydrates along with proteins, hemicelluloses and pectic molecules. These macromolecules influencing the water vapour pressure in the food and vegetables through the interaction with different cellular contains like sugar, salts, hydro colloids, organic acids by polar binding (for small molecules) and surface interaction or capillary action (for large biopolymers) [3].

2.2  Classification of Water in Food Materials Classification of moisture is one of the hard tasks as it is possible to classify water in different points of view. Moisture content in food materials can be classified in the following ways [4, 5]. These classifications also relate to various properties of water, such as structural, dynamic and thermodynamic aspects. (a) Dynamic definition: According to molecular movement and its impact on the hydrodynamic behaviour of the food materials, water is broadly regarded as free and bound water [6]. (b) Energetic definition: Energy requires overcoming the attraction of water molecules with other materials and to transforming liquid water into vapour. The lowest energy value is experienced to be approximately 5 kcal mol−1 within the water-water interaction at liquid states while the maximum energy level is found to be approximately 100 kcal mol−1 in case of the water and solid interaction. Generally, water with 5–20 kcal mol−1 is termed as free water and water which shows higher energy value is called bound water [4]. © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_2

7

8

2  Water in Foods

(c) Spatial distribution: Water can exist in intercellular, intracellular and cell wall in plant tissue. Intercellular water, sometimes defined as capillary water, shows the criteria of free water [7]. Water in inside cell initially maintains turgor pressure with high bond energy. However, this intracellular water becomes free due to cell fracture as a consequence of high vapour pressure during food processing involving heat transfer. In addition of this, bound water is generally classified as physically bound water and chemically bound water. Physically bound water comprises with absorbed, adsorbed, and trapped or liquid- inclusion water. On the other hand, chemically bound water includes coordinated water, lattice water, hydronium-ion water, hydrogen-bonded, decomposition water [4]. Prior to discussion on types of water, the common terminologies related to water in foods are discussed in details. Due to the close meaning of these terms, sometimes early student and researchers in the field of food science and engineering become confused in their applications in proper places.

2.3  Water Related Terminologies 2.3.1  Moisture Content Moisture content is one of the utmost factors that determine the extent of shrinkage or expansion of the food materials during drying. Generally, it can be expressed as the quantitative amount of water present in a food sample on a wet or dry basis. There are approximately 35 methods available to determine the moisture in a sample. However, moisture content can be termed as the mass of water exist per unit mass of dry food materials. Moisture content also dictates the liquid flow in the intercellular and intracellular spaces. At high moisture content, liquid flow is caused under the action of capillary forces, whereas with a decrease in the moisture content in the pores, capillary force is caused and vapour diffusion takes place due to low liquid permeability [8]. Wet Basis =

weight wet − weight dry

Dry Basis =

weight wet

× 100

weight wet − weight dry weight dry

(2.1)

(2.2)

2.3  Water Related Terminologies

9

2.3.2  Water Concentration Knowledge of water concentration is essential to determine the rate of moisture transfer from a sample to the surrounding. Generally, moisture concentration gradient is the driving force for mass transfer. Therefore, moisture information is essential in the process involving mass transfer including drying, and evaporation. Moisture concentration can be determined from the mass of water and volume of sample. Water concentration in food material can be expressed in mass basis (kg/ m3) or mole basis (kmol/m3). Both approaches are equivalent; however, the application of them depends on the nature of process and solution method.

2.3.3  Intermediate Moisture Content Intermediate-moisture foods or shortly IMFs encompasses foods those undergoes several thermodynamic treatments which extends the shelf life for few months without freezing and ensures foods with relatively soft texture [9]. This type of food including soft candies, jams, many dried foods and even some dried meats are consumable without further preparation and rehydration [10]. Moisture content in the meat and fruits generally varies in the intermediate range (20–30%) and they are most likely termed as intermediate moisture content foods. These so-called intermediate moisture foods (IMF) have no precise definition on the basis of water activity. In general, their water activity is related in the range of 0.60–0.90 and water content 10%–40% as shown in Fig. 2.1 whereas dehydrated foods belong to a water activity below 0.6 [11, 12]. For example, pathogenic microorganisms are unable to grow at a water activity below 0.62 [13]. The term intermediate moisture foods or IMF generally implies the utilization of state-of the-art-technologies to develop the most modern techniques in the field of production and distribution of new food products. IMF preserved foods possess a number of privileges over the conventional dry or high moisture foods in terms of less energy consumption during processing and distribution. The quality, nutrition value, shelf life and cost of the IMF foods are good enough to compare with that of the foods processed with dehydration or thermal food processing method. IMF foods can be distributed without any special packaging and can be stored for a sufficient long time without subsequent freezing or canning even in average humid condition. However, as IMFs are not completely dried and there are presence of plasticity and chewability, they are readily consumable without further any processing and can be formed in bulk without affecting the nutrient value. These advantages make IMF much more compatible than the other processes since there is always a load in food supply chain, limited time for food preservation, lack of energy and technologies for food preservation through freezing and drying in developing countries.

10

2  Water in Foods

Food Tissues

1 0.9

Water Activity

0.8

IMF

0.7 0.6

Dehydrated

0.5 0.4 0.3 0.2 0.1

0

1

2

3

Water Content (g H2O/g solids) Fig. 2.1  Typical relationship between water content and water activity in foods

2.3.4  Equilibrium Moisture Content Internal pressure variation and moisture transportation of the foods are strongly related to the permeability of that sample. At higher permeability, the internal pressure decreases and moisture content increases, and vice-versa. At some state, when the vapour pressure of the water content in the food equals to the surrounding, moisture content to that extent is termed as equilibrium moisture content (EMC) [14]. The composition and water binding characteristics of any food materials give clear understanding about the challenges concern with the water removing treatments. This bonds appears as the absorption force exists between the high molecular weight colloids (like cellulose, pectin, proteins, fats, sugars and mineral salts) and water. Further, this force recognised as the as well-known Vander Waals force [12]. EMC is defined as the state of moisture content in the food material at which the vapour pressure of the food material corresponding to the water content is equal to the surrounding ambient condition [14]. Temperature, relative humidity, and water activity act as a predominant factor to control EMC for a material. At a constant water activity, EMC shows an ascending trend with relative humidity but get lowered with higher temperature depending on the food material properties. Clausius-­ Clapeyron equation describes the essential relationship between vapour pressure and temperature in a system excluding bound water content as well as the effect of temperature to maintain MSI.

2.3  Water Related Terminologies

11

The influence of drying method on the moisture sorption isotherm in addition to equivalent moisture content is given in Fig.  2.2. The variation in EMC during adsorption and desorption process is referred to as the hysteresis. The Fig. 2.2 provides significant information regarding the water content in a food material and also the extent to reach equilibrium condition with the atmospheric humidity in a certain environment. For simplicity of calculation, in most of the cases, the EMC can be considered as the final moisture content to determine the moisture ratio for any foodstuff [16].

2.3.5  Critical Moisture Content The drying curves at the time of food drying, initially maintain a constant rate, after that, it shows sharp fall. Critical moisture content refers to the moisture content range below which there is no or little bit water activity can be observed with no more constant rate. It was also defined as tertiary moisture content by Chen et al. and can be determined from the respiration data of the feedstock [17, 18]. They defined that theoretically, the final stage moisture content must occur at the maximum hygroscopic moisture content (EMC at 100% RH). A very close comparison was found between bound water, sorption equilibrium moisture content and tertiary moisture content for some food materials, as the table shows [19]. Microbial activity and the feedstock chemical composition play a dictating role on critical moisture spray dryer

0.6

FreezeDryer

Vibro-fluidized

Xeq (dry basis)

0.5 0.4 0.3 0.2 0.1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Water activity (aw) Fig. 2.2  Variation of the sorption isotherms with different drying methods for passion fruit pulp powder determined at 20 °C [15]

12

2  Water in Foods

content. Micro bacterial growth varies proportionally with the moisture content up to a maximum limit and thus a further increase in the moisture content lowers the effect [20]. An empirical relationship between the critical moisture content and initial Water Soluble Carbohydrate (WSC) is established where the regression of critical moisture and WSC is used to determine the model and parameters from the Kaleida graph [17]. Critical moisture content = 16.0 + 18.9 log(x), where y is the critical moisture content (% db temperature), and x is the initial WSC content (% db temperature). The WSC content of the sample significantly affects the model up to a critical moisture content range between 2 and 50% [17]. However, Table 2.1 shows the comparison of bound water capacity, tertiary moisture content and equilibrium moisture content at 100% RH for seven materials.

2.3.6  Moisture Removal Rate Moisture content in the food materials and the moisture diffusion coefficient are mainly subjected to the moisture removal rate at the time of drying. In case of porous media in the dehydrated foods and vegetables, a rapid decrease in the vapour pressure has been observed followed by the cease of water diffusion [21]. However, moisture diffusion rate is solely dependent on the moisture concentration, permeability of the water content in the food materials and consequently an instantaneous heat and mass transfer effect can be observed during moisture removal [22, 23]. The following equation represents the moisture diffusion rate relationship. D (m) =

ηRT Mγ

 Cmm kaw  2 2  1 + ( C − 1) k aw

   (2.3)

Where D(m) represents the dependence of the diffusion coefficient on moisture [21].

Table 2.1  Assessment of equilibrium moisture content, tertiary moisture content and bound water capacity at 100% RH for seven materials [19] Materials Cellulose Corn Starch High-amylose corn starch Wheat starch Casein Wheat flour Wheat gluten

Sorption equilibrium moisture content at 100% RH (%) 14.4 26.3 27.8

Drying tertiary moisture content (%) 14.1 27.8 26.7

NMR bound water capacity (%) 14.0 26.8 29.0

29.4 31.2 31.7 37.0

30.8 25.3 28.3 51.4

29.6 30.7 32.2 36.9

2.3  Water Related Terminologies

13

Drying methods measure the mass loss of a product not only by the diffusion of water content but also the losses of the volatile substances contained inherently or formed during the drying process itself [24, 25]. Thus, drying methods have the limitation to specifically determine the moisture content, hence different results can be obtained for the same product with different drying techniques.

2.3.7  Moisture Sorption Isotherm (MSI) The absorption of water by or desorption of water from the foodstuff mostly depends on the thermodynamic behaviour, energy requirement and water content that merely demonstrate the vapour pressure of the foodstuff drying and storing [26, 27]. It is widely used and is an accepted method to study the water binding in food materials. This provides important information such as monolayer moisture content and binding energy from the moisture sorption isotherms for a particular sample. Moisture sorption isotherm is defined as the relationship between aw (or corresponding relative humidity) and EMC of a sample at a particular temperature and shows a sigmoid shape for most of the amorphous structure filled with hydrophilic compounds [28]. Temperature has a significant consideration on the sorption isotherm as the range of temperature affects the storing of food materials as well as on the water activity with the same moisture content. Some of the previous studies reported that at elevated temperature, the EMC decreases for the same water content due to the rise of water molecules to a higher energy level followed by the reduction of active water binding sites in the food materials [29]. However, a reverse behavioural pattern of temperature is observed for high Aw with the presence of large sugar crystal in dry food. An EMC level for a date at aw 0.55 shows decreasing trend with increasing temperature and at higher aw it increases with the further increase in temperature [30]. Moreover, MSI shows significant decision making properties in case of low moisture content food to determine the optimum drying or rehydrated properties at the time of storage [26]. Moisture content increase in the multilayer sorption region at low or intermediate water activity and a sharp increase is observed for capillary condensation region with higher water activity. This fact can be recognized as the local dissolution of sugar and inflammation of proteins in the newly formed active sites but a gradual dissolution takes place in the intermediate water activity region with less active sites in food [31]. Water sorption behaviour in the food material shows a complex phenomenon due to the different polar groups of solvent including proteins, starch, cellulose as well as sugar. With the sorption of water, the polymer experiences changes in terms of composition and physical stability whereas phase transition has demonstrated by the sugar molecules. Generally, the Sigmoid shape of the sorption isotherm is observed in case of food materials [32, 33]. Depending on the moisture absorption pattern relative to the water activity and temperature gradient, five types of MSI has been described [34–36].

14

2  Water in Foods

Langmuir Isotherm

Sigmoid Isotherm

• Derived from the monomolecular adhesion of moisture in the porous structure at a finite volume

• Derived form the asymptomic pattern of soluble products with water activity

Flory-Higgins Isotherm

• Derived for the characterstics of a solvent above the glass transition temperature

Adsorption Isotherm

• Derived from the maximum adsorption of moisture by a body filled with inflattable hydrophilic components

BET Multilayer adsorption Isotherm

• Derived from the adsorption of vapor moisture in charcoal and behave as sigmoid and FloryHiggins isotherm

Fig. 2.3  Different Isotherm models [34–36]

Water absorption in food materials mainly involves the progressive combination of water molecules with food matrix through a chemical reaction, physical interaction or by multilayer condensation [3]. The sorption isotherm as shown in Fig. 2.3 can be divided in three regions. Depending on the bound water molecules appearances, isotherm can be divided mainly into three regions. Bound water at region A, represents strongly bound water. This content has a higher enthalpy of vaporization compared to pure water and it cannot be freezed. Structural water and mono-layer water [37] persists at region A, that is easily absorbed by the hydrophilic and polar groups of the foodstuff (polysaccharides, proteins, etc.) and thus there is no available free water for a chemical reaction. At a very lower water activity (aw) the bound water starts to migrate. Region B is regarded as the transition zone from bound to free water due to the action of less binding force compare to region A. Moreover, the value of enthalpy of vaporization at region B is slightly higher than pure water. The water molecules at region C is loosely bound due to the presence of high relative humidity as shown in Fig. 2.4 and shows similar properties of free water [14, 38]. The bound water at this region moves with a small drop in the water activity (aw). From the sorption isotherm, depending on the quality of bonding energy offers by water molecules to other constituents and thermodynamic consideration, water present in the food stuffs can be grouped into three groups namely, Type I: monolayer water content, Type II: multilayer water content, Type III: water in capillaries and condensed form [39].

2.3  Water Related Terminologies

15

Fig. 2.4  Sorption isotherm models for different food stuff [27]

2.3.7.1  Monolayer Moisture Content (MMC) Water acts as a plasticizer as well as a solvent in the construction of the complex biopolymer matrix of polysaccharide and protein in the plant-based food materials. Thus, the extraction or absorption of water by the cell tissue eventually proceeds a leading part in the mechanical and physicochemical behaviour of the cell wall structure. However, the cell stability is controlled by the water content in the vacuoles which stores water. Vascular solutes show high osmotic interaction through the thin-­ walled parenchyma cells and hence turgor pressure is developed to maintain the elasticity and firmness of the tissue [40] and the tissue collapse with the lost in turgor pressure and it cannot be further reestablished again [41]. The heterogeneous spatial distribution of water molecules in the food matrix give rise to the variation in properties and water content depending on the fraction constituents in the free, capillary and bound water domain. Further, the water content in the foodstuff is broadly classified as monolayer and multilayer water content. Also, the availability of water content in the cell and intercellular spaces contribute in defining water as intercellular and intracellular water content [42]. The MMC exemplifies the maximum primary water content in the water sorption isotherm of a dried food sample as shown in Table 2.2 [3]. The MMC in the ­sensitive

16 Table 2.2  Prediction of the BET monolayer moisture value (g water/100 g dry solids) at two temperatures [52]

2  Water in Foods Product Starchy foods Corn Potato Wheat flour Protein foods Chicken, raw Eggs, dried Gelatine Fruits Banana Pitch Pineapple Vegetables and spices Celery Cinnamon Ginger Onion Horseradish Thyme

20 °C 35 °C 2.01 1.83 1.87

1.88 1.76 1.78

1.91 1.30 2.60

1.73 1.27 2.30

1.51 2.39 3.11

1.24 2.05 2.68

1.69 1.87 1.92 1.69 1.86 1.57

1.62 1.67 1.71 1.41 1.68 1.40

sites of the food cells prevents the direct immersion of oxides by the diffusion of free radicals when storing of dried foods [43]. MMC effectively describes the surface moisture content and specifies the least moisture content for the dehydrated food products [44]. Monolayer water content in different fruits has been presented in the following table [37, 45–48]. The rate of quality degradation in most of the dried foods at or below the monolayer water value is found negligible. MMC corresponds to a water activity of 0.2–0.3 [49]. Separation energy of these monolayer water is very high and will cause decomposition reactions [50]. Chemical composition and molecular structure of the food products strongly affect the monolayer moisture content [51]. During food preservation through drying, water content and energy required to remove water are the two key parameters to food stability and quality. The water removal adjacent to the monolayer is the costly and energy consuming process so far where water molecules bind to the solid surface monolayer at higher energy. Moreover, the final water content after drying corresponds to the monolayer moisture content which must have a water activity between 0.1 and 0.4 for the most stable product [53]. 2.3.7.2  Multilayer Water Content The spatial distribution of water content in the cells and between the cells of food products is termed as multilayer water content. Physically the water content of the monolayer region reflects the changes of water content in the region between the

2.3  Water Related Terminologies

17

second and third group of water [44, 54]. The extent of structural changes of food material during drying depends on this water content. The multilayer water layer can be calculated at a water activity of 0.784 whereas with the application of NMR and electron spin resonance causes a change in resonance point that yields a water activity corresponding to 0.79 [44, 55, 56]. Moreover, multilayer water content shows a good relationship with the gelatinization parameter and in an instance, better R2 value has observed is desorption process. This variation can be addressed by the formation of new intimate binding sites during desorption and incomplete hydration in adsorption for hysteresis, although the ratio of monolayer and multi-­ layer water content remain same throughout the process [57]. Figure 2.5 shows the schematic diagram of BET isotherm layers.

2.3.8  Phases of Water Water can exist in one of the three very common form of materials namely, solid, liquid and vapour. As one of the most vital ingredients in foods, water almost exists in the three characteristics forms during food processing, preservation, and consumption. Although water always exist as a mixture with the solid state components which determine its chemical behaviour, the purest form of water can only be obtained during crystalline or vapour phase. Hence, the phase change appearance has a crucial impact on the determination of food process ability and quality improvement at the time of freezing or drying. Moreover, sensory and mechanical behaviour of the foods has been defined by the physical condition of the solid crystalline or amorphous components such as proteins, carbohydrates, minerals, salts, and fats. Phase change for example vaporization needs a specific amount of energy as the latent heat of vaporization or latent heat of sublimation. The amount of this energy

Fig. 2.5  The schematic diagram of BET isotherm layers [57]

18

2  Water in Foods

depends on the nature of initial phase of water. If vapour phase is derived from liquid, energy requirement for this phase changes is equal to the latent heat of vaporization, whereas the quantity of energy required to vaporize solid water is equal to the latent heat of sublimation.

2.3.9  Unfrozen Water Water that maintains liquid phase at a temperature significantly below the equilibrium freezing temperature of the food materials can be categorised as unfrozen water. Many researchers consider the amount of unfreezlable water manifest most accurate definition of bound water [58–61]. On the basis of this definition is that the bound water can be measure as the water that prevails in the liquid state at low temperature down to −80 °C [62, 63].

2.3.10  Water Potential The unified relationship to describe the governing factors to delineate the presence of water in intercellular and intracellular condition and cell wall pressure is termed as water potential. Chemical activity or free energy is also an important factor to define water potential and in case of bound water, negative water potential can be observed due to less free energy as compared to free water. Moreover, solute concentration, the pressure gradient in water content determines the chemical activity for bound and free water content [64]. In other words, water potential can be addressed by the difference between the Gibbs free energy of water at any given condition to the Gibbs free energy of water at the standard condition. This approach can be used to illustrate the mass transfer in hygroscopic materials at the time of drying. The relationship among the water potential with other potentials as the difference between two or more positions can be described by the following equation. In this context, the potentials do not represent the absolute value and can be omitted as negligible values.

Ψ = Ψm + Ψs + Ψp + Ψg



(2.4)

Where, Ψ = total or combined water potential Ψm = matric or capillarypotential Ψs = solute or osmotic potential Ψp = pressure potential or turgor pressure Ψg = gravitational potential. Moreover, the water potentials of organic solution and chemical solution of water is related by the following equation [65]:

2.3  Water Related Terminologies

19

ψ =

µw − µ 0

(2.5)

w

Vw



Where, ψ = water activity potential (Pa) μw = chemical potential of water in the solution (J/mol) μw0 = chemical potential of water in reference condition (J/mol) Vw = mean partial molar volume of water (m3/mol) Again the relationship between water potential and water activity of the solvent is given by:

ψ =

−RTlnaw Vw

(2.6)

Where, R is the universal gas constant (J/mol K) and T is the absolute temperature (K). Considering, pressure and gravitational potential, the equation becomes as follows [66]:

ψ =

RTlnaw + ( P − P0 ) + ρ gh Vw

(2.7)

RTlnaw = osmotic potential Vw (P − P0) = pressure potential ρgh = gravitional potential

Where,

2.3.11  Water Activity Water content is not adequate information while the stability and shelf-life are concerns. Another parameter called water activity is needed to be introduced, as it denotes the availability of water to the microorganisms to growth and reproduction. There is no simple correlation ship exists between water content and water activity as it depends on several other things. For instance, while salami and cooked beef of 60 percent moisture content show water activity of 0.82 and 0.98 respectively.

2.3.12  Water Mobility The state of water in the foods can be described in the form of bound or free water and freezable or nonfreezable water depending on the mobility of water in the intercellular, intracellular and cell wall spaces. Water mobility deals with the molecule

20

2  Water in Foods

level water motion in terms of vibrational, rotational and transitional forms whereas the water activity and glass transition study is limited to the macro level. The mobility of water molecules varies irrespective to the water activity and also shows mobile behaviour below and above the glass transition temperature.

2.3.13  Water Retention Capacity The quantity of water captured by a known weight fibre under given conditions, is measured by centrifugation and is defined as either water-holding capacity or water-­ binding capacity [67]. Hence, WRC measures water held by the insoluble matrix. The nature of hydration of plant is merely defined as water holding/binding capacity. WRC is mainly dependent on the hydration properties, cell wall material, moisture content, oil absorption properties [68, 69]. Also different WRC values for the same sample can be obtained due to the adoption of different sample preparation techniques [68]. Heller and Hackler found the following values of water holding capacities of various fruits and vegetables [70].

2.4  Types of Water Water can be classified based on their spatial distribution and specific properties. The classification varies due to the diverse nature of food materials and their wide range of applications. Taken different classification of water in foods into consideration, Joardder et al. presented different proportions of water in a pyramid-like distribution as demonstrated in Fig. 2.6 [71].

2.4.1  Free Water Free water is the water available in the intercellular spaces of the foodstuff as a mixture of gas and liquid. Free water can be characterised by the crystallization property that turns into the crystal form during first-order phase transition at 0 °C and having a negligible effect on the polymer matrix [72]. With an increasing water activity (aw), free water contents are more available due to the weak binding energy associated with the available water content and thus shows a sharp upward trend in the absorption isotherm [73, 74]. However, free water facilitates internal transport of compounds and microbial growth, and high temperature decomposition reaction during drying without subsequent shrinkage. High energy requirement and damage of the food quality is experienced during the removal of the bound water when all

2.4  Types of Water

21

Fig. 2.6  Different proportions of water in a pyramid-like distribution in food materials [71]

of the free water migrates. Free water sometimes also named as bulk water and capillary water depending on the nature of food material.

2.4.2  Bound Water There is no unanimous definition of “bound water” rather different definitions of bound water are available in the literature. A very limited number of methods is available for measuring tightly bound water [50, 60]. However, this term generally used to define the water closely attached with other compounds of food materials and it shows different properties from remaining “free water”. The hydrated water content in the plant based food materials shows crystallization property depending on the intermolecular interactions with the polymer matrix. Among these water molecules, those that show a high attraction force to the matrix polymer and turn into crystal at a temperature below 0 °C during heating, may be grouped into both freezing and non-freezing bound water content [72, 75, 76]. The idea of bound water depicts the amount of water firmly bound with the colloidal phase in a water solution in such a bond that the separation as a solvent or from the non-water constituents is restricted by the high heat of absorption [77]. From some of the other literature, bound water is merely defined as that water content which does not freeze or remain in the liquid phase at the sub-freezing temperature, although the pressure

22

2  Water in Foods

of salt and sugar pertains the liquid pressure and hence the freezing condition [61, 78]. From the molecular moisture sorption concept, bound water is contributed by the monolayer and multilayer water content and solely depends on the extent of molecular dynamics [79]. 2.4.2.1  Physically Bound Water A considerable amount of water content in foods is bound by molecular attraction with the protein, carbohydrate, and minerals rather than completely surrounded by the water content. This type of bond between the other molecular components and water is totally different from the water to water molecules. Thus a significant change in the physicochemical properties in the form of melting point, boiling point, density, specific heat of vaporization, electro-magnetic absorption spectra, and dielectric properties are observed for this type of bound water. 2.4.2.2  Chemically Bound Water Apart from the physically bound water, a significant fraction of water molecule may be bounded by some other molecules except for water and minerals to form hydrate such as NaSO4.10H2O. This type of bonding is stronger than the water to water bond and gives different physico-chemical properties like lower melting point or higher boiling point compared to the bulk water.

2.4.3  Spatial Water Distribution Water can be categorized on the basis of the spatial location of their existence in the food. For example, in biological tissue, water can be present at intercellular, intracellular spaces and in cell wall as shown in Fig. 2.7. 2.4.3.1  Intercellular Water Water content at the intercellular spaces mainly in the forms of voids, crevices, capillaries and within the cell to cell wall, is referred as the intercellular water content. This type of water content comprises 5–15% by weight basis to the total water content [8]. This water content starts to flow in the vapour form at the early stage of the drying process followed by without any significant shrinkage but after migration of the intercellular water, pore formation starts in an epidemic form which commonly referred to as the falling rate stage of drying.

2.4  Types of Water

23

Fig. 2.7  Spatial distribution of water in cellular foods

2.4.3.2  Intracellular Water Intracellular water refers to the water present within the cell and cell wall which consists almost 78–97% (wet basis) [80] of the total water content in the fresh vegetables and foodstuff. At higher temperature, after migration of the intercellular water content, the falling rate stage of drying takes place. With the beginning of the cell water removal, cell wall gets ruptured and pore formation takes place followed by cell collapse. However, it is hard to measure the intracellular water concisely but steps have made for better approximation [81]. The cell moisture content is the representation of summation of the moisture content by all of the individual cell components. An individual moisture content of the cells pertains a decision making role for the food quality than the overall moisture content of the food products. The moisture content of any cell components can be determined by the vapour pressure of the water content and the relative humidity from the corresponding moisture isotherm of that food material. The moisture index represents an essential correlation between the equilibrium pressure of water vapour and the moisture content of the individual components rather than evaluating for the cell in a whole. 2.4.3.3  Cell Wall Water The hygroscopic nature of the plant based food materials results in the heterogeneous distribution of water in the foods cellular matrix. This complex cellular environment in the food sample can be broadly divided into intercellular environment, the intracellular environment, and the cell wall environment. In terms of water holding capacity, intercellular water is defined as free water (FW), intracellular water is

24

2  Water in Foods

known as loosely bound water (LBW) and cell wall water is termed as strongly bound water (SBW) [82]. Sometimes, a small portion of water held in the micro space in the cell wall environment which can be categorized as extracellular water or cell wall water [83]. The water quality and quantity in different cellular environment causes a variation in the binding energy which considerably affects the energy requirement and optimum drying parameters as well [7]. From the above discussion reveals that there are many terms associated in relation to “water” in food material. Moreover, different types of water make this list even larger. Proper terminology to refer the intended water-related issue is vital in food processing. Therefore, a clear understanding of different concepts in relation with water is essential for the people dealing with food manufacturing, processing, and preservation.

References 1. R. Ergun, R. Lietha, R.W. Hartel, Moisture and shelf life in sugar confections. Crit. Rev. Food Sci. Nutr. 50(2), 162–192 (2010) 2. Y. Pomeranz, Functional Properties of Food Components (Academic, New York, 1991) 3. E. Maltini, D. Torreggiani, E. Venir, G. Bertolo, Water activity and the preservation of plant foods. Food Chem. 82, 79–86 (2003) 4. J.W. Pyper, The determination of moisture in solids: a selected review. Anal. Chim. Acta 170, 159–175 (1985) 5. M.U.H.  Joardder, A.  Karim, C.  Kumar, R.  J. Brown, Effect of cell wall properties of plant tissue on porosity and shrinkage of dried apple, in Proceedings of the 2014 International Conference on Food Properties (ICFP2014) (2014) 6. N. Pan, Z. Sun, Essentials of psychrometry and capillary hydrostatics, in Thermal and Moisture Transport in Fibrous Materials (Woodhead Publishing Cambridge, 2006), pp. 102–135 7. M.I.H. Khan, R.M. Wellard, S.A. Nagy, M.U.H. Joardder, M.A. Karim, Investigation of bound and free water in plant-based food material using NMR T2 relaxometry. Innov. Food Sci. Emerg. Technol. 38, 252–261 (2016) 8. A. Halder, A. Dhall, A.K. Datta, Modeling transport in porous media with phase change: applications to food processing. J. Heat Transf. 133(3), 31010 (2011) 9. P.S.  Taoukis, M.  Richardson, Chapter 11. Principles of intermediate-moisture foods and related technology, in Water Activity in Foods (Blackwell, Iowa, 2008), p. 273 10. W.D. Bascom, R.Y. Ting, R.J. Moulton, C.K. Riew, A.R. Siebert, The fracture of an epoxy polymer containing elastomeric modifiers. J. Mater. Sci. 16(10), 2657–2664 (1981) 11. A.J.  Fontana Jr., S.J.  Schmidt, T.P.  Labuza, Water Activity in Foods: Fundamentals and Applications, vol 13 (Wiley, Hoboken, 2008) 12. G. Barbosa-Canovas, J. Fernandez-Molina, S. Alzamora, M. Tapia, A. Lopez-Malo, J.C. Welti, Technical Manual FAO Agricultural Services Bulletin 149 (Food and Agriculture Organization of The United Nations, Rome, 2003), p. 3 13. E.  Sandulache, Water activity concept and its role in food preservation. Tech. Univ. Mold., 42–43 (2012) 14. A.S. Mujumdar, S. Devahastin, Mujumdar’s Practical Guide to Industrial Drying (Exergex Corporation, Watertown, 2000) 15. M.A.M. Pedro, J. Telis-Romero, V.R.N. Telis, Effect of drying method on the adsorption isotherms and isosteric heat of passion fruit pulp powder. Food Sci. Technol. 30(4), 993–1000 (2010)

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16. L.A. Ramallo, R.H. Mascheroni, Quality evaluation of pineapple fruit during drying process. Food Bioprod. Process. 90(2), 275–283 (2012) 17. F. Rezaei, J.S. Vandergheynst, Critical moisture content for microbial growth in dried food-­ processing residues. J. Sci. Food Agric. 90(12), 2000–2005 (2010) 18. C.S.  Chen, W.H.  Johnson, Kinetics of moisture movement in hygroscopic materials. I. Theoretical considerations of drying phenomena. Trans. ASAE 12(1), 109–113 (1969) 19. H.K.  Leung, M.P.  Steinberg, Water binding of food constituents as, determined by NMR, freezing, sorption and dehydration. J. Food Sci. 44(4), 1212–1216 (1979) 20. J. Trouillet, J. Fagon, Y. Domart, J. Chastre, J. Pierre, C. Gibert, Use of granulated sugar in treatment of open mediastinitis after cardiac surgery. Lancet 326(8448), 180–184 (1985) 21. R.J. Aguerre, C. Suarez, Diffusion of bound water in starchy materials: application to drying. J. Food Eng. 64, 389–395 (2004) 22. Y.M.  Chen et  al., Platelet adhesion to human umbilical vein endothelial cells cultured on anionic hydrogel scaffolds. Biomaterials 28(10), 1752–1760 (2007) 23. L.M.  Sun, F.  Meunier, A detailed model for nonisothermal sorption in porous adsorbents. Chem. Eng. Sci. 42(7), 1585–1593 (1987) 24. H.-D.  Isengard, Water determination  – scientific and economic dimensions. Food Chem. 106(4), 1393–1398 (2008) 25. H.-D.  Isengard, J.-M.  Färber, “Hidden parameters” of infrared drying for determining low water contents in instant powders. Talanta 50(2), 239–246 (1999) 26. A.M. Goula, T.D. Karapantsios, D.S. Achilias, K.G. Adamopoulos, Water sorption isotherms and glass transition temperature of spray dried tomato pulp. J. Food Eng. 85, 73–83 (2008) 27. S. Basu, U.S. Shivhare, A.S. Mujumdar, Models for sorption isotherms for foods: a review. Dry. Technol. 24(8), 917–930 (2006) 28. A.H.  Al-Muhtaseb, W.A.M.  McMinn, T.R.A.  Magee, Water sorption isotherms of starch powders: part 1: mathematical description of experimental data. J. Food Eng. 61(3), 297–307 (2004) 29. K.B.  Palipane, R.H.  Driscoll, Moisture sorption characteristics of in-shell macadamia nuts. J. Food Eng. 18(1), 63–76 (1993) 30. R.M. Myhara, S. Sablani, Unification of fruit water sorption isotherms using artificial neural networks. Dry. Technol. 19(8), 1543–1554 (2001) 31. K.O.  Falade, O.C.  Aworh, Adsorption isotherms of osmo-oven dried African star apple (Chrysophyllum albidum) and African mango (Irvingia gabonensis) slices. Eur. Food Res. Technol. 218(3), 278–283 (2004) 32. M. Mathlouthi, Water content, water activity, water structure and the stability of foodstuffs. Food Control 12(7), 409–417 (2001) 33. D.R.  Heldman, D.B.  Lund, C.  Sabliov, Handbook of Food Engineering (CRC Press, Boca Raton, 2006) 34. S.  Brunauer, P.H.  Emmett, E.  Teller, Adsorption of gases in multimolecular layers. J.  Am. Chem. Soc. 60(2), 309–319 (1938) 35. S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, On a theory of the van der Waals adsorption of gases. J. Am. Chem. Soc. 62(7), 1723–1732 (1940) 36. M. Mathlouthi, B. Roge, Water vapour sorption isotherms and the caking of food powders. Food Chem. 82(1), 61–71 (2003) 37. R.M. Syamaladevi, S.S. Sablani, J. Tang, J. Powers, B.G. Swanson, Water sorption and glass transition temperatures in red raspberry (Rubus idaeus). Thermochim. Acta 503, 90–96 (2010) 38. A.S. Mujumdar, Principles, classification, and selection of dryers, in Handbook of Industrial Drying, 3rd edn. (CRC Press, Boca Raton, 2006) 39. L.B. Rockland, Water activity and strage stability. Food Technol. 23, 1241–1251 (1969) 40. R.  Ilker, A.S.  Szczesniak, Structural and chemical bases for texture of plant foodstuffs. J. Texture Stud. 21(1), 1–36 (1990) 41. R.M. Reeve, Relationships of histological structure to texture of fresh and processed fruits and vegetables. J. Texture Stud. 1(3), 247–284 (1970)

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42. M.U.H. Joardder, M.A. Karim, C. Kumar, Better understanding of food material on the basis of water distribution using thermogravimetric analysis, in International Conference on Mechanical, Industrial and Materials Engineering (ICMIME2013), Rajshahi, Bangladesh (2013) 43. H.  Salwin, The role of moisture in deteriorative reactions of dehydrated foods, in Freeze-­ Drying of Foods (1962), p. 58 44. L.B. Rockland, Water activity and storage stability. Food Technol. 23(10), 1241 (1969) 45. M.E. Katekawa, M.A. Silva, On the influence of glass transition on shrinkage in convective drying of fruits: a case study of banana drying. Dry. Technol. 25(10), 1659–1666 (2007) 46. G. Moraga, N. Martınez-Navarrete, A. Chiralt, Water sorption isotherms and glass transition in strawberries: influence of pretreatment. J. Food Eng. 62(4), 315–321 (2004) 47. G. Moraga, N. Martínez-Navarrete, A. Chiralt, Water sorption isotherms and phase transitions in kiwifruit. J. Food Eng. 72(2), 147–156 (2006) 48. A. Lopez-Malo, E. Palou, J. Welti, P. Corte, A. Argaiz, Moisture sorption characteristics of blanched and osmotically treated apples and papayas. Dry. Technol. 15(3–4), 1173–1185 (1997) 49. T.P.  Labuza, Sorption phenomena in foods: theoretical and practical aspects, in Theory, Determination and Control of Physical Properties of Food Materials (Springer, Berlin, 1975), pp. 197–219 50. H.-D.  Isengard, Water content, one of the most important properties of food. Food Control 12(7), 395–400 (2001) 51. M.S. Rahman, State diagram of foods: its potential use in food processing and product stability. Trends Food Sci. Technol. 17(3), 129–141 (2006) 52. H.A. Iglesias, J. Chirife, Correlation of BET monolayer moisture content in foods with temperature. Int. J. Food Sci. Technol. 19(4), 503–506 (1984) 53. J.  Welti-Chanes, E.  Pérez, J.A.  Guerrero-Beltrán, S.M.  Alzamora, F.  Vergara-Balderas, Chapter 13. Applications of water activity management in the food industry, in Water Activity in Foods (Blackwell, Iowa, 2008), p. 341 54. L. Godbillot, P. Dole, C. Joly, B. Rogé, M. Mathlouthi, Analysis of water binding in starch plasticized films. Food Chem. 96(3), 380–386 (2006) 55. M. Caurie, A practical approach to water sorption isotherms and the basis for the determination of optimum moisture levels of dehydrated foods. Int. J. Food Sci. Technol. 6(1), 85–93 (1971) 56. Y.  Sato, S.  Noguchi, Water sorption characteristics of dietary fibers. J.  Home Econ. Japan 44(8), 625–631 (1993) 57. A.H.  Yukinori Sato, Y.  Wadab, Relationship between monolayer and multilayer water contents, and involvement in gelatinization of baked starch products. J. Food Eng. 96(2), 172–178 (2010) 58. J. Wolfe, G. Bryant, K.L. Koster, What is’ unfreezable water’, how unfreezable is it and how much is there? CryoLetters 23(3), 157–166 (2002) 59. N. Aktas, Y. Tülek, H.Y. Gökalp, Determination of differences in free and bound water contents of beef muscle by DSC under various freezing conditions. J. Therm. Anal. 50(4), 617– 624 (1997) 60. R. Toledo, M.P. Steinberg, A.I. Nelson, Quantitative determination of bound water by NMR. J. Food Sci. 33(3), 315–317 (1968) 61. J. Kuprianoff, Bound water in foods, in Fundamental Aspects of the Dehydration of Foodstuffs (1958), pp. 14–23 62. R.B. Duckworth, Differential thermal analysis of frozen food systems. I. The determination of unfreezable water. Int. J. Food Sci. Technol. 6(3), 317–327 (1971) 63. E.G. Murakami, M.R. Okos, Calculation of initial freezing point, effective molecular weight and unfreezable water of food materials from composition and thermal conductivity data1. J. Food Process Eng. 19(3), 301–320 (1996) 64. O.A. Plumb, G.A. Spolek, B.A. Olmstead, Heat and mass transfer in wood during drying. Int. J. Heat Mass Transf. 28(9), 1669–1678 (1985) 65. D.M. Griffin, Water and microbial stress, in Advances in Microbial Ecology (Springer, Berlin, 1981), pp. 91–136

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66. Z. Liu, Z. Wu, X. Wang, J. Song, W. Wu, Numerical simulation and experimental study of deep bed corn drying based on water potential. Math. Probl. Eng. 2015, 13 (2015) 67. J.A. Robertson, F.D. de Monredon, P. Dysseler, F. Guillon, R. Amado, J.-F. Thibault, Hydration properties of dietary fibre and resistant starch: a European collaborative study. LWT-Food Sci. Technol. 33(2), 72–79 (2000) 68. F.  Figuerola, M.L.  Hurtado, A.M.  Estévez, I.  Chiffelle, F.  Asenjo, Fibre concentrates from apple pomace and citrus peel as potential fibre sources for food enrichment. Food Chem. 91(3), 395–401 (2005) 69. S. Vetter, H. Kunzek, Material properties of processed fruit and vegetables. II. Water hydration properties of cell wall materials from apples. Eur. Food Res. Technol. 214(1), 43–51 (2002) 70. S.N. Heller, L.R. Hackler, Water-holding capacity of various sources of plant fiber. J. Food Sci. 42(4), 1137 (1977) 71. M.U.H. Joardder, C. Kumar, M.A. Karim, Food structure: its formation and relationships with other properties. Crit. Rev. Food Sci. Nutr. 57(6), 1190–1205 (2017) 72. H.  Hatakeyama, T.  Hatakeyama, Interaction between water and hydrophilic polymers. Thermochim. Acta 308(1–2), 3–22 (1998) 73. I.C. Watt, Theory of water sorption by biological materials, in Physical Properties of Foods, ed. by R. Jowitt et al. (1983) 74. M. Caurie, The unimolecular character of the classical Brunauer, Emmett and Teller adsorption equation and moisture adsorption. Int. J. Food Sci. Technol. 40(3), 283–293 (2005) 75. M. Tanaka, A. Mochizuki, Effect of water structure on blood compatibility—thermal analysis of water in poly (meth) acrylate. J. Biomed. Mater. Res. Part A 68(4), 684–695 (2004) 76. T. Hatakeyama, H. Hatakeyama, Thermal Properties of Green Polymers and Biocomposites, vol 4 (Springer Science & Business Media, Berlin, 2006) 77. D.R. Briggs, Water relationships in colloids. II. J. Phys. Chem. 36(1), 367–386 (1932) 78. H.T.  Meryman, Freeze-drying, in Cryobiology, ed. by H.T.  Meryman (Academic, London, 1966) pp. 609–663 79. L.C. Dickinson, P. Chinachoti, Mobility of “ unfreezable ” and “ freezable ” water in waxy corn starch by 2 H and 1 H NMR. J. Agric. Food Chem. 8561(96), 62–71 (1998) 80. A.  Halder, A.K.  Datta, R.M.  Spanswick, Water transport in cellular tissues during thermal processing. AICHE J. 57(9), 2574–2588 (2011) 81. M.U.H. Joardder, R.J. Brown, C. Kumar, M.A. Karim, Effect of cell wall properties on porosity and shrinkage of dried apple. Int. J. Food Prop. 18(10), 2327–2337 (2015) 82. M.U.H. Joardder, A. Karim, C. Kumar, R.J. Brown, Porosity: Establishing the Relationship Between Drying Parameters and Dried Food Quality (Springer, Berlin, 2015) 83. L. Van Der Weerd, M.M.A.E. Claessens, C. Efde, H. Van As, Nuclear magnetic resonanceimaging of membrane permeability changes in plants during osmoticstress. Plant Cell Environ. 25(11), 1539–1549 (2002)

Chapter 3

Characteristics of Bound Water

3.1  Introduction In general, pure water shows many unusual properties in nature including its exceptional boiling point, freezing point, latent heat of evaporation, specific heat, viscosity and universal solvent. These anomalous properties vary significantly once it dissolves with other substance as in food materials. The properties are even more exceptional when water is bound in other substance in food materials. Proper knowledge of not only amount of water but also of the state of water is required in understanding of most of the aspects of food materials including determining processing conditions, stability prediction, and quality retention. The proper insight of the distribution of water can enhance the level of understanding of underlying physics of different phenomena during food processing. Moreover, the changing of physical, chemical and other qualities are strongly influenced by the distribution of water during food processing. Classification of moisture is one of the hardest tasks, as it is possible to classify water in different points of view. According to the interest, moisture content in food materials can be classified in the following ways [1, 2]. These classifications also relate to various properties of water, such as structural, dynamic and thermodynamic aspects. 1. Thermodynamic aspects: At a certain temperature and relative humidity, water is in thermal equilibrium with its surroundings 2. Structural aspects: The orientation and position of water molecules in relation to other substrates and other water molecules. 3. Dynamic aspects: Contribution of water to hydrodynamic characteristics of the system and motions of water molecules. There is no unanimous definition of “bound water”, rather different definitions of bound water are available in literature. There are very limited methods available for measuring tightly bound water [3, 4]. However, this term generally used to define © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_3

29

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3  Characteristics of Bound Water

the water closely attached with other compounds of food materials and it shows different properties from the remaining “free water”. The hydrated water content in the plant-based food materials shows crystallization properties depending on the intermolecular interactions with the polymer matrix. Among these water molecules, some of them show high attraction force to the matrix polymer and turn into crystal at a temperature below 0 °C during heating, may be grouped into both freezing and non-freezing bound water content [5–8]. The idea of bound water depicts the amount of water firmly bound with the colloidal phase in a water solution in such a bond that the separation as a solvent or from the non-water constituents is restricted by the high heat of absorption [8]. From some other literature, bound water is merely defined as that water content which does not freeze or remain in the liquid phase at the sub-freezing temperature, in spite of the pressure of salt and sugar pertaining the liquid pressure leading to freezing condition [9, 10]. From the molecular moisture sorption concept, bound water is contributed by the monolayer and multilayer water content and solely depends on the extent of molecular dynamics [11]. Generally, bound water is estimated by observing the moisture level at which significant inconsistency in behaviour exist, such as: dehydration, solvent properties, binding energy, spectroscopic properties or freezing [12]. These properties, in case of bound water, generally show as followings [13, 14]. It is worthy to mention that not all of the characteristics of bound water classify water as bound water at a same amount moisture content. Therefore, there is no unanimous definition of bound water in food materials.

3.1.1  Lower Vapour Pressure Temperature and water transportation phenomena dictate the vapour pressure of the bound water molecules in the food materials. Moreover, the elevation of the boiling point of a solution varies proportionally with the decrease in the vapour pressure of the solvent. The relationship between the boiling point and solute concentration can be evaluated in the same manner as the calculation of the freezing point. For a given temperature T, the vapour pressure of water P0 can be found by the following equation [15],



 17.502T  p0 ( T ) = 0.611 exp    240.97 + T  (3.1)

Vapour pressure of the moisture content in most of the fresh foodstuff is almost unity because of high free water content. However, bound water is a fraction of total water content, firmly bonded by the solutes which consequently reduces the water activity as well as the vapour pressure in the monolayer water content [16]. Another explanation can be provided from the concept of entropy of the bound water. Higher entropy can be observed in case of bound water which results in higher lower vapour pressure of bound water.

3.1 Introduction

31

3.1.2  Higher Binding Energy Binding energy controls the water content of foodstuff by the latent heat gradient of the water during adsorption and condensation by the solid surface [17]. The three components of bound water namely primary, secondary and tertiary can be calculated by using the equation was developed by Brunauer, Gibbs’ free energy, and the Clausius–Clapeyron equation respectively [18]: For primary bound water, Binding energy,

C = k exp {( ∆H a − ∆H c ) / RT }

(3.2)

Which can be rewritten as:



 ( ∆H a − ∆H c ) ln ( c ) = ln ( k ) +  R 

 1  +  T (3.3)

Gibbs’ free energy and the Clausius–Clapeyron equation:



 ( ∆H a − ∆H c ) ln ( aw ) =  R 

 1     + C  T  (3.4)

Where, ΔHa = Heat of adsorption   ΔHc = Heat of condensation   R = Universal gas constant  K = Constant Many important physical parameters including water and solute activities, partial equilibrium properties can be calculated from the Gibbs’ free energy [19]. The value for C is calculated by using BET equation. The slope of the Arrhenius plot of ln(C) against 1/T multiplied by the value of R gives the primary bound water binding energy content [20]. The free water content in the food matrix lowering down during drying throughout the intercellular spaces and even throughout the cell wall, causes a significant amount of mechanical stress to be stored with the commencement of extraction of bound water [21]. In order to proceed this drying process, a strong heat flux is applied for dehydration followed by moisture vaporization and glass transition of the food matrix [21]. This mass transfer process eventually lowering the mobility temperature to almost zero and eventually decreasing the shrinkage rate with an increase in the collapse rate and urges for higher energy to move the bound water [22, 23].

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3  Characteristics of Bound Water

3.1.3  Lower Mobility of Water Water mobility refers to molecule level motion of water, which associates vibrational, rotational and transitional motion of water considering water activity as a macro-level concept. The intermolecular motion of water molecules changes the molecular shape and is termed as vibrational motion. The spatial position of the water molecules gets altered by transitional motion and describes the flow through cells and capillaries. The change in molecular orientation can be explained by the in-place rotational motion of water molecules [24]. Moreover, mobility in the food matrix can also be defined by either Fickian or non-Fickian or fractal diffusion. The molecules can move locally or as a whole as a reactant for the commencement of a reaction to take place [25]. Generally, water loses its mobility significantly over the course of transition from a rubbery state to glassy one. At sufficient below glass transition temperature, mobility decreases sharply but never tends to a value near-­ zero even for the smaller water molecules. In simpler words, Bound water demonstrates immobilized character at high frequency due to the presence of the adjacent elements which continuously hindered the rotational mobility and reduce the permittivity [26].

3.1.4  Unfreeze-Ability at Low Temperature Bound water is categorized as the water content in the matrix polymer that is strongly held by the molecular interactions and non-crystallisable or non-freezing at a low or sub-freezing temperature [5–7, 9]. Moreover, the fraction proportion of water content that can persist at the liquid state at a very low temperature without being frozen can be calculated as the bound water [9, 11]. Several studies reports that the bound water content is free from the temperature effect and remains unfrozen up to −80 °C; whereas, some NMR results demonstrates that the unfrozen water amount decreases with the decreasing temperature and at about −80 °C all the water content became frozen [3, 27–29]. This nature of the bound water can also be elaborated from the very basic of equilibrium freezing temperature (EFT) of solutions. As in the case of the foods, water acts as the solvent and the freezing temperature is affected by the concentration of the solutes. During the measurement of EFT of water, the temperature at which the last crystals of ice get melts due to heating is taken as the thermodynamic melting temperature of the frozen substances and the value of EFT has a lower value than the freezing temperature of water. Table 3.1 presents an summery of freezing temperatures of some selected food [30–31].

3.1 Introduction Table 3.1  Initial freezing temperature of different foods [30, 31]

33

Food item Gelatin gel Asparagus Spinach Milk, skimmed Milk, whole Cod Tomato Chicken Carp Pork, lean Perch Orion Beef Fructose solution Orange juice Apple Peach Pear

Water content (%) 98.3 92.6 90.2 90.0 87.5 80.3 92.9 76.0 77.8 76.0 79.1 89.7 74.0 90.0 89.0 85.8 85.1 83.5

Tf (°C) −0.13 −0.52 −0.56 −0.60 −0.60 −0.69 −0.72 −0.79 −0.80 −0.82 −0.86 −0.90 −1.00 −1.17 −1.17 −1.45 −1.56 −1.62

3.1.5  Unavailability as a Solvent Pure water is treated as the universal solvent with the exception of hydrophobic molecules due to its high permittivity and small molar volume. These properties allow water towards multiple stable interactions between water molecules and the dissolved ions [32]. As bound water possesses a stronger structural bonding than the free water, it offers less option as a solvent. Physiochemical properties of pure water have a dictating role on the physical state of foods. However, the quality and quantity of the dissolved hydrophilic components exhibits different characteristics of water in solutions or in foods. Moreover, the complex structural form of foods such as the plant-based ones makes it much more difficult to relate the compositional pattern with the phase behaviour of water [8]. Depending on the degree of bound water, some form of bound water shows solvent or liquid-like properties just above the monolayer water content to some extent, and thus it is referred to as the unbound solvent water content rather than the bound non-solvent water [33, 34].

3.1.6  Dielectric Property Dielectric property depend on the ionic polarization or dipole nature of material. Water shows the dipole nature and substantially absorbs microwaves. Free and bound water response differently in the electromagnetic fields that produced by

34

3  Characteristics of Bound Water

microwaves. Bound water orient insignificantly than free water in the presence of microwaves [35]. Even tightly bound water does not demonstrate any significant absorption of microwave.

3.1.7  Higher Boiling Point High cohesive force prevails in liquid water due to the strong hydrogen bond in free water, let alone in the bound water. Consequently, comparatively higher energy is required to release water molecules from the free surface [36]. Boiling of the bound water needs some energy to overcome the strong cohesive force as well as the adhesive force that exists between the water molecules and other components of food materials. Moreover, bound water has a higher boiling temperature compared to the free water depending on the molecular environment. The change of enthalpy of a sample in relation to the temperature gives a clear indication of water content at different molecular environments. A higher boiling temperature of the bound water can be found as the phenomena of boiling point elevation in case of water in a solution. For example, low boiling point solutes or the application of an excess amount of salt has a remarkable effect on the concentration of the solutes as well as on the boiling point elevation. However, the elevation of the boiling point and flow properties of liquid play a crucial role in the design and operation of food process [37, 38].

3.1.8  Higher Density High cohesive nature in water molecules as well as adhesive nature between water molecules and other substance reduces the free volume among bound water molecules [39]. Eventually, a relatively high-density is observed in bound water. The mobility of bound water is higher in comparison to the free water, as the water molecules are densely packed and show a higher structural bonding with the ionic groups, starch, proteins and so on [40, 41]. However, this water has a lower vapour pressure due to a higher strength existing between the cell environment and water molecules. The water molecules of this nature are more tightly packed than in the free water.

3.1.9  Slower Diffusion Diffusion is the net exchange of particles due to the presence of concentration gradient present in two places in a system. Higher density and molecular mobility of the bound water tends to reduce the moisture diffusion rate of the sample [42]. Generally, bound water molecules rustics the movement of each other’s result in slower

3.1 Introduction

35

self-­diffusion. Therefore, bound water removal demands a higher energy and time requirement [43, 44].

3.1.10  Specific Heat Out of the known liquid, water shows the highest specific heat except for ammonia. In case of water, additional heat is utilized to bend and break hydrogen bond and so an increase in kinetic energy needs substantially higher energy. The presence of bound water in the sample makes a significant difference in the specific heat content in food materials. Specific heat of food materials is also affected by moisture content of both types including free and bund water. Specific heat varies linearly with free moisture content; whereas, it varies non-linearly with bound moisture content [45].

3.1.11  Monolayer Moisture Content (MMC) Water acts as a plasticizer as well as a solvent in the construction of the complex biopolymer matrix of polysaccharide and protein in the plant-based food materials. Thus, the extraction or absorption of water by the cell tissue eventually proceeds a leading role in the mechanical and physicochemical behaviour of the cell wall structure. However, the cell stability is controlled by the water content in the vacuoles which stores water. Vascular solutes show high osmotic interaction through the thin-­ walled parenchyma cells and hence turgor pressure is developed to maintain the elasticity and firmness of the tissue [46] and the tissue collapse with the lost turgor pressure and it cannot be further reestablished again [47]. The heterogeneous spatial distribution of the water molecules in the food matrix give rise to the variation in properties and water content depending on the fraction constituents in the free, capillary and bound water domain. Further, water contents in the foodstuff is broadly classified as monolayer and multilayer water content. Also, the availability of water content in the cell and inter-cellular spaces contribute in defining water as intercellular and intracellular water content [48]. The monolayer moisture content demonstrates the maximum primary water content in the water sorption isotherm of a dried food sample [49]. The MMC in the sensitive sites of the food cells prevents the direct immersion of oxides by the diffusion of free radicals when storing of dried foods [50]. Monolayer water content is determined from the BET, Caurie and GAB equation using the relationship of EMC and aw by non-linear optimization. MMC effectively describes the surface moisture content and specifies the least moisture content for the dehydrated food products [51]. Monolayer water content in different fruits is presented in the following table [23, 52–55]. Rate of quality degradation in most of the dried foods at or below the monolayer water value is found negligible. MMC

36

3  Characteristics of Bound Water

corresponds to water activity 0.2–0.3 [56]. Separation energy of these monolayer water is very high and will cause decomposition reactions [4]. Chemical composition and molecular structure of the food products strongly affect the monolayer moisture content [57]. The moisture content in the dry food is maintained in such a manner that the water driven chemical reactions and quality degradation are not affected. The moisture content has a predominant factor on the characteristics of foods at aw of 0.2–0.3 corresponding to the monolayer value. Moreover, the theoretical model of MMC starts from this value and with an incremental change in the aw causes further increase in the mobility, reactivity and above all the quality degradation. Typical monolayer values of some materials are presented in Table 3.2. In order to estimate the water content in the plant-based food materials, different isotherm models have been proposed in different studies where the Aw gets much more emphasised. Among these models Brunauer, Emmett, and Teller (BET) equation, Couries equation, and the Guggenheim, Anderson, and DeBoer (GAB) equation are preferred for the calculation of MMC [59–64]. GAB equation can effectively predict the MMC over a wide range of Aw adoptable for describing different isotherm models for drying food and for selecting physical parameters for BET equation [61, 62]. BET isotherm (Brunjauer, Emmet and Teller) (0–0.5 aw)



a ( C − 1) aw 1 = + w moC (1 − aw ) m moC

(3.5)

Caurie isotherm equation:



 2C   1 − aW  1 ln   = − ln ( Cc mo ) +  c  ln   m  mo   aW  (3.6) GAB equation (Guggenheim, Anderson, de Boer) (0–0.85 aw) m=



C1 kmo aw (1 − kaw ) (1 − kaw + C1kaw )

(3.7)

m = moisture in g/g solid at water activity aw mo = monolayer moisture content C, C1, Cc and k = constants

3.1.12  Water Activity Water activity is a qualitative measure of the energy condition of the water in the sample. Moreover, previously it was widely known as the ratio of free (available for transport) to bound water in a sample. It is an intensive property (does not depend

37

3.1 Introduction Table 3.2  Typical monolayer values of some food material [58]

Food Kidney beans Cocoa

Mo (g water/ 100 g solids) 4.2 3.9

Ground and roasted coffee

3.5

Chicken

5.2

Egg

6.8

Fish meal

3.5

Gelatin

8.7

Meat

5.0

NFDM Potato

5.7 6.0

RTE cereals

5.2

Whole milk powder

2.2

on the amount of sample materials) [65]. Water activity can be defined as the ratio of equilibrium partial vapour pressure, in a given temperature, of a sample to the partial vapour pressure of pure liquid water at the same temperature [66]. a= w

p = RVP po

(3.8)

Where, Po and P are the partial pressures of pure water and water above the food and under identical conditions respectively.

38

3  Characteristics of Bound Water

The bound water in the food material has theoretically zero effective water activity, since it does not have much energy to escape from the food matrix as vapour and thus cannot exert any vapour pressure. The tightly bound does not exert any partial pressure and has an effective water activity of zero as it can escape from a food as a vapour. Mathematically, water activity can be calculated by the above equation when the food is in equilibrium condition with the surrounding atmosphere. It should be noted that RVP denotes the vapour pressure in a closed container containing the foods of the headspaces, whereas water activity refers to the activity of water inside the foods [67]. A deviation of less than 0.5% from the thermodynamic value for water activity has been observed when the food preserves at a constant pressure and the water vapour content in the food is close to the ideal gas [68]. Water activity can be reliably used to predict the food physical states, micro-­ biological stability like oxidation reactions, enzymatic and non-enzymatic actions [67]. Water activity leads a crucial role in food quality and storage stability as most of the food materials is considered to be stable when maintaining at its monolayer moisture content corresponding to a water activity of below or equal of 0.1–0.3 or the equivalent glass transition temperature [69]. Pure or distilled water shows an aw of one and it rapidly falls with the addition of a fraction of solute due to the restriction of water molecule movement and newly binding sites developed with solute and distilled water molecules [70]. One molar sugar solution has aw of 0.98, and one molar solution of sodium chloride has aw of 0.9669 whereas a saturated solution of sodium chloride has a water activity of 0.755 [71]. In case of multiphase, the vapour pressure of liquid water within food at equilibrium is as the same of the vapour pressure of vapour phase of the foods. Water activity depends on the surrounding temperature of food as shown in Fig. 3.1 [24]. The thermodynamic equilibrium between the water content in the solid and the environment is a prerequisite for the measurement of water activity [73]. Thus water activity is a function of temperature as well as melting and freezing point temperature, relative humidity, and turgor pressure. At a higher temperature, the moisture content of the food products decreases with a sharp increase in the vapour pressure inside the food materials which eventually lowers the water activity (aw). Water activity varies with temperature within the permissible range of temperature for microbial growth, although it is independent from the temperature effect for an ideal solution [71]. Water activity can also measure the efficiency of the water present in the food to take part in chemical (physical) reactions, thus on the microbial enzymatic and ­non-­enzymatic growth, lipid based oxidation and texture and taste [67]. If a portion of water is tightly bound to a protein or other molecule, the overall water activity would be reduced.as the water cannot take part in a hydrolysis reaction, The amount of active water has the decision making role regarding the microorganism growth, moisture transportation in multi-domain, physivochemical and enzymatic decomposition reaction compared to total water content in the fresh food materials. This fact can be attributed due to the hydration of polar groups, dissolution of chemical components, increasing diffusion through the new reaction sites and dilution of reactant at high water content [69, 70]. However, it has been sug-

3.1 Introduction

39

Fig. 3.1  Changes of water activity with temperature and water content (adapted from Fontana et al. [72])

gested that this very water activity is not the parameter that directly influences food stability [11]. Relations between water activity and moisture content are the followings: 3.1.12.1  Limitation of Water Activity Water activity is not the only parameter to define the food quality, chemical reactions in foodstuff and the growth of microorganisms. Some of the limitations of water activity are sorted and alternatives are proposed. These limitations are: 1. Water activity is established on the principle of thermal equilibrium between the food materials and its ambient atmosphere, whereas foods are not in a state of equilibrium with its surrounding atmosphere [76]. 2. The critical range of water activity is easily affected by the pH, high molecular weight solvent like starch and salt, temperature, anti-microbial agents. 3. Molecular environment of the solution can also change the water activity of the plant-based foods. 4. Water activity does not indicate the amount of water present in the food sample and its binding action with food matrix [57, 77]. 5. Water activity does not clarify the physical changes of the moisture and the food polymers and the consequences such as crystallization, caking, gelatinization, dissolution [78].

40

3  Characteristics of Bound Water

3.1.13  Glass Transition Glass transition concept was put forward considering the limitations of water activity. Glass transition temperature is an important factor affecting different properties of food solids at lower contents of water. Glass transition relaxation process in food solids occurs during transformation of amour phase state to supercooled liquid as a consequence of rapid moisture removal either by drying or freezing [74, 75]. The basic difference between water activity and glass transition is that water activity is the attributes of water molecules, whereas glass transition is the quantifier of the components of the amorphous foods [80]. Glassy state can be defined with a high viscosity ranging from 1013 to 1014 Pa s and molecular immobility [81]. Molecular mobility increases and viscosity decreases within foodstuff above the glass transition temperature and phenomenon collapse and stickiness come in effect [76, 82]. Glass transition is characterized by a second-order time-temperature dependent transition followed by a change in physical, chemical and electrical properties of the materials. The basic concept of glass transition is, (a) food materials are in the most stable condition at its monolayer moisture content at the room temperature, (b) higher glass transition temperature hinders the physiochemical and transportation properties of foods and thus accelerates the deterioration rate [83]. 3.1.13.1  Importance of Glass Transition The temperature above glass transition demonstrates various time dependent and dynamic viscosity related physicochemical changes such as expansion of free volume, increase of specific heat with a decrease in the viscosity and mobility that consequently affect the structural behaviors like stickiness, crystals formation and often collapse of the cells [49, 79–80]. Glass transition concepts are getting much more attention for the last few decades, because it explains the limitations of water activity regarding non-equilibrium thermodynamic behaviour, low moisture content, starch enriched content, and the browning reactions towards the changes in colour and taste during storing [49]. In this context, glass transition plays a crucial role by identifying and applying the facts regarding glass transition temperature towards storing and preserving the amorphous foodstuff. Unlike water activity, glass transition is related to the variation in food structure, rates of diffusion control reaction, and crystallization [30]. Glass transition is an important determinant of the shelf stability and dehydrated characteristics of high carbohydrate—comprising foods [80]. Glass transition temperature is dependent on water content. The relationship between glass transition temperature and moisture content are, in general, obtained from the Gordon-Taylor and GAB equation depending on the moisture or solid part in the products. Glass transition temperature of some fruits and vegetables are represented in Table 3.3. The glass transition temperature describes the mobility of the cell water content, transportation of cell water to the extracellular spaces, cell shrinkage, pore reduc-

3.2 Limitations

41

Table 3.3  Glass transition temperatures of some selected foods [30, 49, 69] Solutes Sucrose Glucose Fructose Citric acid Malic acid

(Tg °C) 67 31 5 6 −21

Fruits Apricot Pear Apple Strawberry Blueberry

(Tg °C) 18 5 18 29 15

Fruits Raspberry Blackberry Orange Lemon Peach

(Tg °C) 41 22 45 11 20

Vegetables Celery Cabbage Carrot Potatoes

(Tg °C) 58 43 57 71

tion and cell collapse. At a lower temperature than glass transition, water transportation takes place through the intercellular spaces from cell to cell and thus shows a high mobility [86]. The cell membrane and the mobility rate remain convenient to the extent when the glass transition temperature is less than the drying temperature. With the increasing drying temperature of the sample, cell shrinkage rate, as well as the mobility temperature declines, and glass transition temperature above drying temperature is found to be difficult to process and preserve foodstuffs [87]. Glass transition temperature can be measured through the investigation of vitrification phenomena by using rheological techniques in food stuff or the widely used Differential Scanning Calorimetry (DSC) along with a clear statement of the parameters used in the dynamic mechanical techniques to obtain better results [64, 84–85]. In case of amorphous solids, glass transition temperature gives a clear indication of the optimal drying temperature for the fully dehydrated powder. Also, the molecular diffusion rate has been experienced by the glass transition temperature. The molecular diffusion rate of amorphous solids in water slows down at a temperature below Tg and the reaction rate increases in such an extent that it produces water and other volatile materials at a temperature higher than Tg [90]. Using a calorimetric technique like DSC, the change in the specific heat can be obtained which can be used to determine Tg. Also, the molecular mobility of the amorphous sample can be used to determine Tg by NMR or dynamic mechanical thermal analysis method [81, 91].

3.2  Limitations Amorphous solids are found in high immobility during diffusion in water solution and give rise to the glassy state [88, 89].This glassy state cannot be used to recognize and predict the molecular dynamics of water solution and its influence on food stability [57]. Moreover, the measurement of Tg is not as simple as aw and it remains unrecognized in some multi-phase, multi domain complex as well as simple food matrix (fresh fruit juice) where aw and Tg both are asked for attention [49]. Taking the limitations of water activity and glass transition temperature, a combination of glass transition and water activity can provide more stable approach in food preservation.

42

3  Characteristics of Bound Water

In brief, each of the above mentioned characteristics defines bound water for food materials from different perspective. However, the proportion of bound and free water is not given the same from different approaches of characterizing the state of water. Eventually, no unanimous definition of bound water in food materials can be drawn from these characteristics.

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71. E. Sandulache, Water activity concept and its role in food preservation, Technical University of Moldova, Chisinau, pp. 42–43, 2012 72. A.J.  Fontana, G.V.  Barbosa-Cánovas, S.J.  Schmidt, T.P.  Labuza, Water Activity in Foods: Fundamentals and Applications (Wiley, Exeter, 2008) 73. M. Fouskaki, K. Karametsi, N.A. Chaniotakis, Method for the determination of water content in sultana raisins using a water activity probe. Food Chem. 82, 133–137 (2003) 74. T.P.  Labuza, S.R.  Tannenbaum, M.  Karel, Water content and stability of low-moisture & intermediate-­moisture foods. Food Technol. 24, 543–550 (1970) 75. M.  Shafiur Rahman, R.  Hamed Al-Belushi, Dynamic isopiestic method (DIM): measuring moisture sorption isotherm of freeze-dried garlic powder and other potential uses of DIM. Int. J. Food Prop. 9(3), 421–437 (2006) 76. L. Slade, H. Levine, A food polymer science approach to structure-property relationships in aqueous food systems: non-equilibrium behavior of carbohydrate-water systems, in Water Relationships in Foods (Springer, Berlin, 1991), pp. 29–101 77. J.  Chirife, P.  Buera, Water activity, glass transition and microbial stability in concentrated/ semimoist food systems. J. Food Sci. 59(5), 921–927 (1994) 78. M.S. Rahman, Food stability determination by macro – micro region concept in the state diagram and by defining a critical temperature. J. Food Eng. 99(4), 402–416 (2010) 79. Y. Roos, M. Karel, Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. J. Food Sci. 56(1), 38–43 (1991) 80. Y.H. Roos, Water activity and glass transition, in Water Activity in Foods: Fundamentals and Applications (Blackwell, Ames, 2007), pp. 29–45 81. H. Kunzek, R. Kabbert, D. Gloyna, Aspects of material science in food processing: changes in plant cell walls of fruits and vegetables. Z. Lebensm. Unders Forsch. A 208, 233–250 (1999) 82. Y.  Roos, M.  Karel, Applying state diagrams to food processing and development. Food Technol. 45(12), 66–68 (1991) 83. M.S. Rahman, Food Properties Handbook (CRC press, Boca Raton, 2009) 84. Y.I. Matveev, V.Y. Grinberg, V.B. Tolstoguzov, The plasticizing effect of water on proteins, polysaccharides and their mixtures. Glassy state of biopolymers, food and seeds. Food Hydrocoll. 14(5), 425–437 (2000) 85. Y.H. Roos, M. Karel, J.L. Kokini, Glass transitions in low moisture and frozen foods: effects on shelf life and quality. Food Technol. 50(11), 95–108 (1996) 86. H. Levine, L. Slade, A polymer physico-chemical approach to the study of commercial starch hydrolysis products (SHPs). Carbohydr. Polym. 6(3), 213–244 (1986) 87. Y.H. Roos, Effect of moisture on the thermal behavior of strawberries studied using differential scanning calorimetry. J. Food Sci. 52(1), 146–149 (1987) 88. R.K.  Richardson, S.  Kasapis, Rheological methods in the characterisation of food biopolymers. Dev. Food Sci. 39, 1–48 (1998) 89. S.  Kasapis, I.M.  Al-Marhoobi, J.R.  Mitchell, Testing the validity of comparisons between the rheological and the calorimetric glass transition temperatures. Carbohydr. Res. 338(8), 787–794 (2003) 90. G. Vuataz, V. Meunier, J.C. Andrieux, TG – DTA approach for designing reference methods for moisture content determination in food powders. Food Chem. 122(2), 436–442 (2010) 91. J.M.V.  Blanshard, The glass transition, its nature and significance in food processing, in Physico-Chemical Aspects of Food Processing (Springer, Berlin, 1995), pp. 17–48

Chapter 4

Bound Water Measurement Techniques

4.1  Introduction In the literature, a wide variety of definition of microstructure may be found. However, microstructure is well-defined as the spatial organization of components and their collaborations in food materials [1]. Polymers and water are the typically the main constituents of Foods. Generally, the polymers are found as three elementary macromolecules, namely proteins, polysaccharides, and lipids or formed from even simpler repeating components. Food materials also comprise different minerals and gases to develop complex structures in foodstuff [2]. In general, the living tissues of plant-based food materials are oriented in cellular structure to achieve its total functionality. Food structure can be classified to four distinct types, namely cellular, fibrous, crystalline and gel structure [3]. The hierarchical structure of all of these types of food materials is ranging from nano to millimeters scale as displayed in Fig. 4.1. Several food materials show considerably diverse structure from cellular to tissue level. Some of the food materials even encompass the combination of two or more of these basis. Consequently, enormous structural orientations are observed in nature. Higher level structures are expected to gradually assemble in different length of scales to attain the required properties and functions, in natural as well as in processed foods [5]. For example, fibrous materials organized from Amino acid to fiber level with size from Nano to macro scale. Similarly, other categories of food including cellular, crystalline and gel materials maintain the distinct hierarchical structure as revealed in Fig. 4.1. In plant tissue, the cell acts as a building block. Matrices in food materials form by cells integrating with cell walls which then become steady with the support of fibers. Cellulose (fibers), hemicelluloses, matrix pectin and few proteins are existed in plant cell wall. Pectin existing in the cell wall is soluble in water, which is why water migration usually occurs through it. The solubility of pectin performs a substantial role to preserve the intact characteristics of the cell wall [6]. Apple cell, for © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_4

47

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Fig. 4.1  Hierarchical structure of different food materials (Adapted from Joardder et al. [4])

example, becomes spherical rather than polygonal as a consequence of the migration of water through the pectin present in the cell wall [7]. Hence, macroscopic structural behavior relies on dissimilar factors including turgor pressure, cell wall properties and cell size [8–10]. Among the factors, cell wall constituents are the significant structural constituents, as the cell wall properties significantly affect the mechanical properties of tissue [11, 12]. Apart from numerous components, water is one of the main components of food materials as discussed earlier. Water is distributed throughout different lengths of food materials as revealed in Fig. 4.2. In a comprehensive categorization, free or bound water are the two main types that are categorized as the types of water in food materials. Free and bound water correspondingly known as multilayer and monolayer water. Furthermore, water is also designated as intercellular and intracellular water depending on the spatial position, as presented in Fig. 4.2 [14]. Water in the capillaries is known as intercellular water (free water), whereas water inside the cells as intracellular water (loosely bound water). Usually, intracellular water ranging between 78% and 97% (wet basis) in dissimilar fruits and vegetables and the residual water are intercellular water [15]. Moreover, besides these two types of water, water present in the cell wall is known as strongly bound water in cellular food materials. The spatial distribution of water not only significantly influences the food stability but also affects different physical phenomena. Therefore, water distribution can strongly affect the migration characteristics of water and volatile components.

4.2  Water Content Measurement

49

Fig. 4.2  Water distribution of plant-based cellular tissue [13]

Water depicts diverse response in different environmental conditions. For example, water from three dissimilar spaces, namely cells (loosely bound water), cell walls (strongly bound water) and intercellular zones (free water), follows different pathways and takes non-equal energy to migrate to the atmosphere during the drying process [4]. Cell wall water migration necessitates comparatively higher energy as it passes through intermicrofibrillar spaces that are roughly 10 nm in cross section and several times lengthier [16]. On the contrary, free water and loosely bound water requires less energy to migrate during drying. This chapter will focus comprehensively on the measuring techniques of different types of water present in dissimilar types of food materials.

4.2  Water Content Measurement Determination of water in foodstuff is a critical issue, although it is not a trivial analysis [17]. Due to a different theory of different determination methods, results from different methods vary significantly. Methods to determine the moisture content of food materials can be broadly classified into two types: direct methods and indirect methods, as shown in Fig. 4.3. Direct methods deal with the removal of water from the sample and measuring water proportion by either the loss of weight of the sample or collecting the extracting water or other means that directly deal with the water [18]. With an exception in case of thermal methods, most of the methods for determination of moisture in

Fig. 4.3  Different techniques for quantifying water content in food material

Combined Methods 1. Evaporation and diphosphorous pentaoxide method 2. Evaporation and Karl Fischer titration

Chemically Methods 1.Production of C2H2 or H2 2.Karl Fischer titration

Physical Methods 1.Distillation 2.Drying methods/ Thermogravimetric - Oven drying - Combined drying - Microwave drying - Infrared drying - Vacuum Dryinng - TGA - Halogen drying 3. Acoustic method

Direct Methods

Spectroscopic Methods - Nuclear Magnatic Resonance (NMR) - Microwave Resonator - Microwave Spectroscopy - Near Infrared - UV-visible - X-ray - CT Scan

Macroscopic Properties Methods - Densitometry - Polarimetry - Refractometry - Dilatometry (DIL) - Differential Scanning Calorimetry (DSC) - Electrical Water activity

Indirect Methods

50 4  Bound Water Measurement Techniques

4.2  Water Content Measurement

51

foods cannot differentiate between free water and bound water, while indirect methods are quite promising of distinguishing free and bound water. While indirect methods either measure characteristic features of the water molecules or determine properties that influence by water content, these techniques do not calculate the water content directly, rather measure a specific property of food material that is strongly affected by water content [16, 17]. These approaches necessitate an tremendously product-specific calibration that must depends on a direct and selective method [19]. This requirement is obvious as the relationship between the calculated entity and the moisture content is exceptionally complex in maximum of the actual circumstances. In indirect methods, water removal is not a requisite in water content determination, rather depends on the modification or variation of some physicochemical properties with a change of moisture content of the samples, such as electrical conductivity, dielectric constant, or vapor pressure. Prior to discussing techniques available in determining bound water, three very common methods of overall water content are discussed briefly in the following section. The amount of bound water in food materials are described differently by numerous researchers [20–24]. There are several approaches available in quantifying the bound water in the food material. An extensive discussion on the measurement of bound water is presented shortly.

4.2.1  Distillation Distillation method is one of the standard techniques for measuring the water content in a sample directly. In this method, the water in the food sample is co-distilled with a high boiling point solvent without mixing each other which enables to find out the volume of co-distilled water easily. The distillation techniques can further be divided into direct distillation and reflux distillation techniques, depending on the pattern of solvent utilization. In direct distillation, the sample with a high boiling point solvent is heated. The amount of vapor represents the water content after condensation. On the other hand, in reflux distillation, the solvent is recirculated and extracted from the system periodically to ensure better collection of the moisture content in the food material sample. A lower temperature, isolation from contamination, and moisture absorption need to be confirmed to attain a better result in water measurement by this method [19].

4.2.2  Drying Methods Drying techniques are probably the oldest and easiest methods of moisture determination in food materials. Water migrates due to the temperature and concentration gradient during the drying process. Therefore, an appropriate amount of heat and dry atmosphere are essential for an effective drying process. Different factors

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including atmospheric pressure or adjacent humidity, forms of energy transmission and temperature significantly influence the time required to measure water content using drying method [19]. Distinctions between the types of water (bound or free water) are barely possible in drying methods [18]. Apart from water, the loss of other volatiles during drying, incomplete removal of water content, case hardening which usually causes slowing down of releasing water from the sample are the main disadvantages of these measuring techniques. Moreover, Maillard reaction and decomposition of products is a remarkable limitation of these methods. In the industry, moisture content is mostly determined by drying method by varying the temperature from 70 to 135 °C for 2–24 h [23, 24].

4.2.3  Karl Fischer (KF) Method The Karl Fischer method is a widely used chemical technique to determine water content in a sample through titration reaction [25]. KF is the most widely used methods for the moisture determination in food [26]. KF method uses a reagent that reacts with water and transforms water into a non-conductive chemical. Volumetric Karl Fischer (VKF) and Coulometric Karl Fischer (CKF), two of the common methods, are based on KF method. Prior to the beginning of titration, the sample is dissolved in a solvent in VKF method. Following this, an appropriate reagent is added to the sample until all the water removes. On the other hand, electric current is used to induce the dissolved sample during titration. The calibrated measurement of the current required to induce all of the water represents the amount of water present in the sample [27].

4.2.4  Spectroscopy An electromagnetic spectrum is typically used to investigate the moisture content in the sample, as the water molecules give different spectrum depending on the molecular environment. The hydrogen nuclei of the water atoms have magnetic properties which act as a small magnetic bar during electromagnetic spectroscopy. The strong interaction between the electromagnetic ray and hydrogen atom acts as a measure for determining water content.

4.2.5  Inferred Infrared heating involves drying through radiation, which is faster than the drying techniques using conventional conduction and convection method and save the drying time. The water content is measured by a change in weight or volume after

4.3  Bound Water Measurement Techniques

53

drying. It is mainly used in the process of monitoring in case a rapid change in the thermal gradient occurs. It requires a low pressure as well as a low temperature that reduces the chance of thermal degradation. Water absorbed at an infrared wavelength, usually from 1.45  μm to 6.1  μm, depending on the water absorption frequency, is used to investigate the moisture content in the sample. FTIR spectrometer collects data for each of the wavelengths which are more robust than by DSC. The infrared spectrum is calibrated using a sample with known moisture content and frequency of absorption to avoid any interference [27].

4.2.6  Microwave Some of the food component including water, salt and fat present in food material can absorb microwave (MV). However, water shows the highest absorptivity of microwave due to its dipole nature. Microwave absorption is taken as a function of moisture content of the food material. Dielectric properties of the food material greatly influence the absorption of the microwaves [18]. Only the water at free state can be determined precisely using this method [28]. Prior to conducting the measurement, the followings are the considerations regarding microwave [29]: (a) MW Absorption capacity of water: The frequency of electromagnetic waves is increased with water to a value of almost 100GHz. In addition, it provides the first peak at around 25 GHz. Remarkable attenuation to water is found from the microwaves frequency adjacent the 25 GHz region, therefore these microwaves are very much appropriate for the moisture meter [29]. (b) MW absorption by dissolved components: Some of the food components mainly salt has remarkable influence on the absorption of microwave having frequency less than 9  GHz. Therefore, the frequency of microwave must be above of 9 GHz to avoid significant amount of absorption by salt contained in foods. (c) Effect of size and shape of the sample: Microwave comprises 0.1–100 cm wave length. Therefore, sample size and shape is vital in choosing the frequency of microwave applied for the measurement of water content of food. For example, MW of 25 GHz has wavelength of close to 2 cm. Therefore, the sample thickness should be maximum of this wavelength.

4.3  Bound Water Measurement Techniques There are several concepts practiced in bound water measurement. Individual methods detect bound water using special characteristics of water in response to different conditions. There is no guaranty that the bound water found from different methods would be equal in quantity. Figure 4.4 shows different properties of water utilized in

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Fig. 4.4  Bound water measurement techniques (At a glance)

dissimilar techniques of measuring bound water. It is worthy to mention that water distribution in different types of food varies significantly with the diversification of constituents and structural formulation. In this chapter, bound water present in agro-­ based food materials has been attracting more attention due to their complex nature, meaning understanding the presence of bound water in these type of food materials give a clear insight on the distribution of water in types of food materials.

4.3.1  Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) is an advantageous technique to determine the amount of both free and bound water content in food materials. DSC generally, uses the phase transition of water in food material to quantify the free and bound water. In other words, the energy required to melt or crystallize the water

4.3  Bound Water Measurement Techniques

55

content of the food materials reflects the bonding energy of water with surrounded components. While measuring the bound water using DSC, the free water is considered having the same property of pure water. The crystallization temperature of the bound water in food materials are considered to be less than that of free water [21, 30–33]. In Differential Scanning Calorimetric, the peaks of enthalpy in DSC curves for free water is equal to that of pure water, and the phase transition temperature of the bound water is less than that of pure water. In DSC, the transition of non-­freezing water, considered as the bound water, is not detected in the first order, occurring at a temperature of 273 K. 4.3.1.1  Measurements To quantify the phase transition of moisture content in food materials DSC is used which is excellently equipped with a cooling cell. In DSC, the curves are attained by cooling at the rate of close to 8 K per minute from room temperature to 200 K for some food materials depending on the nature of food material. After this, the sample is heated to room temperature with the similar heating rate. Initially, the crystallization and melting temperature of pure water are observed for the calibration purpose. The food materials are typically dried in a vacuum desiccator before putting it into the DSC. To do this 5 mg sample is taken and is weighted in an aluminum pan for a volatile sample. However, the desiccator drying is continued for a time period of 1 week. Before putting the sample on an aluminum plate, the plate is heated up to 373 K in order to exclude any reaction between the aluminum surface and water. To ensure the identical thermal history, all of the sample pans are required to heat at 70 °C for 1 h and to stand for a few days. Figure 4.5 displays the heating and cooling curves of food materials found from the DSC. If a food material is cooled down from room temperature to 200 K, the first order transition of water in food materials is not recognized due to the type of water content. This occurrence is correspondingly long-established by cooling the food materials to a certain temperature of 130  K using a DSC instrument. Food components and structure substantially affect the first order transition of water in food materials. In DSC curve, a broad crystallization peak (Peak II) seemed at around 230–250 K, whereas a new sharp peak (peak I) appears at the right side of Peak II which is the high-temperature side. The enthalpy calculated from the area of Peak II varied with the type of food material. Figure  4.5 demonstrates that the shape and temperature of Peak I ­consensuses excellently with that for the crystallization of pure water, which is indicated by dotted curves. Even though Peak I is extended marginally in the low-temperature side, for the presence of higher amount of free water, Peak I upsurges, while the enthalpy of peak II remains constant. The schematic heating curves of water in food material begins at a lower temperature than that of pure water as revealed by the dotted line in Fig. 4.5. Generally, asymmetry nature of the curve is found as a distinguishing pattern of the melting of

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Fig. 4.5  DSC curve of water in food material (=water in food material, =pure water) [34]

the water existing in food materials. By using DSC, the amount of dissimilar categories of water in food material can be calculated, only if the amount of total water is known. So, if the total weight of a food material is T.W.,

T.W. = ( WS + WWT )

(4.1)



Where, WS and WWT are the weight of the solid portion of food material and weight of the water content in food material sample, respectively. In general, from the crystallization curve of water during heat flow, the total detectable water can be calculated from the enthalpies of peak I and peak II. Sometimes a fraction of water cannot be frozen in this process due to their distinct non-freezing feature. This type of water can be labeled as strongly bound water. Following equations can be used for determining the amount of dissimilar categories of water present in food materials.

WWT = ( WP1 + WP2 + WSB )



WM = ( WP1 + WP2 )





WWT = ( WM + WSB )





(4.2) (4.3) (4.4)

Where, WWT = Weight of the water content in food material sample, WP1 = Weight of water calculated from the enthalpy of crystallization peak I, WP2 = weight of water calculated from the enthalpy of crystallization peak II, WSB = weight of non-freezing water or strongly bound water, and.

4.3  Bound Water Measurement Techniques

57

WM = weight of water calculated from the enthalpy of melting. Here, in DSC curves WP1 is also termed as the weight of free water and WP2 is termed as weight of loosely bound water. The weight of total bound water may be calculated from the subsequent equation WB = ( WP2 + WSB )



(4.5)



Where, WB = weight of total bound water. Therefore, by using the subsequent equation the percent bound water content in a food material is usually calculated:

CB = ( WB / T.W. ) ∗ 100%



(4.6)

Where, CB = percent bound water and T.W. = the total weight of a food material. The quantities of bound water WB revealed in Eq. (4.6) may be attained directly from a DSC cooling curves, while WP2 crystallization peak II is challenging to estimate from a DSC heating curve due to the trouble in splitting the melting peaks as symbolized by WM from WP1 and WP2 of Eq. (4.5). Hence, the proportion of bound water content in food material may be only calculated by means of DSC cooling curves. 4.3.1.2  Data Interpretation Figure 4.6 demonstrates DSC heating and cooling curves of a typical food material having different moisture content for the clarification of the nature of peaks and their corresponding water distribution. From Fig. 4.6, it is found that no first-order transition of water is documented till the proportion of water surpasses X% by weight. At a temperature around 230–250 K in the water content region from X% to Y%, the broad crystallization peak (peak II) appears. Similarly, a new sharp crystallization peak (peak I) found at around 255 K once the water content surpasses Y%. The DSC heating curve of water on food materials correspondingly displays a comprehensive peak in the region of Y% of water. The coinciding with broad and sharp peaks is noticeably recognized if the water content surpasses Y% by weight.

4.3.2  Bound Water Estimation from SEM Image Bound water estimation from SEM images is another emerging technology that can be efficaciously used to quantify the extent of bound water in food materials. For applying this technique, the whole cell wall water is assumed to be strongly bound water. Therefore, this method is suitable for determining the cell water of cellular

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Fig. 4.6  DSC cooling and heating curve of food materials having a different amount of water content

food materials. Assessing the cell wall thickness before and after drying may provide the quantity of strongly bound water in cellular food materials. However, some additional assumptions, as mentioned below, should also be made prior to calculating the amount of bound water [35]: 1. The shape of the cell of food material is considered uniform. The error of this assumption may be nullified by measuring the ratio of intracellular and cell wall water, instead of only cell wall water. 2. Constant and same density of the water present in the cell wall and intracellular water are needed to be considered in this method. This assumption may potentially lead to a sample where the densities of different water are not the same. 3. Both the cell wall water and intracellular water have identical densities. 4. An average cell dimension of the food material is considered here, although anisotropy in cell dimension is found in plant-based food materials. 5. An ideal shrinkage of cell walls is considered to take place during drying. Therefore, by using this technique, it is predicted that the difference in the cell wall thicknesses of fresh and completely dried food materials signifies the quantity of strongly bound water.

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4.3  Bound Water Measurement Techniques

4.3.2.1  Measurement FSEM images of both fresh and completely dried samples are needed to measure cell wall bound water. Fresh fruits are necessitated to keep in refrigerator until the time of experiment execution. After that, the samples are needed to slice in to appropriate sample dimension. Following proper coating, the sample can be placed in an SEM to get the required image of a fresh sample. The similar procedure needs to be followed to get SEM images of dried samples. The samples can be dried in a conventional dryer at 50–70 °C until complete removal of water. After getting good quality SEM images of the fresh and dried sample, image analysis software such as ImageJ can be used for measuring the cell wall thickness. In addition to this, cell dimension of the samples also needs to be measured to get the fraction of intracellular and cell wall water. After measuring all the dimensions, the following equation is used to evaluate the quantity of strongly bound water or the cell wall water:



1   M cw = 4πρβ t  r 2 − β rt + β 2 t 2  3  

(4.7)

Where, r = radius of the fresh food material r1 = radius of the dried food material t = initial thickness of cell wall t1 = Final thickness of the cell wall. All the dimensions are represented in Fig. 4.7. Shrinkage coefficient of the cell wall thickness can be found by using the following equation:



β=

t1 t

(4.8)

Cell dimension, Cell wall thickness and cell wall shrinkage coefficient are the different factors upon which the quantity of cell water relies directly, which is also noticeably apparent from Eq. (4.8). More insight concerning the overall bound water existing in the food material can be obtained from the ratio of intracellular and

Fig. 4.7  Approximation of strongly bound water or cell wall water by SEM image [35]

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cell wall-bound water. Therefore, the ratio of intracellular and cell wall water may be calculated from Eq. (4.9).

(r − t ) Miw = 2 M cw β t 3r − 3β rt + β 2 t 2 3



(

)

(4.9)

4.3.2.2  Data Interpretation Figure 4.8 shows an SEM image of fresh and dried apple samples which are required to investigate the bound water in this study [35]. Cell wall thickness analysis by using SEM image, represented in Table  4.1 reveals that in a fresh state the Red Delicious has thicker cell walls than the Granny Smith apples. Granny Smith and

Fig. 4.8  Microstructure of fresh and dried apple samples (500×) [35]. (a) Fresh Granny Smith apple. (b) Fresh Red Delicious apple. (c) Dried Granny Smith apple. (d) Dried Red Delicious apple

Avg. Min. Max.

Granny Smith apple Cell wall thickness Fresh Dried Cell wall shrinkage (μm) (μm) coefficient, β  9.312 4.685 0.4969  6.734 3.769 0.4403 11.785 6.281 0.4670 Ratio of intracellular and cell wall water 393.02

Table 4.1  Cell wall thickness of fresh and dried apple [24] Red Delicious apple Cell wall thickness Fresh Dried Cell wall shrinkage (μm) (μm) coefficient, β 11.41 2.432 0.786  7.678 1.65 0.785 14.46 3.527 0.756

Ratio of intracellular and cell wall water 203.44

4.3  Bound Water Measurement Techniques 61

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Fig. 4.9  A diagram of a common dilatometer (Adapted from Imran et al. [36])

Red Delicious have cell wall thickness of 9.31  ±  3.1  μm and 11.405  ±  3.2  μm respectively. Conversely, the shrinkage of cell wall is more (on average 78.68% shrinkage) in Red Delicious when compared to Granny Smith (50% shrinkage) after drying as presented in Table 4.1. Therefore, it can be said from the results of Table 4.1 that the Red Delicious apple contains more strongly bound water or cell wall water than the Granny Smith apple.

4.3.3  Dilatometry Dilatometry is a thermo-analytical approach that may also be applied for quantifying bound water in food materials. This method is based on thermal enlargement or reduction of food material while exposed to a controlled temperature. Dilatometry works on the basic expansion property of pure water. It is well established that when pure water undergoes through a low-temperature region, it freezes and expand almost 9% in volume [36]. By observing the change in the level of the meniscus of the indicator fluid, the change in volume of the freezing of water in a food sample is calculated. All through the Dilatometry measurements, it may be anticipated that proportion on of bound water is detained by forces larger than those which act to orient water molecules into the crystal lattice of ice. It is already mentioned that Dilatometry works on the principle of thermal expansion or contraction of water existing in food material. The bound water proportion can be found from the thermal expansion or contraction from the subsequent equation [36]:



 T2  ∆V = exp  ∫αV ( T ) dT  − 1 T  V  1 

(4.10)

4.3  Bound Water Measurement Techniques

63

Where, V is the volume of the material, αv is the volumetric thermal expansion coefficient as a function of temperature, T1 and T2 are the initial temperature and final temperature respectively. For anisotropic food structures, thermal expansion coefficients can be non-­ uniform in various directions. Therefore, the total volumetric expansion depends on this uneven three-dimensional expansion. In this type of circumstances, it is indispensable to consider the coefficient of thermal expansion as a tensor with up to six independent elements. Thermal expansion usually reduces with rising bond energy. The bound water that has remarkably higher bonding energy with a solid matrix can be found using this principle. 4.3.3.1  Measurement In order to measure bound water in food material using Dilatometry, the prepared food material must be positioned into the sample holder which is typically known as the dilatometer tube as shown in Fig. 4.9. The sample holder can be positioned horizontally or vertically in the Dilatometry setup. The food material must be in connection with the pushrod to convey the signal to the displacement sensor or transducer. The pushrod must be in contact with the food material throughout the process of having an accurate result. Push or pull occurs in the tube when the sample enlarges or contracts. A displacement sensor is used to detect this movement of the pushrod. An expansion curve while heating or contraction curve while cooling the food material, may be found from the transducer signal. An expansion or contraction coefficient (ξ) may easily be determined using this curves. The free water may be measured by using the subsequent equation [37, 38]: FW =

D + ( W ∗ A ∗ ∆T )

ξ

(4.11)

Where, D = the level difference in the dilatometer from ∆T, W = the weight of the reference material, A = the concentration coefficient of reference material, ∆T = the temperature difference, and. After calculating the free water present in the food material, bound water can be measured by subtracting the free water from the known total water content of the sample.

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4.3.4  CT Scan CT scan is one of the non-destructive methods of measuring the bound water in food materials. X-ray μCT uses x-rays to generate 2D cross sections of a food material and a virtual 3D object is reproduced with sophisticated image analysis process without destroying the original object. By using a typical μCT instrument, it is possible to scan the whole food sample at a time. From the scanning, a number of micro-level information including moisture distribution, cells dimension, and pore size distribution can be attained without the requirement of cutting of sample. Even the selected food material may be investigated at various phases of the drying process. The scanning is typically done at 55 kV, and the images are taken through a rotation of 0–360°. The X-ray tomography has a 2D high-resolution camera to get an image of the region of interest. For developing the 2D scans at diverse vertical positions, the samples are rotated horizontally between 0° and 360° angle. For performing the analysis efficiently, the images of the middle part of the food material are taken. By utilizing a built-in software package that normally uses the back-­projection algorithm, the reconstruction of the 2D images is done. There is many a software available to construct 3D images from the 2D images [39]. 4.3.4.1  Measurement For measuring bound water using a typical CT scanner, such Scanco μCT-40 can be deployed. Sample slices with appropriate dimensions are needed to be prepared prior to further processing to image acquisition. Even food sample with different level of water content may be analyzed to investigate the water distribution and its transportation during drying using CT scan. In this case, a selected drying is performed in a well-designed dryer [40]. Sample with different moisture content is taken out from the dryer for Xray μCT experiments. Before performing the X-ray μCT experiment, the food materials are needed to be kept in a desiccator to evade the natural rehydration from the ambient humidity. Then the food sample are scanned in a typical μ-CT system with a region of interest close to 6 mm. The preceding steps are performed again and the scanned images are saved for additional investigation [41]. 4.3.4.2  Image Processing The 2D or 3D images obtained from the CT-scan do not provide the amount of moisture content at different state directly; rather further image processing is required in order to measure the proportion of free and bound water. Generally, the greyscale images attained from the X-ray μCT are processed in MATLAB to investigate the water distribution in food materials. Noise elimination and segmentation

4.3  Bound Water Measurement Techniques

65

are normally done in this part of image processing. By using the intensity thresholding and filtering, noise elimination can be done, whereas, segmentation is done with the watershed algorithm [42]. Different degrees of grayscale are achieved for different components of food materials. For instance, a greater gray intensity is depicted by cell wall of plant-based tissue in compared with the water-filled cells. By utilizing the manual segmentation process, dissimilarities between the intracellular and intercellular water can successfully be classified. In addition, the categorization of the cell wall, water-filled cells and intercellular spaces can also be accomplished using this very process. 4.3.4.3  Determination of Moisture Content in Cellular Level The grey-level intensity of every unit of the processed image is investigated in order to calculate the moisture content from the micro-tomographic images. The moisture content with different proportion of free and bound water may be determined using the subsequent equation [43]. Moisture Content =

( Grey level )Dry solid − ( Grey level )wet solid ( Grey level )Dry solid − ( Grey level )water

(4.12)

Where, (Grey level)Dry solid = the grey level of the cell wall, (Grey level)Wet solid = the intracellular water and (Grey level)Water = the intercellular water. The intracellular and the intercellular water contents are attained from the X-ray μCT images by choosing the region of interest (ROI) physically and putting the values in Eq. (4.12). The particular ROI in the cellular area delivers the data of intracellular water and the designated ROI in the intercellular area supplies the data of the intercellular water. By using the Eqs. (4.13) and (4.14), the proportion of intracellular water and intercellular water may be calculated: Proportion of Intracellular water =

Intracellular water ∗100 Intraccellular water + Intercellular water (4.13)

Proportion of Intercellular water =

Intercellular water ∗100 Intraccellular water + Intercellular water (4.14)



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4.3.4.4  Data Interpretation For the data interpretation, the study carried out by Rahman et  al. for analyzing moisture distribution in apple slice with different moisture content is presented here [39]. A correlation between the moisture distribution in cellular level and total moisture content for the period of drying in a food material at 60  °C is displayed in Fig. 4.10. Maximum of the water stays inside the cells at the initial stage of drying, and the water present in the intercellular spaces migrate at this very early stage of drying. This is because of the lower energy requirement in migrating intercellular water from the sample than the energy needed to escape the intracellular and cell wall water to the surrounding [36]. From the Fig. 4.10 it is found that as drying advances, the proportion of intracellular water correspondingly reduces. At the preliminary phase of the drying, the overall moisture removal rate is high. At the first falling rate, the transference of cellular water takes place through the cell membrane and cell wall. With further progressing of drying, heat transfer from the peripheral cells to the internal cells; consequently, the rupture of the cell membranes in the food material is occurred due to the formation of pressure gradient between intracellular and intercellular spaces [44]. Migration of cellular water occurs quickly after the rupture of cell membranes. The distribution of intracellular and intracellular water can be attained by this method very successfully. Figure 4.11 displays the distribution of intercellular and intracellular water in the food material over the course of at a temperature of 60 °C [15, 44, 45]. Intracellular water migrated throughout the drying period may be caused due to the concentration gradient of

Fig. 4.10  The distribution of moisture content in cellular level inside the apple slice at various drying time (at 60 °C) [39]

4.3  Bound Water Measurement Techniques

67

Fig. 4.11  Progression of intracellular and intercellular water content at 60 °C drying temperature [39]

intercellular spaces and intracellular spaces at the preliminary phase of drying. This results in the conversion of intracellular water to intercellular water, whereas at a final phase of drying, cell rupture leads to rapid water migration from cell towards intracellular spaces. For this very reason, intercellular water content seems to be higher at this stage.

4.3.5  Thermogravimetric Analysis (TGA) In Thermogravimetric Analysis (TGA), at a constant heating rate or time the food material is analyzed where the mass of the food material acts as a function of temperature. In TGA, the food material is exposed to a temperature controlled environment. The food material is generally, heated at a constant heating rate: this process is occasionally known as dynamic measurement process. Furthermore, it may correspondingly be conceivable to accomplish TGA at isothermal process where the temperature will be fixed. Even though it can be exposed to non-linear temperature programs like those used in controlled TGA (so-called SCTA) experiments [46]. The selection of temperature program varies with the types of food materials. To minimize oxidation and other undesired reactions during the TGA, an inert environment needs to be confirmed. Firstly, the sample needs to be placed on the pan of the TGA system. By using a precision balance the sample pan is supported and is positioned in a heating source

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to heat the sample up or to cool the sample down throughout the experiment. A built-in weight monitoring system is available in most of the TGA devices for continuous and precise measurement of sample weight. The outcomes of a TGA measurement are typically presented by a TGA curve where mass or percentage of weight is plotted against temperature or time. A general trend of TGA curve is shown in Fig. 4.12. There is another important parameter known as differential thermogravimetric or DTG curve that delivers the rate at which the mass changes. Figure 4.12 represents the first derivative of the plotted TGA curve, which is known as DTG curve. Furthermore, a point of inflection can be found from the DTG curve, which may deliver the indepth understandings of water status in dissimilar biological tissues. The point of inflection is known as a point where the greatest change occurs in DTG curve. 4.3.5.1  Measurements The food sample must be prepared with appropriate care. Size of the sample must be small enough to fit in the pan of TGA setup. Typically for TGA experiments, the food material weight is taken between 2 and 50 mg [36]. Several small pieces of

Fig. 4.12  A common trend of TGA curve

4.3  Bound Water Measurement Techniques

69

food sample are better than a larger piece of the sample to increase the mass flux. To quantify the temperature precisely, TG analyzer commonly comprises of several important equipment. In TG analyzer, there is a high-precision microbalance with a pan which is positioned in a small electrically heated oven with a thermocouple. The prepared food material is loaded into the pan and uninterruptedly heated generally from room temperature to 1000 °C. The rate of heating must be constant during the TGA experiment. Numerous constituents of the food material are transformed owing to the various chemical reaction when subjected to the continuously increased temperature. Besides, the weight of the food material reduces when the temperature in the TGA analyzer increased uninterruptedly. This occurs due to the moisture migration from the food material by evaporation. With the change in temperature, the weight of the food materials is also changed which is precisely and constantly documented by the TGA analyzer for the further investigation. In order to analyze the raw data of TGA, the temperature must be plotted on the X-axis and moisture loss on the Y-axis. In addition to this, the first derivatives plotted to find out the points of inflection can be used for further in-depth interpretations. In TGA analyzer, it is considered that at a specific temperature the free water is evaporate quickly, whereas the residual water is considered as bound water. A typical trend of TG and DTG curve is presented in Fig. 4.12 which interprets the free and bound water presented in a food sample. At the initial phase of heating, no water migration is occurred from the food material due to the higher level of micro-­ molecular bonding strength of bound water. It clearly appears from the figure that the free water migrates steadily towards an exact moisture content, whereas the bound water remains stable at a lower temperature. Presence of free water is referred up to the point of inflection in DTG curve, and beyond that point; rest of the water can be defined as bound water either strongly bound or loosely bound [46, 47].

4.3.6  Bioelectrical Impedance Analysis (BIA) Bioelectrical impedance analysis (BIA) provides information pertaining to the proportion of intracellular and intercellular water present in the plant and animal-based food materials. The method is based on the relationship between water content and impedance. Various frequencies of current encounters different responses in flow through intracellular water (ICW) and intercellular water content in the food material. A tissue of a sample food material that will undergo the investigation of moisture content by BIA is an imperfect conductor of electrical current. Because of its easiness and measurement of moisture content without cutting or crushing the sample food material, BIA has been extensively used to evaluate the physiological state of various biological tissues in medical science [48]. In 1992, the first efficacious validation of measurement of moisture content by using BIA was done in animal tissues

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[49]. Currently, quantifying of intracellular and intercellular water in living animals by means of BIA is becoming a regular issue for identifying the immunological diseases. In addition, BIA has been extensively used for determining the intracellular and intercellular water content in plant tissues including fruits, vegetables, roots, and leaves [50]. BIA not only can determine the proportion of different types of water but also can trace the injury in food materials during processing. For instance, injury results in chilling and bruising have been investigated in the literature using BIA [36]. BIA is also used to underpin the relationship between the electrical and rheological properties of dissimilar kinds of plant-based food materials [51, 52]. The BIA approach is established on the principle that electricity flows through a food material by electrolyte-containing fluids and usually follows the pathway of the smallest resistance. Figure  4.12 displays the simplified equivalent circuit for current flow through a food material. The total impedance, denoted by Z has two parts, namely resistance (real part), and reactance (complex part). The electrical current in the food material encounters two types of obstacles from different spaces of the food material: (a) from the fluids in the food material (intercellular and intracellular fluids), which has low resistance; (b) From the cell membrane that has a high-resistance and acts as a capacitor. In Fig. 4.13, the resistance owing to the intercellular fluid is represented as Re whereas the resistance resulted by the intracellular fluid is shown by Ri. The impedance because of the cell membrane capacitance is as follows: Zc =

1 jωC

(4.15)

Where, Zc = impedance due to cell membrane capacitance, ω = 2πf, f is the frequency of the signal, and. C = capacitance of the cell membrane. Consequently, the overall impedance of the electrical circuit of the food material is



 1  Re  Ri +  jωC  Z=  1 Re + Ri + jωC

(4.16)

4.3  Bound Water Measurement Techniques

71

Fig. 4.13 (a) The pattern of current flow in food material during BIA analysis, (b) An equivalent circuit for current flow through intracellular water and intercellular water (Adapted from Halder et al. [15])

4.3.6.1  Measurement Bioelectrical Impedance at Low Frequencies It is apparent from Fig. 4.14 that if the current having low frequency is applied to the food material, it travels through the easiest pathways such as the spaces where intercellular water is present. On the other hand, this low-frequency current encounters resistance substantially once it tries to pass through cell wall and membranes of the food tissue. At low frequencies, due to the low RC time constant cell membranes shows higher electrical resistance. The potential gradients through the cell membranes thoroughly maintain the external potential gradient and consequently the amount of the current in the cell capacitor is small. At this circumstance, most current conducts through the intercellular region. At zero frequency, the conducted current is considered as direct current while the measured resistance is R0. This is a paramount theoretical value of the impedance of the intercellular region, as at zero frequency maximum portion of the current travels through this region.

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Fig. 4.14  Responses of different frequency signals in food material

Bioelectrical Impedance at High Frequencies The current having high frequency can travel through all pathways in food tissue including the cell wall and spaces where intracellular water is present, as an increase in frequency causes a significant decrease in the impedance of the membrane. In AC current the polarity is changed continuously and as a consequence the cell membrane charges and discharges at the rate of the frequency. The amount of charge and discharge converts in such a way that the impedance owing to the capacitor reduces to inconsequential proportions, while the frequency increases. The current conducting through both the intercellular and intracellular sections is typically dependent on their relative volumes and conductivities [53]. A high-­ frequency signal can go through both the cell membrane and intracellular water. Thus, Rinf is the impedance calculated at an infinite frequency, when current passes directly through the cell membranes. Therefore, at this high frequency the reactance component of the impedance vector is zero. Moreover, Rinf represents the total impedance encountered during the flow of current through tissue, and it can be expressed by the following equation: Rinf =

Ri Re Ri + Re

(4.17)

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73

Table 4.2  Intracellular Water Content (%) in several food materials [14]

Sample Cucumber (Sample 1) Cucumber (Sample 2) Tomato (Sample 1) Tomato (Sample 2) Apple (Sample 1) Apple (Sample 2) Eggplant (Sample 1) Eggplant (Sample 2) Potato (Sample 1) Potato (Sample 2)

Intracellular water content (%) 95.3 95.8 84.7 78.7 90.2 89.9 96.8 97.3 95.3 96.0

Ratio of Intracellular to Intercellular Water The proportion of intracellular and intercellular water may be figured out by the amount of their corresponding volume obtaining the values of R0 and Rinf [54]. The ratio of intracellular and intercellular water may be calculated by the subsequent equations [55].



 VIntraCW  1 +   VInterCW 

5

2

=

Re + Ri Ri

 VIntraCW 1 + k VInterCW 

  

(4.18)

By using Eq. (4.18) and the assumption of R0 = Re, Eq. (4.19) may be written in terms of R0 and Rinf as:



 VIntraCW  1 +   VInterCW 

5

2

=

R0 Rinf

 VIntraCW 1 + k VInterCW 

  

(4.19)

Finally, the ratio of intracellular and intercellular water in food material can successfully be measured using Eq. (4.19). This very method is implemented in literature to calculate the proportion of various kinds of water exist in biological materials. For instance, Datta et al. measured intracellular water content using BIA at room temperature for five selected ­plant-­based food materials and found the result as presented in Table 4.2 [14]. It is clearly observable from the table that most of the selected food materials contain a significantly high proportion of intracellular water ranging 84.7–96.8% in weight basis.

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4.3.7  Nuclear Magnetic Resonance (NMR) Nuclear magnetic resonance (NMR) is one of the widely used systems in determining the proportion of free, weakly bound and strongly bound water in food materials [56, 57]. As previously mentioned, the binding energy varies with the variation of the nature of water molecules with solid matrix and other components. Due to this, water at different location shows different mobility in the response to nuclear magnetic resonance. Low-field Nuclear Magnetic Resonance (NMR) analysis techniques utilize the relaxation time of hydrogen protons in a constant magnetic field of low intensity usually 0.55 T to investigate the migration pattern of water molecules in a food material [58, 59]. Different levels of nuclear magnetic energy environment can be produced in NMR in order to analyze the mobility of the hydrogen protons presented in biological tissue. The hydrogen protons in a low-energy stage may jump to a higher energy stage if a pulse of an electromagnetic wave is provided at right angles of the magnetic field which finally causes resonance [60, 61]. Conversely, the hydrogen protons relapse to their original low-energy stage, while the influence of pulse electromagnetic wave removes. The time necessary for the additional energy to relapse to the original (unexcited) energy stage in every hydrogen proton is called relaxation time T1. The time necessitates for two actuated hydrogen protons having an identical frequency, but dissimilar moving tends to change their directions at the similar level is called relaxation time T2 [62, 63]. Determination of free or bound state of water of fresh and dried food materials can be investigated from the relaxation time T2 [64, 65]. Presently, NMR is widely used to quantify the moisture content and the state of water (bound or free) in drying processes [66, 67]. This technique facilitates the observing of water status during drying and the modifying of drying parameters effectively. 4.3.7.1  Measurements Online Detection of the State of Water Using NMR To measure the state of water and amount of moisture content the relaxation time T2 at the shortest possible time is selected as the main detection parameter in NMR [63, 68]. The attainment of T2 necessitates almost 0.5–1 min. Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence is an efficient device that is extensively used to collect the T2 value of the food material during NMR ­detection [69]. In a typical NMR investigation, a magnet temperature of 32 °C, a magnetic field intensity of 0.55 T, resonant frequency of 23.137 MHz and coil diameter of 40 mm is usually maintained. While the NMR investigation carrying out for monitoring real-time water distribution, it is found that certain range of temperature difference between 50–60  °C of the food materials has no noticeable effect on the assessed result.

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4.3  Bound Water Measurement Techniques

Using the relaxation model specified by Eq. (4.20), the attenuated curve as represented in Fig. 4.15a of total signal amplitude is inverted to Fig. 4.15b of T2. n



 −t   T  2i 

M ( t ) = ∑ i =1 Ai e

(4.20)

Where, T2i = distribution of relaxation time T2 in (ms), t = sampling time in (ms), Ai = (arbitrary unit, a.u.) is the distribution of relaxation time, M (t) = total amplitude of the entire signal. The T2 relaxation of food material in the course of drying may be achieved by this technique, and an inversional curve of T2 from the complete signal is formed as represented in Fig. 4.15b [59]. Overall, if T2 relaxation time is smaller, the peak of T2 spectrum moves to the left that designates the constricted binding of the protons with the dry matter of food material. In contrast, if the T2 relaxation time is lengthier, the peak of T2 spectrum moves to the right, which indicates the open binding of the protons with the dry matter of food material. In the drying process of food material, the attenuation signal of T2 relaxation relies upon the state of water (bound or free) and the solid content [71]. Up to the present time, there is no standard technique to distinguish among diverse states of water in NMR [72–74]. Different authors used different ranges to distinguish among diverse states of water [73, 75]. For the subsequent study, the relaxation time segments T21 (0.01–5  ms), T22 (5–50 ms), and T23 (50–10,000 ms) represent the strongly bound, loosely bound, and free water respectively. Correspondingly, the amplitudes of the signals of these dissimilar states of water may be incorporated into the predetermined relaxation time domains. Here, the A21, A22, and A23 denote the signal amplitudes of strongly bound, loosely bound and free water respectively.

Fig. 4.15  Total relaxation curve and corresponding T2 inversion curve of fresh food material (corn). (a) Total relaxation curve of fresh food material: The abscissa displays the relaxation time from 0 to 3000 ms. The ordinate displays the total amplitude of the signal as M (t). (b) T2 inversion curve of fresh corn: The abscissa displays the transverse relaxation time T2 from 10−2 to 104 ms in logarithm scale. The ordinate displays the amplitude of signal corresponding to the relaxation time T2 (Adapted from Weiqiao et al. [70])

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4.3.7.2  Data Interpretation For the clarification of the complex water measuring technique using NMR, it is worthy to mention a couple of reported studies concerning bound water measurement during drying. Measurement of the Sate of Water During Microwave Drying Zhang et  al. found that NMR is capable to investigate the spatial distribution of water in food materials during drying [64]. The T2 relaxation time versus amplitude curves of NMR demonstrates two peaks. These peaks are pretty noticeable, particularly in high moisture content as demonstrated in Fig. 4.15a [70]. Figure 4.16a displays that the peaks of strongly bound water and peaks of loosely bound water are mixed together in the fresh food material, as the moisture content is high [76]. As presented in Fig. 4.16a, the peak to the right reduced quickly and the peak into the left reduced gently that specifies the presence of free water content at the initial stage of drying, and the proportion of dehydration is the fastest at the initial stage of drying. As the drying progresses, the evaporation rate of strongly bound and loosely bound water increases rapidly while a lower amount of free water shows the opposite phenomena. After 40 min of drying, a new peak seems rather than the two other peaks. At the final stage of drying, the peak value of the left side is greater than that of the other two peaks and the value of the rightmost peak is the smallest. Investigation reveals that at the ultimate stage of drying, the dissimilarity between signal amplitudes of strongly bound water and loosely bound water becomes flawless and their corresponding peaks progressively detaches from each other. The signals of free water, practically extinct and simply weak signals of strongly bound water and loosely bound water persist at the finishing stage of drying.

Fig. 4.16  T2 and A2 curves of NMR of corn kernels obtained from MVD (a) T2 curves with the microwave power of 100 W, (b) A2curves with the microwave power of 100 W [70]

4.3  Bound Water Measurement Techniques

77

By using MATLAB2011b and Process_Peak_QY prepared by Suzhou Niumag Co. the integral area of different types of water as a function of time is calculated. To do this, the magnitudes of peak amplitude as found in Fig. 4.16a and set relaxation times of free water, loosely bound water, and strongly bound water is used. By indicating to the dissimilar states of water, the overall signal amplitudes as a function of time is represented in Fig. 4.16b. A21, A22 and A23 curves characterize the aggregate signal amplitude of strongly bound water, loosely bound water, and free water [69]. From Fig. 4.16b it is observed that, at the initial phase of drying, the free water reduces quickly, while the bound water persist nearly the same. The strongly bound water has the strongest amplitude signal among different states of water at the 30 min of drying. The A23 (free water) and A22 (loosely bound water) reduces quickly at the initial phase of drying, while A21 (strongly bound water) reduces quickly at the concluding stage. Figure 4.17a displays a contrast of the proportions of the dissimilar categories of water in diverse food materials [45]. About 80–90% Loosely Bound Water (LBW), 2–5% Strongly Bound Water (SBW) and 10–20% Free Water (FW) has been observed in different food materials depending on the category of them, and their cell structures, diversity of food composition, orientation and the solute content. Bound Water in Fresh Plant-Based Food Materials Another study conducted by Khan et al. investigated free and bound water in ten selected fruits and vegetables as presented in Fig. 4.17 [32]. Among the selected fruits, apple comprises the maximum amount of loosely bound water (LBW) whereas kiwi fruit confirms the minimum amount of the same result has been previously found by other authors while using different techniques [15]. Conversely, the SBW measured for kiwi fruit is greater than rest of the fruits which is merely because of its high proportion of solid material content. Similarly, Fig. 4.17b displays the contrast of the quantity of different types of water in numerous vegetables. The proportion of LBW and SBW varies significantly in individual types of vegetables. This may result due to the diverse combination of compositions and orientation of those components with water molecules. The result found from NMR is consistent with the study made by the other methods such as BIA [15].

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LBW

Percentage of water (%)

a 100 90 80 70 60 50 40 30 20 10 0

Percentage of water (%)

b

Apple

120

FW

Apricot LBW

SBW

Kiwi FW

Nectarine

Pear

SBW

100 80 60 40 20 0

Fig. 4.17 (a) The proportion of dissimilar categories of water in different fruits. (b) The proportion of dissimilar categories of water in different vegetables [45]

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2 7. D. Reid, Water determination in food, in Encyclopedia of Analytical Chemistry (2006) 28. M. Mathlouthi, Water content, water activity, water structure and the stability of foodstuffs. Food Control 12(7), 409–417 (2001) 29. T. Okabe, M.T. Huang, S. Okamura, A new method for the measurement of grain moisture content by the use of microwaves. J. Agric. Eng. Res. 18(1), 59–66 (1973) 30. M.F. Froix, R. Nelson, The interaction of water with cellulose from nuclear magnetic resonance relaxation times. Macromolecules 8(6), 726–730 (1975) 31. E.L.  Andronikashvili, G.M.  Mrevlishvili, V.M.  Sokhadze, K.A.  Kvavadze, Thermal properties of collagen in helical and random coiled states in the temperature range from 4° to 300° K. Biopolymers 15(10), 1991–2004 (1976) 32. J.M.  Preston, G.P.  Tawde, 10—Freezing point depression in assemblages of moist fibres. J. Text. Inst. Trans. 47(3), T154–T165 (1956) 33. S.  Nomura, A.  Hiltner, J.B.  Lando, E.  Baer, Interaction of water with native collagen. Biopolymers 16(2), 231–246 (1977) 34. K. Nakamura, T. Hatakeyama, H. Hatakeyama, Studies on bound water of cellulose by differential scanning calorimetry. Text. Res. J. 51(9), 607–613 (1981) 35. M.U.H. Joardder, R.J. Brown, C. Kumar, M.A. Karim, Effect of cell wall properties on porosity and shrinkage of dried apple. Int. J. Food Prop. 18(10), 2327–2337 (2015) 36. M.I.H. Khan, M.A. Karim, Cellular water distribution, transport, and its investigation methods for plant-based food material. Food Res. Int. 99(Pt 1), 1–14 (2017) 37. C. Reh, S.N. Bhat, S. Berrut, Determination of water content in powdered milk. Food Chem. 86(3), 457–464 (2004) 38. J. Vogl, M. Ostermann, On the measurement of the moisture content in different matrix materials. Accred. Qual. Assur. 11(7), 356–362 (2006) 39. M.M.  Rahman, M.U.H.  Joardder, A.  Karim, Non-destructive investigation of cellular level moisture distribution and morphological changes during drying of a plant-based food material. Biosyst. Eng. 169, 126–138 (2018) 40. E.  Tsotsas, A.S.  Mujumdar, Modern Drying Technology, Volume 3: Product Quality and Formulation, vol 1 (Wiley, Hoboken, 2011) 41. J.Y. Ahn, D.Y. Kil, C. Kong, B.G. Kim, Comparison of oven-drying methods for determination of moisture content in feed ingredients. Asian-Australas. J. Anim. Sci. 27(11), 1615 (2014) 42. H.P.  Ng, S.H.  Ong, K.W.C.  Foong, P.S.  Goh, W.L.  Nowinski, Medical image segmentation using k-means clustering and improved watershed algorithm, in 2006 IEEE Southwest Symposium on Image Analysis and Interpretation (2006), pp. 61–65 43. A. Léonard, S. Blacher, P. Marchot, J. Pirard, M. Crine, Moisture profiles determination during convective drying using X-ray microtomography. Can. J. Chem. Eng. 83(1), 127–131 (2005) 44. M.I.H. Khan, R.M. Wellard, S.A. Nagy, M.U.H. Joardder, M.A. Karim, Experimental investigation of bound and free water transport process during drying of hygroscopic food material. Int. J. Therm. Sci. 117, 266–273 (2017) 45. M.I.H. Khan, R.M. Wellard, S.A. Nagy, M.U.H. Joardder, M.A. Karim, Investigation of bound and free water in plant-based food material using NMR T2 relaxometry. Innov. Food Sci. Emerg. Technol. 38, 252–261 (2016) 46. R. Bottom, Thermogravimetric analysis. Princ. Appl. Therm. Anal. 3, 87–118 (2008) 47. J. Biscarat, C. Charmette, J. Sanchez, C. Pochat-Bohatier, Preparation of dense gelatin membranes by combining temperature induced gelation and dry-casting. J. Membr. Sci. 473, 45–53 (2015) 48. H.C. Lukaski, P.E. Johnson, W.W. Bolonchuk, G.I. Lykken, Assessment of fat-free mass using bioelectrical impedance measurements of the human body. Am. J. Clin. Nutr. 41(4), 810–817 (1985) 49. J.R. Matthie, P.O. Withers, M.D. Van Loan, P.L. Mayclin, Development of a commercial complex bio-impedance spectroscopic (CBIS) system for determining intracellular water (ICW) and extracellular water (ECW) volumes, in Proceedings of 8th International Conference on Electrical Bio-impedance (1992), pp. 203–205

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Chapter 5

Challenges in Bound Water Measurement

5.1  Introduction Water measurement of any kind is a complex task. In general, water molecules shows several exceptional characteristics while they make bonds with other molecules. For instance, water in food materials is physically and chemically connected with hundreds of other components. Moreover, different amount of bonding energy in different combinations of water and other substances has been reported in the literature. Therefore, tracing the amount of water in complex materials such as food is one of the complicated tasks encountered by the research of different fields. In order to detect or measure water in food materials, several methods are available. Each of the approaches is based on the response of water molecules in a specific measuring condition. Moreover, the response of water in a specific condition is quite different than that of pure water. In addition to these, water measurement by different methods does not necessarily demonstrate the same amount and distribution of water in the same sample. Therefore, the result of one method shows significant discrepancy with that of other methods in many cases. Most of the water determination methods have remarkable limitations and challenges if applied to food materials. In this chapter, the overall challenges in the determination of water content are discussed. Following this, very specific discussion on the drawbacks associated with different methods of water determination has been presented briefly. Finally, specific challenges in bound water measurement techniques including DSC, SEM, DIL, TGA, BIA, CT-scan and NMR are discussed extensively.

© Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_5

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5.2  General Challenges in Determination of Water in Foods • Water is not only the volatile component Several volatile components alongside water can be found in different types of food materials. The volatile components including alcohol, and chemicals of flavors response to some extent similar to water in a specific measuring environment. However, this types of errors vary with the variation of food materials. For instance, herbs and spices encompass a significant portion of volatile compounds. Therefore, for water measurement of these types of materials, thermogravimetric analysis such as dryings and TGA should not be carried out. • Chemical reactions during water removal Chemical reactions during water measurement such as thermal decomposition of unstable volatile components or interaction between the components are prevalent is some water measurement techniques. Methods associated higher processing temperature and chemical catalyst are subject to this type error from this source. Different types of methods including distillation, oven, and chemical method are few of the methods that encounter such type of error. However, this error can be reduced by controlling low process temperature, although it is difficult to ascertain such safe temperature for particular food materials. This safe zone of temperature can be based on the stability of major volatile component present in food materials [1]. • Complete removal of moisture from the sample Direct methods of measuring water are associated with this type of removal of water from food materials. In these methods, complete removal of water is essential in order to determine total moisture content present in food materials. However, there is no indication that can ascertain of removal of all of the water from food materials during measurement. Therefore, high possibility of error can be associated in direct methods for this particular complexity in water measurement. • Diversification in food structure and compositions Food materials are not always homogeneous in component distribution and structural formation. In most of the cases, amount, availability and distribution of a particular component in food materials vary within a sample. This complex nature of food materials makes the measurement more difficult. The water content in the selected portion cannot be same in comparison with another part of the chosen food. Therefore, a discrepancy in the amount of water can easily be found in two separate measurements of samples. Multiple observations may reduce this type of error in water determination. • Lack of calibration reference Calibration using reference materials is vital in water determination of a selected sample. Unavailability of proper reference material results in calibration error in water determination.

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• Selection of standard Similar to the availability of different water measuring method, hundreds of standard pertaining procedures to moisture content determination are available in the literature. These standards are proposed by ASTM, ISO, and CEN for different methods of moisture measurement. Remarkable variation of procedures in different proposed standards causes difficulties in choosing the appropriate standard for a selected food sample. • Improper handling and sampling Water in food material can be lost during improper handling and sample preparation. Different operations including cutting, grinding during sample preparation cause migration of water. Similarly, inappropriate handling can absorb water from food materials. However, besides these above mentioned general complexities in water determination, there are limitations found in all of the direct techniques including distillation, drying, Karl Fischer method and Infrared.

5.3  Specific Challenges in Water Content Measurement 5.3.1  Distillation • Mixing of the co-distilled water with the solvent causes errors in the determination of water volume. • There is a high chance of mixing the sticking condensed water molecules in the apparatus with the main the water content of the sample. This gives an increase of water content of the sample. Care must be taken in removing condensed water prior to starting water measurement of food material. • Formation of the emulsion is very frequent during distillation which leads to the tendency of mixing with solvent. This also contributes erroneous results. • In many cases such high temperature co-distillation, sample undergoes chemical degradation and form unavoidable water content. This additional water is also treated as the water of the sample. Therefore, a suitable temperature must be maintained for avoiding this type of unexpected water formation. • Incomplete separation of water from the sample in this process is the major challenge over course of distillation. It is very difficult to choose perfect concentration and amount of solute for a specific sample. A significant error in water content may occur while failing to ascertain complete water separation from the sample.

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5.3.2  Drying Method • The residual moisture content in the sample in the last part of the drying process depends on the temperature and relative humidity of the surrounding temperature [2–4]. In other words, different measurement conditions can provide varied moisture content for the same sample. • A complete dried sample cannot be obtained at the end of the drying if proper drying conditions are not maintained. Presence of moist air in drying chamber can be a potential source of error of this kind. Vacuum drying may one of the suitable options in order to avoid this complexity in water measurement. • Drying operation takes place at higher temperature leads to evaporation of volatile components alongside water and intensifies thermal degradation. In general, drying is performed at a temperature between 70 °C and 105 °C for food materials [5]. However, the magnitude of error due to volatile non-aqueous components depends on process temperature, compositions of food, and pressure of the system. The error found negligible in vacuum drying at 70 °C [1] and even more accurate result is reported at 40  °C in oven drying [6]. Moreover, changes in functional and in the content of different components such as proteins, anthocyanidins, tannins and polyphenols can be avoided during the process if it is carried out under 65  °C [7]. In the drying process, some researchers argued that it is erroneous to consider the difference in mass after drying from before drying as ‘water content’, rather it should be termed as ‘mass loss’ on drying [8]. • Despite having a high potential for errors in drying methods, these methods are preferred due to their simplicity and low-cost requirement to perform.

5.3.3  Karl Fischer Method • As the water which is needed to be measured must come into interaction with the reagent during titration, Karl Fischer (KF) method is only feasible while water in foodstuff is freely available [9, 10]. Bound water cannot be measured by this method as this type of water is unavailable for the reagent. • Interference of oxidizing regent results in unnecessary reaction during KF method in some food materials [5]. Consequently, an erroneous determination of water can occur in this case.

5.3.4  Infrared Spectrometry • The infrared method measures the surface moisture content of food materials. Therefore, bulk moisture content cannot be obtained from this method.

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• Apart from characteristics of the irradiated electromagnetic spectrum, color, particle size, and texture of the food sample affect the results in determining moisture content. Selecting appropriate wavelength of the infrared rays and proper sample preparation can significantly minimize the discrepancy in water content measurement. • As the surface temperature sometimes reaches to over 200 °C in infrared spectrometry, a rapid thermal degradation is observed in the food material. • The infrared spectrum data have to be correlated with the direct measurement of water content data to minimize errors in measurement. Accurate calibration needs to be established in order to correlate infrared spectrum data and moisture content.

5.3.5  Microwave Absorption • Microwave absorption significantly depends on the types of sample and sample temperature. Even sample dimension especially thickness influences result. • Uneven distribution of electromagnetic field produced in microwave system results in errors in water determination.

5.4  C  hallenges Associated in Bound Water Measurement Techniques Investigation of water distribution is more challenging than to total water content determination. The techniques that are usually used to investigate the proportion of water content (bound and free) in food materials such as DSC, SEM image processing, DIL, TGA, BIA, CT-scan, and NMR encounter specific challenges during water measurement. The challenges of the above mentioned techniques are given below:

5.4.1  Differential Scanning Calorimetry (DSC) While measuring the bound water using DSC, the free water is considered having the same property of pure water, whereas the crystallization temperatures of the bound water in food materials are lesser than those of free water [11–14]. In Differential Scanning Calorimetry, the peaks of enthalpy in DSC curve for free water is equal to that of pure water and the phase transition temperature of bound water is lower than that of pure water. The proportion of intercellular water (FW) and intracellular water (LBW) in Food material can be measured by using this

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hypothesis. Conversely, the problem is that the investigation of Cell wall water (SBW) is not possible by this method. However, the exact amount of the bound water cannot be measured directly from this method. From the relaxation nature of different water, only the proportion of dissimilar categories of water can be investigated. The exact amount of dissimilar kinds of water in food material can only be investigated if the total water content is known from another direct method. Real-time measurement of water distribution during the process where a higher temperature is required such as convective drying is not possible in this method. The volume of total bound water may be only attained directly from a DSC cooling curves, as the development of crystallization peak II is challenging in DSC heating curves. As the melting peaks do not split into two peaks, it is obvious to investigate the bound water. As a result, the proportion of bound water content in food material may be only calculated by means of DSC cooling curves [15]. Moreover, due to the temperature limitations, DSC is inappropriate for hot air drying setup but significantly appropriate to investigate subcellular water changes for the period of freeze drying. However, in the time of freeze drying of food materials DSC can be used as a suitable tool to investigate cellular water distribution.

5.4.2  SEM Image Processing Bound water estimation from SEM image is another emerging technology that can be used to quantify the volume of bound water in food materials. But, to implement this technique, some assumptions are needed to be considered, and that is the main constraint of this method [16]. 1. Ideal shrinkage takes place during the water removal from the cell wall. That means that volume shrinkage of the cell wall must be equal to the volume of water migrated from the cell wall during drying, which may not be followed in different food materials. 2. Constant and same density of the water present in the cell wall and intracellular water are needed to be considered in this method. This assumption may potentially lead to a sample where the densities of different water are not the same. 3. Bound water presented in micro-level elements such as cell wall and fibers can only be determined in this method. Bound water in food materials other than animal and plant-based ones cannot be measured by this method. In addition to these, image acquisition for the fresh and dried sample in the same place of the sample is a complicated task. Moreover, complete removal of water from the cell wall needs to be assured prior to image acquisition of dried sample using SEM. This issue has been discussed at the beginning of this chapter.

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5.4.3  Dilatometry Dilatometry technique is widely used by the investigators who study about the bound water in sludge and soil [16–18], and polymers [19]. Nevertheless, it is infrequently used to calculate the amount of bound water in food material due to some of the limitations of this method discussed below. The food material must air free before doing the investigation for quantifying the amount of bound water using the Dilatometry analysis. This is one of the challenging parts of this method [19, 20]. As this method is based on the volume expansion of water during the phase change from liquid to solid, presence of air gaps mean a less macro-level expansion of the sample. However, hygroscopic nature is present in maximum of the food materials and therefore it encompasses porosity filled with air and vapor that consequently makes it inappropriate to use the Dilatometry analysis for investigating the amount of bound water in porous food materials [21].

5.4.4  CT Scan This method is appropriate to investigate the instantaneous morphological fluctuations for the period of drying [22]. However, the 2D or 3D images attained from the CT-scan do not provide the quantity of moisture content with different state directly; rather further image processing is required with the intention of calculate the proportion of free and bound water. Over the course of image processing, several steps are needed to be followed carefully which are very much challenging. Firstly, the greyscale images attained from the X-ray μCT are needed to be processed in MATLAB to investigate the water distribution in food materials. Then, noise is eliminated: normally using the intensity thresholding and filtering approaches. Moreover, segmentation is needed to be carried out deploying the watershed algorithm. In addition, as the dissimilarities between Free water and Bound water are usually categorized by utilizing the manual segmentation method which eventually results in significant errors in distinguishing water distribution.

5.4.5  Thermogravimetric Analysis (TGA) TGA is extensively used to measure the moisture content of different food materials [22–24]. However, this method encounters the same challenges that are associated with determining water content using drying method.

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In addition, the processing of raw data of heat, and mass flow demands an expertise in data analysis. The interpretation of data in the TG technique is also not a straightforward task. Moreover, the size constraints of pan limits the application of some food materials can be cut into as small as required to make 2–50 mg.

5.4.6  Bioelectrical Impedance Analysis Bioelectrical impedance analysis (BIA) is a standard technique for investigating the water composition because of its noninvasive characteristics. Moreover, the time requirement by this process is significantly low and above all it is really simpler than other approaches. BIA is generally used for investigating the percentage of fat in animal tissue [25, 26]. For the postharvest quality assurance purposes, numerous researchers have used BIA [27–29]. Likewise other techniques, some challenges prevail during BIA in determining the quantity of bound and free water in food materials. Firstly, calibration is a significant factor before beginning any experiment by implementing a specific technique. Nevertheless, calibration process prerequisite in BIA is extremely complex, and that confines BIA’s effectiveness for food materials analysis [30]. Secondly, several factors substantially including the geometry of the food material affect in impedance analysis [31]. Thirdly, this method is less reliable for heterogeneous food materials such as fruits and vegetables as the ratio of intercellular and intercellular water is not consistent throughout the food sample.

5.4.7  Nuclear Magnetic Resonance (NMR) NMR is one of the most reliable approaches to investigate the bound water in food materials, although it has some drawbacks such as low sensitivity of the signal. Substantial signal averaging is essential to attain a reasonable Signal to Noise ratio (SNR), owing to the low number of nuclei aligned with the magnetic field. The SNR measurement is a vital standard for precise integrations and is correspondingly one of the paramount means to measure the sensitivity of a NMR spectrometer. Overall, to ensure that the instrument has a higher sensitivity, the SNR specification must be also higher. Moreover, an intense concentration of the calculated nuclei, or extended attainment times, or an amalgamation of both is required for the measurement of this type of sample. Besides this problem, there are also couples of minor limitations of NMR that are mentioned here: Firstly, free water content cannot be estimated directly by NMR as it is not contributed to radio frequency. Secondly, the results are needed to be calibrated for the specific food material. Thirdly, there are huge complications in measuring bound water: it takes massive time [1, 31] and the accuracy is also poor [32]. Finally, the cost of purchasing and retaining of the NMR instruments is very high.

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Accurate water measurement is so difficult to such an extent that not a single method can guarantee to measure the exact moisture content let alone bound water. The pervasive nature of water makes it difficult to trace the water separately in solid food materials.

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22. M.M.  Rahman, M.U.H.  Joardder, A.  Karim, Non-destructive investigation of cellular level moisture distribution and morphological changes during drying of a plant-based food material. Biosyst. Eng. 169, 126–138 (2018) 23. J. Biscarat, C. Charmette, J. Sanchez, C. Pochat-Bohatier, Preparation of dense gelatin membranes by combining temperature induced gelation and dry-casting. J. Membr. Sci. 473, 45–53 (2015) 24. H. Feng, J. Tang, S. John Dixon-Warren, Determination of moisture diffusivity of red delicious apple tissues by thermogravimetric analysis. Dry. Technol. 18(6), 1183–1199 (2000) 25. V.M. da Silva, L.A. Silva, J.B. de Andrade, M.C. Veloso, G.V. Santos, Determination of moisture content and water activity in algae and fish by thermoanalytical techniques. Quim. Nova 31(4), 901–905 (2008) 26. H.H. Webber, P.A. Dehnel, Water balance of whole animal, muscle tissue, and muscle cells in the prosobranch gastropod, Acmaea scutum. J. Exp. Zool. A Ecol. Genet. Physiol. 168(3), 327–335 (1968) 27. D.A. Dean, T. Ramanathan, D. Machado, R. Sundararajan, Electrical impedance spectroscopy study of biological tissues. J. Electrost. 66(3–4), 165–177 (2008) 28. P.  Dejmek, O.  Miyawaki, Relationship between the electrical and rheological properties of potato tuber tissue after various forms of processing. Biosci. Biotechnol. Biochem. 66(6), 1218–1223 (2002) 29. M.A. Cox, M.I.N. Zhang, J.H.M. Willison, Apple bruise assessment through electrical impedance measurements. J. Hortic. Sci. 68(3), 393–398 (1993) 30. M. Dehghan, A.T. Merchant, Is bioelectrical impedance accurate for use in large epidemiological studies? Nutr. J. 7(1), 26 (2008) 31. P. Deurenberg, Limitations of the bioelectrical impedance method for the assessment of body fat in severe obesity. Am. J. Clin. Nutr. 64(3), 449S–452S (1996) 32. R. Toledo, M.P. Steinberg, A.I. Nelson, Quantitative determination of bound water by NMR. J. Food Sci. 33(3), 315–317 (1968)

Chapter 6

Bound Water Removal Techniques

6.1  Introduction Different food products having different moisture distribution cause variation in shelf life after processing. For a stable food product, water content needs to be kept in an appropriate range depending on the nature of food materials. For example, in processed fish and dairy products, about 3% moisture content is treated as the safe zone, whereas 5% and 12% moisture content are the allowable range for in fruitsvegetables and cereals respectively to attain equilibrium in the drying process. Water removal from foods involves simultaneous heat and mass transfer during most of the food processes including drying and frying. Three modes of heat transfer, namely conduction, radiation, and convection take place to some extent in food materials during food processing. The contribution of individual heat transfer mode depends on the nature of food process. On the other hand, mass transfer phenomena demand closer attention for a particular unit process during removal of water from food. Different types of water may follow different pathways to migrate from food materials to the surrounding. Knowledge of moisture transfer mechanisms is essential prior to designing the unit process to remove bound water from foods. The first part of this chapter is dedicated to discussing the mass transfer mechanisms associated with bound water removal. Following this, some of the common food processing that may be deployed to bound water removal has been discussed extensively.

6.2  Mechanisms of Moisture Transfer An internal mass transfer takes place in the solid phase or void spaces in the porous media and controls the moisture removal rate [1]. The internal mass transfer takes places in the form of liquid and vapor phase encompasses surface diffusion, © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_6

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hydrodynamic or bulk flow, and capillary flow during food process. Table 6.1 presents the common internal mass transfer mechanisms that take place during food processes such as drying or frying. Bound water migration can be observed micro-and-macro level phenomena and the most of the above listed internal mass transfer mechanisms. After migrating the free water, complexity arises to remove the remaining bound water content. This bound water content requires a continuous energy of vaporization along with the molecular diffusion and pressure force to remove water from the solid matrix [18]. Generally, bound water migration is far more complex than free water removal as it follows relatively complicated pathways along with taking a higher amount of energy. For example, moisture migration from the plant cells and cell walls has a direct impact on the cell collapse during dehydration. This movement of moisture content mainly takes place through the cell, cell membrane or cell wall and intercellular spaces during drying and varies with the food structure, quality, and composition (Joardder et al. 2015b). In Fig. 6.1, the transportation of the water molecules from a different portion of the plant cells and ultimately dispatched to the environment. In the following Fig. 6.1, water molecules have been considered to be transferred from the cells and cell walls to the intercellular spaces through capillaries, vacuoles. Consequently, a thermodynamic equilibrium of the dehydration process is obtained with the atmosphere depending on the conditions and requirements of drying and can be continued up to a temperature at which the integrity of cell wall membrane persists. However, the driving force varies with the variation of food materials and process conditions. Flow path and governing equations of some common mass transfer mechanisms have been presented in Table 6.2. Multiple mass transfer mechanisms may be involved during drying or frying of food. However, an individual mechanism dominates the major transport of water. Three most common internal mass transfer mechanism involved in bound water migration is discussed in the following section.

Table 6.1  Water transport mechanism inside food materials during dehydration [2] Moisture transfer during drying Diffusion Vapor Surface Liquid Capillary Evaporation condensation Hydraulic flow Molecular diffusion Knudsen flow Mutual diffusion Slip flow

Scale Microscale ✓ ✓

✓ ✓

Macroscale ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

References [3–5] [6, 7] [8–10] [11] [12] [11] [13, 14] [15, 16] [17] [1]

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Fig. 6.1  Conceptual maps of different pathways of water migration from plant tissue

6.2.1  Diffusion Concentration gradient in a system gives rise the diffusion process to transfer mass and proceeds until the equilibrium of the concentration [19]. Evaporation of water from the surface to air, mass diffusion of sugar in the water solution or diffusion of body spray in the still air are some of the practical examples. Mass diffusion is the prime mass transfer mechanisms taking place within food materials during all of the mass removal process [20–22]. The analytical solutions of the diffusion equations have been derived after considering the temperature and concentration distribution within a body at any available time and keep the material properties constant. In diffusion mass transfer, the molecules move from one place to other by the random molecular motion of one or more components [23]. This phenomenon can be addressed as the molecular motion initiated and controlled by the concentration gradient in a system and the diffusing molecules experience resistance after a collision from molecules of the medium rather than self-collision between the diffusing molecules [21]. In highly concentrated solution or in solid substance, the diffusion molecules encounter resistance to flow both from the neighboring diffusion molecules and form the medium as well. As a result, the concentration coefficient largely depends on the concentration and the solution of the diffusion equation governs by the constant diffusivity, initial and final boundary conditions which are complex in nature [23]. Diffusion rate can be used to analyze the dehydration process [4]. Where Fick’s second law has a significant role to predict the moisture loss and subsequent drying time for food materials. The removal of bound water from food can be considered under hydrophilic or hygroscopic materials where the diffusion model has been designed in the sorption isotherm region after considering Fick’s law [24]. This model suffers from ­predicting

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Table 6.2  Mechanism of moisture transfer in solid food materials in liquid and vapor phase Mass transfer mechanisms Knudsen diffusion

Flow path

Governing equation nwd =∈ τ r3 Dkw ⋅∇Pw V2

Dkw =

2  2 RT  d  3  n Mw 

(m / s) 2

Stefan diffusion nwd = − ∈ τ Dwg

P M ⋅∇Pw P − Pw RT

Dwg = 2.5 × 10 −5 [m2/s] Poiseuille flow nwd = − ∈ τ

d2P ⋅∇P 32 µ

Condensation evaporation nwd = nm ( ∇T ..…) T1 < T2 Capillary flow

Liquid diffusion

Surface diffusion

nw′ = − PL K ∇ew

nw′ = − PL Dwe M

nw′ = − PL Ds ∇M

accurate moisture distribution and diffusion coefficient at low moisture content [25] and the inclusion of concentration diffusivity or constant diffusivity along with the heat conduction equation puts better prediction with more complex physics and solution. Liquid state diffusion deals with the internal moisture transfer during falling rate period and can be represented as rate limiting mechanism during the drying process of foods [26–38]. Moreover, many researchers opined the vapor diffusion as the drying mechanism for the hygroscopic and porous materials by replacing the moisture concentration gradient in the Fick’s law with the vapor pressure gradient can be written as follows [39–44]:

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∂P ∂  ∂p  = PD ∂t ∂x  ∂x  (6.1)



Where, P = vapor pressure of the moisture content in the sorption isotherm PD = vapor diffusion coefficient indicating the rate of moisture movement under vapor pressure gradient Thus the internal moisture transfer significantly varies with the vapor diffusion and nature of the porous food structure.

6.2.2  Capillary Flow The flow induced by the molecular attraction forces between the solids and liquids to move the liquid between the intercellular spaces, interconnected pores as well as capillaries, termed as capillary flow [45]. This attraction forces merely termed as an adhesive force which moves the liquid through the channels from some high concentration region to low concentration region. Liquid flow through the wick, circulation of blood throughout the body of water from the plant root to all of the leaves and branches can be demonstrated as capillary flow [46]. The capillary flow can be introduced by the following equation in a similar manner to the diffusion equation [47]:



hs ( T − Ts )  M − M∞  ln  t =  Mi − M ∞  ρ s L λ ( Mi − M ∞ ) (6.2)

Food materials act as porous media with interconnected pores and get much more emphasis for liquid transport and heat transfer at different thermodynamic processing [48]. This behavior of foods can be represented by the spatial model of food structure where the mass transfer is dominated by the capillary flow of moisture content during dehydration, rehydration, frying and also in minerals and fats migration process [18, 49, 50]. Moreover, capillary force results as an interfacial pressure difference between the adjacent pores and strongly hold the water in the region of low water content and implies less force in the region of high water content [51]. Thus, capillary force differs from region to region and resist the total drain out of the water content from foods. However, Darcy’s law helps to illustrate capillary flow in the porous foods sample during different thermodynamic processing as [48, 49, 52]: u=−

kl ∂P µ ∂x (6.3)

Where, u = Darcy velocity or average fluid velocity over a cross section P = fluid pressure

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μ = dynamic viscosity kl = permeability of the porous medium

6.2.3  Evaporation-Condensation Evaporation-condensation is a coupled heat and mass transfer mechanism, where the rate of heat energy input in condensation is equal to the rate of heat loss from the surface at evaporation. The mass transfer occurs at the vapor state so that water content does not have a chance to precipitate near the pores near the surface. Evaporation-condensation controls the falling rate drying through the continuous diffuse of heat and mass through the porous media [53] and tortuosity of the pores become significant to account for the heat and mass balance [54–56]. Enthalpy of vaporization effects the evaporation-condensation mechanism of the food. Evaporation-condensation also controls the increasing moisture content at the center while the center pseudo wet bulb temperature remains constant during convective hot air drying or decreasing moisture content during isothermal drying such as dehydration of bread isothermally at 210 °C [57].

6.3  Types of Food Processing to Remove Bound Water 6.3.1  Frying Frying describes the process of submerging the food products in oil or fat and heats up at a higher temperature than the boiling point of water. Simultaneous heat and mass transfer occur when the heated oil enters into the food and evaporates the moisture content in food [58, 59]. In some way, frying can be considered a drying process as it also removes water from foods. The migrated oil into the food attributes to the quality and taste of the fried products along with the consumer concern about the excessive fat consumption [60]. Thus the frying mechanism concerning the fat gain and moisture become a major food processing issue [61]. The frying process can be illustrated as a moving boundary layer where the moisture migrates through diffusion from the core to the surface and vaporizes with the trace of pores on the surface [59, 62, 63]. This pore acts as the pathways for oil to enter into the foods during frying whereas the moisture and oil migration takes sequentially [61, 64]. Oil uptakes is a surface phenomenon which enables the oil droplets to enter into the food products from the outer surface at a greater extent after taking from the fryer compared to the time of frying depending on the product composition and structure and variation in the product, oil and heating medium properties which enables [62, 65]. The frying mechanism can be further described by the two major

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steps namely: continuous replacement of moisture by fat or droplets and absorption of oil after frying [66]. The oil uptakes at the time of frying are relatively less because of higher vapor gradient sets up with the transformation of moisture into the vapor [59], which consequently hinders the oil uptake rate [61]. However, the cooling effect after the frying process insists oil to transfer from outer surface to inner of the foods by the negative pressure driven force followed by the capillary action [61]. The frying process can be broadly classified into two categories; atmospheric frying and vacuum frying [67]. Moreover, an additional frying type generally integrated to the frying approaches where large vapor pressure is released during frying and termed as high-pressure frying. The boiling temperature acts as a function of the high-pressure release during frying and the food process parameters and consumer satisfaction depend on this build up high-pressure release [68]. This high pressure frying yield juicer and crispy products with less energy consumption compared to the atmospheric frying [69, 70]. The frying process can be characterized as the simultaneous heat and mass transfer process along with the formation bubbles in the solid-liquid interface which drastically effects the heat transfer process at different steps of frying [71, 72]. The four major frying steps can be listed up as follows [71]: 1. Initially heating the immersing the food products to an extent that the surface temperature of the products become equals to the boiling point of the oil or fat. The heat transfer mechanism is governed by the natural convection 2. After initially heating the surface, evaporation of moisture takes place with the subsequent bubble formation at the outer surface and surface boiling happens. The heat transfer coefficient increases and the modes of heat transfer changes to boiling from the natural convection [73] 3. At this stage foods internal temperature becomes equal to the boiling point of the immersing oil and the formation of relatively thick lower conductivity curst decreases the mas transfer which initiates the falling rate steps. 4. At the last stage of frying, bubbles moves in a random fashion and with the bubbles collapse in the solid-liquid interface, turns the end of the frying process.

6.3.2  Drying The term drying reflects that food loose water in order to dry to some extent. The sole purpose of drying is to remove required amount of water from food using appropriate conditions. Most of the drying process shows simultaneous heat and mass transfer over the course of water removal. Water evaporates at the exposed surface to leave from food sample to the surrounding. Whereas, various internal mass transfer mechanisms push the water towards the surface of food. Driving the water from the inner part of foods needs energy. Depending on the inclusion of energy to the food, types of food and application of final product, more than 200

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types of dryers are available [74]. Process condition such as temperature, pressure, humidity of the drying system varies significantly in different drying system. This variation eventually control the water transport mechanisms as well as the quality of dried food [75, 76]. Prior to selection of a particular dryer, several factors including application of end product, expected quality, available energy and even requirement of bound water removal need to be taken into consideration [77]. In addition, diverse types of food materials demands various drying conditions, in turn, a single dryer can be used in order to eliminate water from all types of food [77, 78]. A broad classification of dryer has been presented in Fig. 6.2 [76]. Despite the difference in the process conditions in different drying system, almost same pattern of drying kinetics can be observed [39]. In general, typical drying process periods can be broadly divided into constant rate period and falling rates periods. The difference in nature of moisture removal from the inner region to surface, and surface to surrounding clearly divide the drying periods [43, 74]. During constant rate period, the evaporation occurs from the surface at a constant rate and the internal moisture transfer mechanism is well enough to balance surface

Conduction Convection

Radiation

Cold Air Hot Air

Drying Unit

Mode of Heating • Conduction • Convection • Radiation • Infrared Heating • Microwave Heating • Electric Heating Conventional single Dryer • Sun Drying • Solar Drying • Hot Air Drying • Heat Pump Drying • Freeze Drying • Microwave Drying • Appearance Hybrid Drying • Microwave Assisted Freeze Drying • Microwave Assisted Vacuum Drying • Microwave Assisted Convection Drying • Osmotic -Convection Drying • Osmotic -Microwave Drying • Ultrasonic - Convective Drying • Solar - Heat pump Drying

Fig. 6.2  Classification of drying processes [76]

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equilibrium. The constant rate period drying can be characterized by the external surface evaporation rate based thermodynamic process. This period is significantly affected by the temperature gradient between the dry atmospheric air and wet surface, outer exposed area, and heat and mass transfer coefficient. As the internal moisture transport to the externally exposed surface is equal to the evaporation rate so that the energy input and heat loss during dehydration remains constant. Also, the rate of evaporation has been balanced by the moisture transfer rate from the inner region of food that causes the surface temperature below the wet bulb temperature of air [20]. In this stage of drying, only free water effectively migrates from foods. Whereas, falling rate periods are the internally controlled drying process stages where the rate of evaporation from the surface is less than the moisture transfer from the core to surface [78, 79]. Further, the falling rate period is divided into two sub categories namely first falling rate and second falling rate. In the first falling stage, the internal moisture flow rate falls; hence the evaporation rate decreases as well as the heat flux from the surface increases [39]. At the same time, the internal temperature of the sample does not vary significantly compared to the wet bulb temperature of the air and a large fraction of the sample remains non-porous due to low moisture transfer rate. In this stage of drying both loosely bound and free water migrate from food material. On the other hand, when the partial pressure of the internal water content falls below the saturation level, the second stage of falling period starts to take place. Also, the heat flux to the sample decreases due to the low temperature gradient between the sample and hot air, highly porous structure has been developed which insists internal evaporation in the vapor form [20]. The required heat of vaporization has been transferred to the interior through the low conducting dry matrix and porous medium. This second stage falling rate is relatively more time consuming than the first one. Second falling period of drying takes roughly the same energy to transfer last 10% moisture compared to the first 90%. This kind of drying behavior can be experienced in most of the food materials and biological plants [31, 43, 80]. Therefore, the falling rate periods dominate the whole drying time. Apart from the constant rate and falling rate period drying process, critical moisture condition can be observed immediately after the constant rate period depending on the external drying conditions and material internal properties [43, 74]. The critical moisture content of some fruits as 5.5–7.7 g/g dry solid which corresponds to a similar value at the very beginning of drying [81]. Sometime the surface moisture migrates during preparing and handling the sample and seemingly no constant rate period is observed in that case. That is why some researchers strongly opined the absence of constant period drying and identified the drying process as the falling rate period drying [32, 34, 80–82]. As drying is the prime unit processing to remove water from foods, an extensive discussion is required to get the insight of both free and bound water migration during drying.

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6.3.2.1  Convective Drying Convectional hot air drying associated convection heat transfer from air to food surface followed by conduction towards sample core. This combination of heat transfer mode results in slow water removal from food core to the surrounding. Convective drying is highly efficient in removing free water from the food materials. Due to continuous heating of food surface, case hardening phenomena occurs during convective drying. Low heat and mass transfer characteristics of the external surface, it takes exceptionally long time to remove interior water let alone relatively bound water from the food materials. Therefore, traditional convective drying is not a right choice in removing relatively bound water from food materials. 6.3.2.2  Microwave Drying Microwave drying takes advantages of the response of water molecule in the electromagnetic field with a wavelength of 1 m–1 mm [83]. Microwaves are generated by stepping up the frequency of 60 Hz up to 2450 MHz using vacuum tube devices such as magnetron [84]. Microwave penetrates towards an appreciable extend of thickness of foods, resulting volumetric heating [85]. Volumetric heating refers internal heating process that distributed throughout the volume of food as opposite of heating only on the exposed surface. Apart from water, fat and sugar present in food also contributes in up the food materials [86]. In practice, microwave interaction with the dipole molecules causes tremendous motion, producing faster heat energy. In food materials, mainly water molecules absorbing microwaves transfer heat to the rest of components of foods. Eventually, volumetric heating takes place in the sample where water is available in food material [87]. In volumetric heating, temperature gradient exist even in the inner part of the sample, forcing the moisture out [88]. Moreover, the puffing effect developed due to rapid vapor generation accelerate relatively bound water migration from food materials [89, 90]. Internal volumetric heating in microwave highly accelerate the moisture migration from food, turn in, substantially decrease in processing times, leading to higher product quality food [91–95]. However, the uneven distribution of electromagnetic field leads non-uniform heating in foods. Muti-magnatron can be used to minimise this problem. Having these exceptional heating characteristics, microwave drying can remove all sort of water except tightly chemically bound water. 6.3.2.3  Infrared Drying Application of infrared (IR) energy in food industry become popular due to the advance in the production technology of infrared energy emitters. The basic difference between conventional hot air drying and infrared drying is the mode of heat

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transfer. Radiation heat transfer govern in the moisture migration from the surface of food to surroundings whereas convection heat dominates in hot air drying. Therefore, less thermal resistance is observed infrared drying for a food sample with higher emissivity [96, 97]. Moreover, direct contact of oxygen with food material can be avoided in infrared drying as no medium is required in infrared drying [98]. Moreover, the control of temperature and reduction of loss of heat energy is possible in this process as IR radiation energy transfer directly from the IR emitter to the surface of food material [99]. Moreover, water present in food materials show high absorptivity of long electro-magnetic wave (wavelength more than 3  mm). Eventually, the emitted thermal radiation directly interacts with the internal structure and heat up the adjacent water of both free and bound state quickly. Bound water starts evaporating along with free water and migrate from the sample at the very beginning of the drying process [100]. Therefore, IR drying is pretty much faster than hot air drying and allows migration of bound water at the earlier stage of drying [100]. Due to the intrinsic advantages of radiation heat transfer, IR offers easy bound water removal, energy savings and short drying time [101–103]. 6.3.2.4  Vacuum Drying Vacuum drying (VD) process is mainly adopted for the preservation of heat sensitive materials including fruits with higher sugar content [104]. Vacuum drying needs assistance of other types of drying such as hot air or microwave drying for complete drying of food materials. In vacuum pressure condition, water can evaporates at a temperature significantly lower than that is needed at atmosphere pressure [105]. The vacuum in the drying chamber has a twofold functions on the under process products; one is to reduce the surface water content of the products and the other is to reduce the boiling point of the inner water content. Hence, a higher vapor pressure gradient can be observed between the interior and the surface of the product which ultimately stimulates the drying rate at a lower product temperature and atmospheric pressure. Generally, in the third phase of drying when there is no unbound water left, bound water starts moving towards the surface by diffusion or capillary action. Relatively lower amount of heat energy than convectional drying is required to eliminate bound water during vacuum drying [106]. 6.3.2.5  Freeze Drying Freeze drying (FD) is a thermodynamic process that involves removal of the ice crystals from frozen foods through sublimation. In freeze-drying, the pressure-­ temperature relation is a crucial factor to maintain the physical state of water. The pressure in the freeze-drying maintains in such a fashion that it keeps the sublimating ice pressure as well as the pressure required for the direct transformation of

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solid ice to water vapor below the triple point of water. However, Fig. 6.3 shows Phase changes of water. The first stage of the freeze-drying process starts with the quick freezing of raw materials. In this stage, the raw materials need to keep at −18 °C necessarily after harvesting to ensure that there is no change in the structure of raw materials. The second stage of freeze drying acompasses with the removal of free water from the raw products through air drying. This process is termed as sublimation where the products are kept in the vacuum chamber and the frozen water in the products is drained out in the form steam under very low pressure condition. The heat energy require to carry out the sublimation process can be obtained either from the temperature gradient between vacuum chamber and products or using integrated heating chamber as well. To ease the flow of water vapour towards the condenser can be ensured by maintaining the drying chamber as well as the condenser area in vacuum condition. After passing the condenser area, steam of water accumulated as ice. Moreover, the vacuum maintained sufficiently below the sublimation temperature and pressure whereas the freeze drying commences from 35 °C. After the migration of free water followed by the sublimation process, the remaining water content is bound water which imparts products quality and stability with further processing. Further, complete sublimation of the primary ice, the rest of the water molecules remain bound to the product and urges secondary drying for thorough removal. However, a sudden increase in the product temperature can be observed after removing the free ice. Secondary drying takes place immediately after the primary drying, moreover in the primary phase at an elevated temperature between 30  °C and 50  °C.  Secondary drying depends on the product temperature and proceeds at a faster rate with subsequent increase in the system

Fig. 6.3  Phase changes of water

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vacuum level. In case of amorphous products, a sharp increase in the temperature profile from the primary to secondary drying may cause rupture or collapse. In addition, the extent of secondary drying depends on the storage condition and in most of the cases for long-term preservation, the secondary drying continues up to a moisture content of 0.5–3%. 6.3.2.6  Spray Drying Spray drying (SD) is the process of converting the feed fluid flow into the dry particulate form in the presence of a hot medium. In case of hot air spray, the drying temperature can be kept between 150 °C and 200 °C whereas the final shape of the product depends in the physical and chemical characteristics of the feed flow and can be obtained in the shape of granules, powder or agglomerates together [76]. Figure 6.4 represents the spray drying process which consists mainly of the following three steps: the atomization of the feed sample, liquid samples drying and recovery of powder [107]. In the first step, the liquid is fed to the drying chamber after atomization through an atomizer where the liquid atomizes into tiny fine droplets with the maximum exposed surface area for efficient drying. Hence, the quality of final products depends largely on the performance and design of atomizer [96].

Fig. 6.4  Typical spray drying setup [107]

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After then the atomized droplets passed through the drying chamber where the atomized particles mixed up with the hot air in a co-current or counter or in a mixed flow manner at a temperature between 150 and 200 °C. Meanwhile, moisture evaporation occurs at the surface of the droplets and a dry layer forms on the surface. This process continues until the tiny droplets reach the critical moisture content level and form powders of spherical and oval shape with smooth or rough surface depending on the atomizing as well as drying conditions [97]. 6.3.2.7  Electrohydrodynamic Drying Electrohydrodynamic (EHD) drying is mainly a nonthermal food processing technology used for the drying of heat sensitive materials and getting much more attention in the recent time because the processed foods possesses high quality and the processing involves less energy consumption [108]. In EHD process, high velocity air flow at an elevated temperature is used to enhance the mas transfer by rupturing the boundary layer upon which the food product is kept. The electric or corona wind develops the ambient condition under the application of high voltage on an electrode having small radius of curvature [109]. EHD has some advantageous features over the traditional drying techniques in terms of ambient pressure and temperature use for the food processing along with the simple design without any movable parts as well as no adhesion wear [110]. The effectiveness of the EHD largely influenced by the formation of corona wind under the application of high voltage electric field. This electric field imparts accelerating force to the gaseous ions to convey momentum to the noncharged molecules through the ionic collision which results in ionic wind. This well form ion drag sensations has its effect on the evaporation process by impinging on the food materials after rupturing the boundary layer. Moreover, the ionic flow sets the water molecules in line with the electric field direction that consequently reduced the system entropy and hence the temperature of the food materials under processing [111, 112].

6.3.3  Hybrid Drying Individual drying process sometimes not as effective as expected in bound water removal. Longer drying time, high cost and poor quality may be associated in case of executing individual drying in bound water removal. The combination of the individual drying outcomes represents the hybrid drying technology as a better option for food processing. Different individual processes offer distinct benefits over other unit operation. Combination of two or more drying systems allows effective moisture migration from food materials than individual drying system. The combination depends on the types of food materials, final application and quality

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expectation of the final product. In hybrid drying, all of the individual processes may be applied simultaneously or intermittently depending on the nature of food materials and expected quality of food materials [113]. 6.3.3.1  Hot Air-Freeze Drying The combined application of the hot air along with the freeze drying offers some advantageous feature on the quality improvement and shelf life of the biological heat sensitive materials [114, 115]. With the rapid microwave drying, foods aroma and texture experiences a sharp degradation in association with the high cost of processing. Experimental comparison between the combination of hot air-freeze drying with the freeze drying alone for carrot and pumpkin showed that the dehydrated foods has same quality for the two processes while the combined process consumes 50% less energy than the freeze drying [116]. Former research strongly support that, a pre convective drying before microwave drying will help to minimize the drying time, cost, energy and keep the food quality as it is [81, 117]. For instance, applying hot air at a temperature of 60–70 °C along with the microwaves at 0.4 W/g in case of garlic drying reduces the drying time almost 80% with a better quality assurance [118, 119]. 6.3.3.2  Microwave-Assisted Freeze Drying Microwave-assisted freeze drying or MAFD getting emphasized for the dehydration of the biological food materials and sea foods for its shorter processing time with minimum energy consumption and also provides improved product quality [120, 121]. Low temperature and oxygen free drying process in addition to the superior protection against the physical and chemical properties, composition, structure for the preservation of biological heat sensitive, fruits, vegetables and pharmaceuticals materials makes FD a promising food processing technique [122]. However, the low dehydration rate in consort with the high installation cost of refrigeration units restricts the use of FD in the large scale industrial applications and found mostly suited for the small scale domestic, laboratory and pharmaceuticals plants [123, 124]. Moreover, previous experimental results showed that the, MFD offers better options to reduce drying time and to retain the internal quality of the foods with lower energy consumption [125]. Most of the research concludes that, MFD reduces 50–75% of the drying time compared to the conventional FD systems and in case of sea cucumber the drying time reduces to almost 50% than the FD whereas the food quality remains almost same in both cases [122]. However, the control of MAFD is relatively complex. The melting of the ice which forms at the different local positions in foods matrix offers thermal runway as the dielectric factor of ice is higher than water and offers uneven heating [126].

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6.3.3.3  Microwave-Assisted Vacuum Drying Microwave energy insists the moisture migration by increasing the interior pressure of the food products by the continuous absorption of heat energy at the outer surface. In addition, the evaporation temperature of any materials decreases if it maintains at a temperature below the atmospheric pressure. These two principles are adopted for the microwave assisted vacuum drying or in the MAVD process. In this system, the vacuum chamber maintains at a lower pressure than the atmospheric that consequently decreased the evaporation temperature of the product placed in the drying chamber and require sufficiently less temperature to assist the microwave drying [127]. This microwave assisted vacuum drying (MAVD) or microwave vacuum drying (MVD) combines the most attractive features of microwave drying such as high drying rate, easiness of controlling, good quality product with the less energy requirement by maintain negative pressure and lower boiling point for the products being processed [84]. The energy efficient MAVD techniques reduce the chances to form spot on the surface of the products after drying due to excess heating [128]. In addition, the expansion ratio and the texture of the dried food noticeably increased which causes puffing in the fruits and vegetables after dehydration [129]. Investigation has been carried out to compare among the MAVD, FD, hot air freeze drying and MAFD in terms of quality parameters vitamin C, moisture and chlorophyll contents, surface texture, aroma, shrinkage tendency, color and rehydration capacity. After freeze drying, best quality dehydrated products can be obtained whereas hot air assisted products loose the quality at higher rate than MAVD. Also the subsequent energy requirement is lower in case of MAVD compared to FD or hot air drying [129, 130]. The product inherent quality remain intact in case of banana slice drying through the MAVD [131] and a sorer with softer texture have been observed during drying of the cranberries with MAVD operated with both in pulse and continuous modes [132]. The same results for drying cranberries have been observed with relatively lower energy consumption and higher drying performance when compared to the microwave assisted convective drying [133]. A comprehensive comparison have been carried out between the MAVD and FD by assessing the food quality, color, texture, density of the dried carrot sample. The food sample processed in these two processes illustrates almost similar quality in case of rehydration potentials, softer texture, color, appearance whereas MAVD yields product with less density than the FD [134]. Although, the MACVD processed biological products possesses almost similar food quality parameters compare to the FD products but the drying rate is around 20–30% higher in case of MAVD process with much lower energy consumption [135–137]. 6.3.3.4  Microwave Convective Drying Microwave heating causes the center of the product sample to be heated at a higher temperature than the outer ridge. So, that a temperature gradient exists in the sample which acts as a driving force to remove water content from the center to the

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atmosphere through food matrix [88]. It is clear from the drying kinetics of the microwave heating that, most of the drying mechanism occurs at the falling rate period [138]. Hence, microwave drying becomes convenient at the last stage of drying and a combination of initial convective drying followed by the microwave drying at the last stage will accelerate the drying rate and improve the product quality [88, 138, 139]. Moreover, the addition of microwave energy at the final stage of drying has an efficient action on the bound water removal as the bound water is the last water content exists in the food matrix [140]. In addition to the bound water removal microwave energy significantly effects the free water removal as the polarized ions of water molecules vibrates and collides with each other due to the electromagnetic field set up by microwave energy and accelerates the water removal rate [141]. The previous researches on the carrot sample through microwave drying showed a pretty quality improvement in the colour, rehydration and shrinkage factors [142]. An experimental study of the quality assessment of kiwifruits sample processed with the microwave, hot air and microwave assisted-hot air drying process showed that both of the microwave heating and microwave assisted hot air heating increased the drying rate and reduced drying time [115]. Also less shrinkage was taken place for the microwave assisted-hot air drying compared to the other two processes [143]. Dehydration of raisins through microwave heating yield better results in the food quality, shrinkage, crystalized sugar and density whereas most of the literature strongly supports the application of microwave heating at the falling rate period from the economic point of view [95, 144]. Also the drying rate in microwave assisted hot air drying increase upto five times compare to the hot air drying and reduce almost 80% of the corresponding drying time [145]. Microwave energy can combine with the hot air drying by following any of the three methods explain below [146]: 1. Microwave energy can be applied at the beginning of the drying process which heats up the inner to the evaporation temperature and causes the vapour to escape though the outer surface. Better drying rate and formation of porous structure can be found during drying to convey moisture transport. 2. At the beginning of the falling rate period or at the early stage of falling drying rate, the moisture accumulated in the inner core whereas the surface remains dry because the rate of evaporation is high compare to the moisture supply rate at the surface. With the application of microwave energy, simultaneous increase in the internal heat as well as the vapour pressure happens which assists the moisture removal from the surface. 3. At the last stage of drying or the falling rate period when the moisture content is below 20%, microwave energy can be applied to finish the drying process. The drying rate and products quality sharply degraded with the hot air drying at the final stage whereas microwave energy allows easier way to remove bound water [142, 147]. However, hot air drying becomes supportive at high moisture content and reduce the drying cost almost 30% compare to the microwave drying [140]. Moreover, drying with hot air restricts the diffusion by shrinking the cells and

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reduce the drying rate at the last stage of drying. The application of microwave energy at the final stage increases the inner vapour pressure that promotes the diffusion process and carryout the drying process with improve product quality [135, 148]. 6.3.3.5  Microwave–Vacuum Combined with Infrared Drying The combination of microwave energy and infrared energy for the drying of biological foods in the vacuum chamber emerges as a new food processing technology. The vacuum chamber lowers the boiling point of the foods and the application of microwave shorten the drying time and improves the drying rate by increasing internal temperature. While the infrared heating changes the molecular momentum and positions of atoms by applying the infrared energy through a lamp and significantly increase the evaporation rate from the outer surface [149]. Combination of infrared heating with the other drying methods like microwave heating, hot air drying getting much importance in the agro based food preservation due to the improve product quality, shorter drying time, high thermal efficiency, reduce the risk of appearance and colour change, less [150–152]. Microwave heating has a promising impact on the rehydration of foods at falling rate period by creating puffing in the food sample. The temperature gradient between the core and outer surface helps rapid rehydration at the final stage [153]. This rehydration ratio has a linear relationship with the applied infrared heat energy and the application of 100 W and 200 W infrared energy causes an increase in the rehydration ratio from 1 to 10% and 2 to 20% respectively [152]. Higher infrared substantially increase the surface temperature and drying rate. When the microwave vacuum drying applies with the infrared energy, the colour of dried mushrooms appears lighter when applying 200  W as compare to the 100  W surface heating. Similar effects can be observed in case of apple slice heating where the enzymatic browning reaction reduced at high infrared energy [154].

6.3.4  Pre-treatment There are several water removing process including ultrasound waves, osmotic dehydration and blanching are deployed in food processing as pre-treatment. Some selected pre-treatment has been discussed in the following section. 6.3.4.1  Osmotic Dehydration Osmotic dehydration is one of the zero energy required drying techniques. The driving force of osmotic drying is the difference of concentration of water between the water sample and osmotic solution [101–103, 155–158]. When a solid product with

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high content is submerged in a concentrated aqueous solution of sugar or salt, osmotic dehydration takes place. During this process, mass transfer follows three different pathways. Water transfer from product to solution whereas solute transfer from the solution to the product. In addition to these, some of other components of the product leach out from the sample [159]. Loose water in food materials can be displaced by osmosis. However, bound water, especially in biological tissue, cannot be taken away using osmotic drying [160]. As relatively free water migrates from the food sample to the solution during osmotic drying, this process generally associated simultaneous transfer of water and solutes [108]. Therefore, this process is widely deployed to attain intermediate dried product or used as pre-treatment of different types of the food process. 6.3.4.2  Blanching Thermal blanching is mainly carried out prior to food processing including freezing and canning. Blanching is mainly applied to fruits and vegetables for increasing tissue softening Wang et  al. [109]. Moreover, blanching significantly enhance cell membrane permeability [110–112]. Generally, long chain with several large branch chains structure prevail abundantly in cell wall of plant-based food materials [114, 161]. These prolonged chain structure of cell wall hinders water migration significantly. Blanching can change the size of the long chain components into smaller chains with small branch chain. Eventually, the cell wall become more permeable for the migration of bound water (intracellular water) and drying rate increased significantly [109, 111, 115–117]. 6.3.4.3  Ultrasound Pre-treatment Application of ultrasonic wave in removing water from food material is a relatively new approach. Ultrasonic waves causes series of alternative expansion and compressions within the solid structural of food materials. This resultant of this force subsequent compression and expansion is pretty higher than adhesive force works between water and other solid molecules. Eventually, water migrates from food materials through mico-channel during ultra sound drying. As ultrasound waves effectively produce large enough cavities that allows migration of bound water from enclosure [109, 110]. Frequency of the ultrasound significantly affect the rate of compression and expansion. Ultrasound basically increase the effective moisture diffusivity of the food sample by modifying structure substantially. For instance, applying ultrasound wave prior to convection drying result in 10–70% increase in effective moisture diffusivity. Ultrasonic pre-treatment is suitable for food with high amount of bound water such as fruits and vegetables [162]. Several pathways are developed in food materials to enhance the bound water transport during ultrasound process as given follows [162]:

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(a) Reducing diffusion boundary layer due to the turbulence developed during alternative compression and expansion. This thinner boundary layer leads to significant increase in force convective mass transfer during drying in the sample [119]. (b) Production of several cavitation in the enclosure surface such as cell wall. Eventually, the cavitation produced by ultrasound allows the migration of relatively bound water in food. To put it briefly, food processes described in the above sections have their own distinctive features with respect to the removal of bound for any specific food materials. Therefore, the selection of any methods to eliminate bound water from food should be based on the nature of foods, processing energy requirement, quality of final product and processing time.

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95. T. Tulasidas, Combined convective and microwave drying of grapes. Dry. Technol. 13(4), 1029–1031 (1995) 96. N. Phisut, Spray drying technique of fruit juice powder: some factors influencing the properties of product. Int. Food Res. J. 19(4), 1297–1306 (2012) 97. O.A. Caparino, J. Tang, C.I. Nindo, S.S. Sablani, J.R. Powers, J.K. Fellman, Effect of drying methods on the physical properties and microstructures of mango (Philippine “Carabao”var.) powder. J. Food Eng. 111(1), 135–148 (2012) 98. D. Nowak, P.P. Lewicki, Quality of infrared dried apple slices. Dry. Technol. 23(4), 831–846 (2005) 99. R. Khir, Z. Pan, A. Salim, B.R. Hartsough, S. Mohamed, Moisture diffusivity of rough rice under infrared radiation drying. LWT-Food Sci. Technol. 44(4), 1126–1132 (2011) 100. A.R. Celma, S. Rojas, F. Lopez-Rodriguez, Mathematical modelling of thin-layer infrared drying of wet olive husk. Chem. Eng. Process. Process Intensif. 47(9–10), 1810–1818 (2008) 101. A.B. Jemai, E. Vorobiev, Enhanced leaching from sugar beet cossettes by pulsed electric field. J. Food Eng. 59(4), 405–412 (2003) 102. L. Khezami, A.B. Jemai, R. Capart, E. Vorobiev, Drying kinetics study of food pulps by continuous relative humidity measurements: air flowrate and electric field effects. Chem. Technol. 5(1), 45–50 (2010) 103. P.P. Lewicki, Design of hot air drying for better foods. Trends Food Sci. Technol. 17(4), 153–163 (2006) 104. M. Zhang, H. Jiang, R.-X. Lim, Recent developments in microwave-assisted drying of vegetables, fruits, and aquatic products—drying kinetics and quality considerations. Dry. Technol. 28(11), 1307–1316 (2010) 105. A.K.S. Chauhan, A.K. Srivastava, Optimizing drying conditions for vacuum-assisted microwave drying of green peas (Pisum sativum L.). Dry. Technol. 27(6), 761–769 (2009) 106. V.S. Kishan Kumar, N.K. Upreti, S. Gupta, Scope of vacuum press drying for fast removal of moisture below fiber saturation point. Dry. Technol. 34(10), 1204–1209 (2016) 107. M.R.I. Shishir, W. Chen, Trends of spray drying: a critical review on drying of fruit and vegetable juices. Trends Food Sci. Technol. 65, 49–67 (2017) 108. A.W. Galston, R.K. Sawhney, Polyamines in plant physiology. Plant Physiol. 94(2), 406–410 (1990) 109. J. Wang et al., Effect of high-humidity hot air impingement blanching (HHAIB) on drying and quality of red pepper (Capsicum annuum L.). Food Chem. 220, 145–152 (2017) 110. J.-W. Bai, D.-W. Sun, H.-W. Xiao, A.S. Mujumdar, Z.-J. Gao, Novel high-humidity hot air impingement blanching (HHAIB) pretreatment enhances drying kinetics and color attributes of seedless grapes. Innov. Food Sci. Emerg. Technol. 20, 230–237 (2013) 111. J. Wang et al., Effects of various blanching methods on weight loss, enzymes inactivation, phytochemical contents, antioxidant capacity, ultrastructure and drying kinetics of red bell pepper (Capsicum annuum L.). LWT-Food Sci. Technol. 77, 337–347 (2017) 112. M. Blasco, J.V. García-Pérez, J. Bon, J.E. Carreres, A. Mulet, Effect of blanching and air flow rate on turmeric drying. Food Sci. Technol. Int. 12(4), 315–323 (2006) 113. C. Kumar, M.A. Karim, M.U.H. Joardder, Intermittent drying of food products: a critical review. J. Food Eng. 121, 48–57 (2014) 114. M.F. Basanta, M.F. de Escalada Plá, C.A. Stortz, A.M. Rojas, Chemical and functional properties of cell wall polymers from two cherry varieties at two developmental stages. Carbohydr. Polym. 92(1), 830–841 (2013) 115. F. Xu, X. Jin, L. Zhang, X.D. Chen, Investigation on water status and distribution in broccoli and the effects of drying on water status using NMR and MRI methods. Food Res. Int. 96, 191–197 (2017) 116. G. Bingol, B. Wang, A. Zhang, Z. Pan, T.H. McHugh, Comparison of water and infrared blanching methods for processing performance and final product quality of French fries. J. Food Eng. 121, 135–142 (2014) 117. L.M. Ruiz-Ojeda, F.J. Peñas, Comparison study of conventional hot-water and microwave blanching on quality of green beans. Innov. Food Sci. Emerg. Technol. 20, 191–197 (2013)

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144. A.V. Mahn, P. Antoine, A. Reyes, Optimization of drying kinetics and quality parameters of broccoli florets. Int. J. Food Eng. 7(2) (2011) 145. Saguy IS, Marabi A., Wallach R, Water imbibition in dry porous foods, in Proceedings of the 9th International Conference on Engineering & Food, Montpellier, France. 146. S.J. Kowalski, D. Mierzwa, Hybrid drying of red bell pepper: energy and quality issues. Dry. Technol. 29(10), 1195–1203 (2011) 147. A.A. Adedeji, G. Tanya, M.O. Ngadi, R. GSV, Effect of pretreatments on drying characteristics of Okra. Dry. Technol. 26, 1251–1256 (2008) 148. M. Zhang et al., Trends in microwave-related drying of fruits and vegetables. Trends Food Sci. Technol. 17(10), 524–534 (2006) 149. J. Yongsawatdigul, S. Gunasekaran, Microwave vacuum drying of cranberries, part I: energy use and efficiency. J. Food Process. Preserv. 20(1), 121–143 (1996) 150. A.M. Goula, K.G. Adamopoulos, Retention of ascorbic acid during drying of tomato halves and tomato pulp. Dry. Technol. 24(1), 57–64 (2006) 151. P.H.M. Marfil, E.M. Santos, V.R.N. Telis, Ascorbic acid degradation kinetics in tomatoes at different drying conditions. Lwt-Food Sci. Technol. 41(9), 1642–1647 (2008) 152. B. Zanoni, C. Peri, R. Nani, V. Lavelli, Oxidative heat damage of tomato halves as affected by drying. Food Res. Int. 31, 395–401 (1999) 153. V. Lavelli, S. Hippeli, C. Peri, E.F. Elstner, Evaluation of radical scavenging activity of fresh and air-dried tomatoes by three model reactions. J. Agric. Food Chem. 47, 3826–3831 (1999) 154. N.S. Kerkhofs, C.E. Lister, G.P. Savage, Change in colour and antioxidant content of tomato cultivars following forced-air drying. Plant Foods Hum. Nutr. 60, 117–121 (2005) 155. M.M. Khin, W. Zhou, C.O. Perera, Impact of process conditions and coatings on the dehydration efficiency and cellular structure of apple tissue during osmotic dehydration. J. Food Eng. 79(3), 817–827 (2007) 156. M.L.M. Lopes, V.L. Valente Mesquita, A.C.N. Chiaradia, A.A.R. Fernandes, P. Fernandes, High hydrostatic pressure processing of tropical fruits. Ann. N. Y. Acad. Sci. 1189(1), 6–15 (2010) 157. G.E. Lombard, J.C. Oliveira, P. Fito, A. Andrés, Osmotic dehydration of pineapple as a pretreatment for further drying. J. Food Eng. 85(2), 277–284 (2008) 158. L.F. Machado, R.N. Pereira, R.C. Martins, J.A. Teixeira, A.A. Vicente, Moderate electric fields can inactivate Escherichia coli at room temperature. J. Food Eng. 96(4), 520–527 (2010) 159. A.-L. Raoult-Wack, Recent advances in the osmotic dehydration of foods. Trends Food Sci. Technol. 5(8), 255–260 (1994) 160. E.A. Disalvo et al., Structural and functional properties of hydration and confined water in membrane interfaces. Biochim. Biophys. Acta (BBA) Biomembr. 1778(12), 2655–2670 (2008) 161. H. Liu, F. Chen, S. Lai, J. Tao, H. Yang, Z. Jiao, Effects of calcium treatment and low temperature storage on cell wall polysaccharide nanostructures and quality of postharvest apricot (Prunus armeniaca). Food Chem. 225, 87–97 (2017) 162. G. Musielak, D. Mierzwa, J. Kroehnke, Food drying enhancement by ultrasound – a review. Trends Food Sci. Technol. 56, 126–141 (2016)

Chapter 7

Significance of Bound Water Measurement

7.1  Introduction Status of water is important over the course of harvesting, processing and storage of food materials. Fraction of bound water dominates in several key phenomenon including ripening, microorganism growth and energy requirement in whole food processing [1]. Therefore, fraction of bound water is not only vital for processed food rather it is equally important for fresh food during storage [2]. However, the scope of the book allow the discussion on the significance of bound water during food processing. Therefore, in the following section an extensive discussion on the significance of bound water removal has been discussed. Water content in the form of bound water and free water plays a significant role during the water removing process including drying and frying. Bound water is captured by the solid matrix of water in the insoluble compounds in the fruit cells and the migration of bound water urges higher energy, once all the free water is migrated, whereas free water gives support to the chemical reactions, metabolism activities and acts as a medium for internal transport. Moreover, product quality is negatively affected by the high temperature drying process over the course of moisture removal from the food product [3]. This process causes changes in the food quality like discolouring, aroma loss, textural changes, nutritive value, and physical properties significantly [4]. Drying conditions and time have a great effect on these quality changes of dried product. Mild drying conditions with lower temperature can improve the product quality but increase the drying time, whereas severe drying conditions with higher temperature reduces drying time and provides low quality dried food. Therefore, quality, energy consumption, processing time and cost need to be optimised carefully [5]. Bound water removal contributes in all of the aspects including quality, energy consumption, processing time, cost and even environmental pollution. There is no straight forward answer whether removal of bound water is beneficial, since removal of bound water needs more energy, time, and cost along with promising higher © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_7

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s­ tability. Moreover, quality in different aspects is affected negatively with the migration of bound water. In the following section, a critical discussion on the significance of bound water removal has been discussed in details.

7.2  Energy and Time Water migration from food materials requires a significant amount of energy. The energy mainly provided in the form of heat with maintaining different processing conditions. For example, approximately 1.5 × 106 kJ/t energy is required to remove water if the drying temperature is 50 °C [6]. However, the required energy varies with the variation of food sample and process conditions [7–9]. Moisture removing processes such as drying and frying are very complex phenomenon involving simultaneous heat and mass transfer along with changing physical and chemical aspects of the material. Figure 7.1 shows a typical drying curve of hygroscopic food materials. Constant drying rate is observed at the first stage removing the surface free water from the food sample [11]. After this period, loosely bound and capillary water diffuses from the inside of the material to the surface. Here the drying rate is mainly controlled by internal parameters such as size, shape, and collapse of internal tissues. Heat damage may occur at the surface in this stage due to lack of moisture content at the surface. Following this, concentration gradients between the deeper parts and the surface causes the movement of moisture through the solid matrix [12, 13]. At the end of the drying, only bound water needs to be removed. This falling stage takes significantly higher energy to the low thermal conductivity of solid matrix of food materials [14]. Moreover, a strong bond of water with the adjacent food matrix requires more energy to evaporate towards surrounding. Removal of water from food materials needs a substantial amount of energy depending on the nature of food samples and types of food processes. Removal of final 10% of Fig. 7.1  Typical drying rate curve at constant drying conditions [10] Drying Rate

First stage of drying Second stage of drying

Third stage of drying

Time

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­ oisture content takes almost equal energy that is necessary to remove first 90% of m water from the food materials [15]. The amount of energy for the bound water varies due to the diversification of components and structure of foods as shown in Table 7.1. From the Table 7.1, it is clearly depicted that different amount of energy and time are required to dry the food sample to dry properly during solar drying [6]. This is mainly due to the distribution of water in food materials. For instance, potato takes significantly higher drying time than foods like apple. From the drying curve, it is clearly depicted that much more time is required to remove bound water in comparison with free water removal time [20]. For example, removing even loosely bound water (close to one third of total water) in fruits and vegetables completely takes two third of total drying time. For this very reason, foods with significant amount of bound water take relatively higher drying time from several hours to days in a moderate processing conditions. On the other hand, foods with less bound water can be completely dried within couple of hours. For instance, leafy vegetables including spinach, and mint leaves can be dried within an hour or two [21]. Therefore, bound water removal is very energy intensive part of food processing. Evaporation of bound water increases with the intensity of binding energy of water with other components. As different types of food comprise different levels of bound water, the energy requirement for removal varies significantly [22]. For example, drying of grain having lower than moisture content below 12% db is due to higher intra-particle resistance to moisture migration [13, 14]. Drying can be categorized as an energy intensive industrial processes that accounts for approximately 15% of all industrial energy usage [10, 15]. Essentially, an increasing energy efficiency by as less as 1% in drying, turn in, as much as 10% increase in profit [23]. Therefore, improvement in energy efficiency even to some extend in food drying process will result in a sustainable development to global energy. As stated earlier that industrial dryers consume about 15% of all energy used in food manufacturing, energy efficiency is vital in removing bound water. Energy Table 7.1  Specific energy consumption of different foods [16–19] Final Initial moisture Approximate moisture energy required Required drying content, content, Food (MJ/kg) temperature (°C) (%wb) (%wb) material Apple 80–85 20–24 23–50 45–70 Banana 70–80 7–15 16.79 45 Cassava 62–75 7–17 23–62 30–60 Pepper 75–80 5–14 16.1 40 Mango 80–85 12–18 15.64 55 Potato 70–75 8–13 14.53 50–70 Azarole 65–70 8–9 27–42 60–70 Garlic 65 15–20 85–90 70 Berberis 60–65 10 67–80 50–70

Required drying Time (h) 8–10 8–10 7–10 5–6 10–15 10–13 50 9–10 6–8

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optimization and unit process design have been extensively discussed in the literature [21–34]. It is worthy to mention that cost and time required in removing almost propositional to the energy requirement. Higher time and cost are associated with removing bound water in conventional drying means higher energy needs to accomplish the process [35]. Moreover, Greenhouse Gas emission should take into consideration prior to drying an additional 1% of bound water.

7.3  Quality Food processing and preservation focuses mainly on the microbial, physical and nutritional quality stability. However, the quality varies with moisture content significantly during processing. Effect of bound water removal on different quality aspects has been discussed in details in the following section.

7.3.1  Structure Structural changes are indispensable due to the moisture removal from the food materials. Regardless of the variation of the food processing, changes in structure take place to a varied extent. Porosity increases with the removal of water in most of the plant and animal-based food materials. Whereas, considerable shrinkage of the food sample also increases with the removed water volume in many cases. These two opposite physical changes determine different patterns of structural evolution. The domination of the contradictory these phenomena depends on the food composition and structural matrix. Taking both contradictory phenomena into consideration, it is really difficult to predict what type of structural changes will take place during water removal. On the other hand, manipulating of the food microstructure may ensure the chemical and microbiological Stability. The expected internal food structure can be attained using different food processes such as freezing, different types of drying [36–38]. Water confined in differently sized food matrix shows disparate behaviour. In other words, water behaviour in larger diameters is not same as the one in nano-­pores [39].

7.3.2  Texture The quantity and distribution of water possess by the foodstuff has their influences in the rheological properties in the form of viscoelastic and textural deformation. Water content significantly affects the textural properties of food materials. Low

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water content leads hard, crisp or crunchy products, whereas a product is found soft, juicy, moist, tender and chewy at intermediate to high water content as shown in Fig. 7.2. If the same amount of energy supplied throughout the drying, the surface undergoes damage as there is no sufficient moisture at the surface to be removed in the falling rate drying period. In this stage of drying, case hardening may occur on the surface although the water content of at the centre is still high. The very early stage contribution of the food preservation techniques aims to keep the texture of the food materials intact by giving much more importance to crispness. The texture, as well as crispness, makes an appeal to the consumers to its consumers through its sharp crushing sounds [41]. However, these crispy foods have their water content below 10% and aw below 0.55 which keeps them brittle in nature. In addition, Fig. 7.3 shows the effect of water activity on sensory crispness of food material.

7.3.3  Collapse Migration of free and bound water from the cells contributes to the pore formation and shrinkage over the course of food processing [16]. The structural modification of the tissues depends on the bound and cell water content at different stages of drying rather than the bulk water content. Figure 7.4 illustrates a hypothesis of different steps of pore formation depending on the cells microstructure, composition, and moisture migration phenomenon. 1. Drying starts with the migration of intercellular or free water content that cannot make any significant deformation of the sample

Fig. 7.2  Water and mechanical properties [40]

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Fig. 7.3  Effect of water activity on sensory crispness score (on a 1 –to-10 scale) determined at first bite for extruded maize-based puffed snacks with (°) 0% and (•) 20% sucrose (% drying basis) [42]

No shrinkage

Intercellular water Cellular shrinkage

Air Material Properties

Water Solid

Structural changes

Moisture content

Moisture transport

Pore space shrinkage

Intracellular water

Cell collapse Pore collapse

Heat transfer

Cell wall water

Overall tissue shrinkage

Processing time

Fig. 7.4  Structural changes during the time of drying [43]

2. Falling rate stage takes place immediately after the migration of intercellular water content. At this stage, intracellular water content begins to migrate with the progression of the drying process and the amount of water removed can be taken as the volumetric shrinkage of the cells. 3. Lower turgor pressure is obtained after migration of a considerable amount of intercellular water and consequently, cell shrinkage is observed. The water density and particle density affects the void formation and the porosity vary linearly with the water migration rate [44]. 4. The tendency of cell collapse mostly depends on the drying kinetics and water migration rate, whereas the gradual cell shrinkage with the drying process has a little effect. After analysing the plant cell microstructure, it is found that the thin walled parenchyma tissues covered maximum volume in the foods with vacuoles in the cell wall to store water. Due to the vascular solutes, cell membrane exists close to the cell wall that gives rises the turgor pressure to maintain the cell

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shape, elasticity, and crispness of the tissues [45]. Turgor pressure falls with the advancement of water migration; hence the cells collapse and the turgidity cannot be restored after that [46]. Moreover, the cell membrane collapse behaviour can be attributed to the migration of bound water from the cell with the passage of time [47]. To illustrate this fact, a curve is drawn and divided into four respective zones depending on the free water (FW) and bound water (BW) content. At the early stage of consideration, it is assumed that the sample contains 100% moisture and undergoes the drying process. The maximum point corresponds to each of the zones represents the cell collapse point. The drying process at the Zone I of Fig. 7.5 has been observed for 100 min time period where the BW percentage remain 80–85% for the first 55 min and the reduces to 50–55% with the passage of time and increasing the surface temperature of the object. After 55 min of drying, the surface temperature goes up to 53–54 °C and most of the literature reported that the cell membrane starts to collapse around 50 °C with a simultaneous effect of the pressure and temperature gradients [1, 21]. The cell membrane next to the drying sample is affected by the gradual thermal stresses and starts the rapturing phase that penetrates the intracellular spaces to mix the BW in the intercellular space with the FW as shown in Fig. 7.5. With the continuation of this process, FW content has a tendency to migrate at the upper surface and the quantity of BW increases as compared to the FW in the intercellular space. However, the moisture transportation takes place due to the internal mass diffusion and a decreasing trend of moisture content has been observed up to 170 min of the drying process. At the same time, the heat flows from the surface at a temperature of 56–57 °C to the core of the object at the maximum temperature of 58 °C. At the end of the Zone II, the temperature of the surface increases up

Fig. 7.5  Cell collapse with water migration [47]

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to 2 °C higher than the core which again initiates the cell rupture and BW squeeze from the cell to fuse with the FW [47].

7.3.4  Chemical Reactions Water presents as the vital constitutes of any food materials and controls most of the water driven chemical reactions as well as the food stability. Food chemical property directly varies with the moisture content and water activity as shown in Fig. 7.6. However, dielectric properties and molecular mobility influence some of the reactions and physical state along with the water content. Acting as the parameter to describe physical stability, water content controls the reaction rate and reaction type depending on the appropriate moisture content. Moisture content has a dominating role in the chemical reaction and reaction rate determination by reacting water with the solvent to dissolute, diffuse and react in a controlled way. 7.3.4.1  Vitamin Vitamins are generally heat sensitive and diluted easily in water. Water content in food materials significantly affects the retention of vitamins. During the process of water removal leads to degradation of vitamins [48]. Moreover, migration of water

Fig. 7.6  Variation of stability with the water activity of food

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gives rise to the access oxygen in the food sample which account for oxidation of the vitamins. Consequently, further degradation of vitamins occur [49–51]. In case of ascorbic acid stability in foodstuff with constant aw, the moisture content increases when the system is prepared through dehydration rather than humidification. In dehydration process, more stable ascorbic acid properties can be attained due to less moisture content available that offers viscous medium to react with the solvent, whereas the humidification gives a rise to the amount of moisture for the reaction which dilutes the solvent and pulls down the reaction rate. Most of the reaction rate decreases with a decrease of water activity. This correlation better fits than the amount of moisture content present in foods sample [52–56]. Moreover, internal temperature of food materials substantially increased at the latter stages of drying along with developing porous structure. These phenomena remarkably decrease the retention of many types of vitamins [57, 58]. Therefore, bound water removal negatively affects the ascorbic acid retention. Essentially, bound water removal is not favourable for vitamin retention. 7.3.4.2  Maillard Reaction Millard reaction can be characterized as bimolecular reaction where the concentration and the molecular mobility of the participating reactants possess vital roles on the reaction rate. In case of brown pigment formation by Millard reaction, aw controls the reaction rate upto a maximum level and further increase of aw after the maximum value, the reaction rate decreases [59–63]. During absorption of the moisture by the reactants in the dry system, the reactants mobility and dissolution of the reactants increases up to a critical moisture content. Further, an increase of the water activity beyond this level causes dilution of the reactants concentration and a decrease of the reaction rate by limiting the availability of mobile water for the mobility of the system. The maximum browning rate for most of the fruits varies from an aw of 0.65–0.75 n where the critical condition observed at an aw of 0.3 [64]. Moreover, lower temperature, water content, heating time, pH level are important process parameters for controlling Millard reaction and visual aspects of the foods [65]. 7.3.4.3  Phenol Phenolic compounds are the important nutrients for human. All foods including fruits, vegetables, content to some extend of these [66]. Phenolic compounds is distributed unevenly in the same way of water with free and bound forms [67]. In the raw foods like fruits and vegetables, the bound phenolic compounds generally not estimated as these are not at the released form. Food process that associates to the breakdown of cellular constituent might accelerate the release of phenolic compounds [68]. For instance, drying at high temperature (150 °C for 40 min) causes significant release of bound phenols in citrus peels [69]. Similar to the process that

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remove bound water effectively also increase the possibility of releasing phenolic compounds. For example, Hayat et al. found that bound phenolic compounds significantly released during appropriate microwave treatment [70, 71]. Therefore, it can be assumed that the condition that allows removal of bound water may increase the chance of releasing bound phenolic compounds of food materials. 7.3.4.4  Lipid Oxidation (Aroma) Flavour of the food materials is considered as one the most important parameter to define food quality. More than 200 volatile aroma components are present in the fruits among which a single group is responsible for making a particular smell. These components are highly volatile in the presence of moisture and readily get evaporated during processing. However, lipid oxidates vary inversely with the aw and reach a minimum value near the monolayer and the reaction rate increases with the additional moisture absorption above this layer [72–74]. As the water content is absorbed from the dry state, water quenching effect decreases the catalytic initiation of free radicals formation and hinders the oxidation rate. Moreover, oxygen diffusion rate remarkably decreases with the replacement of water by air in the pores and capillaries at the moisture sorption process. Low water activity state lowers the oxidation reaction rate. Consequently, fewer aromas prevail from the food where bound water is replaced by air molecules.

7.3.5  Water and Food Appearance A physical view of the food products not only stands for the chromatic distribution and shininess but also the transparency to the spatial distribution of light and glow has similar effects on the appearance. The luminosity of light creates the visual sensitiveness of the colour of the food and this visual perception due to varying spatial distribution of light is termed as “cesia” by the architect Cesar Janello in 1965 [75–78]. The chromatic distribution pattern defines the colour of the food by means of hue, saturation and luminosity which has been further alleged by cesia as permeability, darkness and diffusivity for the vividness and sharpness of the image in case of generating human visualization by transmission and reflection [78]. However, the cesia behaviour gets less impact than the chromatic aptitude due to the deficiency in specifying and precisely measuring the ingredients solely responsible for these consequences. Apart from the change in colour or glow due to structural changes during food processing and preserving, the light emits from the object or the light reflects or transmits from the object causes the variation in colour and the later stimuli can be attributed as cesia effect. This variation in colour mainly depends on the micro-textural pattern of the surface exposed to light or the volume of the surface [78]. As the plant-based fruits and vegetables have higher moisture content (>85%), during drying this moisture content drastically reduces and contributes to

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the intense chromatic distribution and affect the colour, whereas in freeze drying, air cavities forms by replacing the intercellular and intracellular water content by air and influences the spatial distribution of the chromatic stimuli.

7.4  Stability Microorganism growth: Water activity, in foods, is an established factor for controlling the reproduction and growth of microorganism. There are certain values of water activity, critical water activity that ascertain the resistance in the growth of microorganism. Two of the most important critical points are observed to be 0.6 and 0.84 for the growth of any microorganism and pathogenic bacteria have respectively. Shelf life can be extended by removing water from the food and making water unavailable for the growth of microorganism. In the following ways water can be unavailable for microorganisms: • Solutes and ion such as sugars or salt can tie up water in solution. Available water of the food is tied up by solutes and water inside the microbial tends to leave due to osmosis. • Hydrophilic colloids or gel such as agar can make water unavailable for microorganisms. • Water at the crystallization phase also becomes unavailable to microorganisms. Shelf life significantly lowers at a higher water activity of the food materials. A decrease in water activity, for most of the dried foods, by 0.1 increases shelf life about 2–3 times as demonstrated in Fig. 7.7 [50, 51]. Figure 7.7 demonstrates that if the value of water activity is less than 0.6, there is a very little chance of microbial growth. Moreover, foods with different ranges are susceptible to different types of microbial growth. Food processing and preservation focus on the assurance of retention of physical, chemical, microbial and nutritional stability. However, process conditions significantly dominate the level of stability of processed foods. Along with the process conditions, the moisture content of the food at a given time is vital in retention of different qualities of the food. Moreover, chemical and microbiological stability of foods depend on the bonding of the water in food materials rather than bulk water content. However, food stability does not depend on the water activity; rather it does on other aspects such as glass transition concept. Throughout the history of food stability, many concepts of water are taken into consideration as shown in Fig. 7.8. These concepts deal with the stability of food differently. With the progress of research in food research, all of these concepts show some form of limitations in explaining the stability of food materials. However, researchers are coming with the micro-macro region concept in determining food stability. Interested readers can go through excellent work of Safiur [79].

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Fig. 7.7  Water activity limit for the growth of microorganisms of food [51]

7.4 Stability

131

Fig. 7.8  Food stability determinant relating water

7.4.1  Is Bound Water Removal Beneficial? As mention earlier that, bound water removal affects energy consumption, processing time, food stability and overall quality. In the production of dried foodstuffs, minimizing drying cost is often the most highly regarded criterion. However, the conditions which produce minimum costs are unlikely to also result in the lower quality as the optimum requirements of heat and mass transfer do not necessarily match with those for optimum quality. The key challenging issue of food processing is the selection of appropriate process parameters to attain high quality of finished product [80]. Eventually, optimization in process condition and quality expectation is essential. Optimization in bound water removal decision involves selection of proper processing conditions, product quality, food stability and productivity. In general, mild process conditions and moderate level of bound water removal favors quality optimization. However, this results in higher processing times along with less productivity. The conflicting influences on the productivity, quality, and energy needs to be addressed prior to bound water removal. The conflicting issues in optimization in bound water can be presented in Fig. 7.9. In addition to optimization of process conditions, critical analysis on benefits of bound water removal needs to be carried out. There is no single answer on the issue of bound water removal. Due to the variation in food materials types in composition

132 Fig. 7.9 Optimization factors in bound water removal

7  Significance of Bound Water Measurement

Expected

Unexpected

and structure, bound water removal affect either positively or negatively. The decision on bound water also depends on the end use of processed food. Taking all into consideration, bound water removal demands more consumption of higher energy, time and cost than free water removal does. Most of the quality attributes also deteriorate with removing bound water. However, stability in microorganism growth increases substantially in case of bound water removal from food materials. Therefore, critical assessment needs to be carried out for individual food prior to deciding bound water removal as it affects shelf life, cost, processing time and energy and overall quality.

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Chapter 8

Conclusion

Food materials are complex in nature as it has heterogeneous, amorphous, hygroscopic and porous properties. Comprehensive understanding of the water distribution of the raw food material and its evolution during processing is critical in order to design optimum processing conditions and quality anticipation. Depending on the heterogeneous distribution of water and voids, foods can be broadly classified into hygroscopic-non hygroscopic and porous-non porous materials respectively. This spatial distribution of water content in the intercellular and intracellular spaces allows it to significantly control the internal transport, chemical reactions and foods structure. Several important conclusion can be drawn from the discussion of the previous chapters of this book as discussed below. Water is one of the main components of food material. The amount of water present in water unevenly distributed due to diverse interaction of water and other components. Broadly, water in food materials can be divided into free and bound. The degree of strength in bonding widely varies in different situation. Therefore, nature of water distribution need to be taken into consideration rather merely accounting water content in food stability. It is very hard to define bound water as there is no unanimous definition of bound water in literature. Bound water shows many unusual properties including exceptional boiling point, freezing point, and latent heat of evaporation, specific heat, and viscosity. These distinct properties of bound water vary significantly in individual food materials. Each of the distinct characteristics defines bound water for food materials from different perspective. Eventually, the proportion of bound and free water varies remarkably in materials. Accurate determination of total water is a critical task as it involves many assumption. Many methods can be deployed in order to measuring water content; however, there are very few method available for measuring bound water. Bound water measurement techniques are mainly non-destructive and indirect in nature. These method can distinct the presence of bound and free water with acceptable accuracy. As individual method takes a single property of water, the proportion of bound and free water obtained varies with the variation of method. In other words, © Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6_8

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8 Conclusion

two different approaches of bound water measurement may provide result with remarkable discrepancy. Despite the acceptable level of accuracy, indirect water measurements that are used for bound water distribution has some shortcomings. These shortcomings needs to be considered during the interpretation of the result. Likewise the measurement of bound water, its removal from food materials is quite challenging task. The removal of bound water during drying involves simultaneous heat and mass transfer at micro or macro levels. Many internal moisture migration mechanisms including molecular diffusion, evaporation-condensation or in the form capillary flow during food processing take place. This types of water can be removed through different types of food processing such as drying and frying processes depending on the food composition and structure. In general, rapid processing accurses the loss of sensory, nutrition and physical appearance of the food products. Slow processing keeps the quality of foods with subsequent longer drying time and cost. Hence, the foods stability and prolonged shelf life functionally dictates the associated optimum food processing conditions and quality requirement. Therefore, the necessity of bound water removal should be done with right approach to ensure quality as well as drying cost, energy and time. Bound water plays a significant role during the food process that involved the removal of water. Bound water is captured by the solid matrix of water in the insoluble compounds in the foods needs higher energy to migrate from food structural matrix. This higher energy negatively affects in the overall quality of food material along with higher energy and time requirement. However, stability in microorganism growth increases substantially in case of bound water removal from food materials.to sum up, bound water removal demands more consumption of higher energy, time and cost than free water removal. Whereas, most of the quality attributes also deteriorate with removing bound water. However, stability in microorganism growth increases substantially in case of bound water removal from food materials. Therefore, critical analysis of optimum water content needs to be performed for individual food to ensure adequate amount of water for better quality and stability with less energy consumption during food processing.

Index

A Adsorption isotherm, 14 Amorphous, 2, 3, 13, 17, 40, 41, 105, 137 Aroma, 107, 108, 119, 128 Atmospheric frying, 99 Atomizer, 105 B BET multilayer adsorption isotherm, 14 Binding energy, 13, 20, 24, 30, 31, 74, 121 Bioelectrical impedance analysis (BIA), 69–71, 73, 77, 83, 87, 90 Boiling point, 22, 29, 30, 34, 51, 98, 99, 103, 108, 110, 137 Bound water, 2, 4, 7, 8, 10–12, 14, 15, 18, 20–22, 24, 29–42, 47–77, 83–91, 93–112, 119–132, 137, 138 Bound water measurement, 47–77, 83–91, 119–132, 137, 138 Bubble formation, 99 Bulk flow, 94 Bulk water, 21, 22, 123, 129 C Canning, 9, 111 Capillaries, 3, 7, 8, 13–15, 18, 21, 22, 32, 35, 48, 94, 97, 99, 103, 120, 128, 138 Capillary flow, 94, 96, 97, 138 Capillary water, 8, 21, 120 Cavitation, 112 Cell collapse, 23, 41, 94, 124, 125 Cell membranes, 41, 66, 70–72, 94, 111, 124, 125

Cell stability, 15, 35 Cell water, 23, 40, 57, 59, 123 Challenges in bound water measurement, 83–91 Chemically bound water, 8, 22, 102 Chemical reactions, 14, 36, 39, 69, 84, 119, 126–128, 137 Clausius–Clapeyron equation, 10, 31 Complete removal of moisture, 84 Complex polymer, 15, 35 Concentration gradient, 9, 34, 51, 66, 95, 96, 120 Constant diffusivity, 95, 96 Constant rate period, 100, 101 Convective drying, 88, 102, 107–110, 112 Crispness, 123–125 Critical moisture content, 1, 11, 12, 101, 106, 127 Crystal growth, 1 CT scan, 64–66, 83, 87, 89 D Darcy’s law, 97 Dehydration, 9, 30, 31, 76, 94, 95, 97, 98, 101, 107–111, 127 Dielectric properties, 22, 33, 53, 126 Dietary fibre, 3 Differential Scanning Calorimetry (DSC), 41, 53–58, 83, 87, 88 Diffusion, 8, 12, 13, 16, 32, 34, 35, 38, 40, 41, 94–98, 103, 109, 110, 112, 125, 128 Diffusion coefficient, 12, 96, 97 Dilatometry (DIL), 62, 63, 83, 87, 89

© Springer Nature Switzerland AG 2019 M. U.H. Joardder et al., State of Bound Water: Measurement and Significance in Food Processing, https://doi.org/10.1007/978-3-319-99888-6

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Index

140 Direct distillation, 51 Direct water content measurement, 49, 84, 87 Distribution of water, 1, 4, 8, 15, 16, 22, 23, 29, 35, 48, 54, 57, 64, 66, 74, 76, 83, 88, 89, 121, 122, 137, 138 Drying rate, 103, 108–111, 120 temperature, 41, 67, 105, 120 Dynamic aspects, 29 Dynamic definition, 7 E Electrohydrodynamic (EHD) drying, 106 Energy required, 7, 13, 16, 18, 20, 24, 54, 66, 104, 108, 110, 112, 119, 121, 122 Enzymatic browning reaction, 110 Equilibrium freezing temperature (EFT), 18, 32 Equilibrium moisture content (EMC), 10–13, 35 Extracellular, 3, 24, 40 F Falling rate periods, 96, 100, 101, 109, 110, 123 Falling rate stage, 22, 23, 124 Fick’s law, 95, 96 First falling rate, 66, 101 Flory-Higgins isotherm, 14 Food processing, 1, 2, 8, 9, 17, 24, 29, 34, 93, 94, 98–104, 106, 107, 110–112, 119–123, 127–129, 131, 138 Food stability, 4, 16, 39, 41, 48, 126, 129, 131, 137 Food structure, 1, 40, 47, 63, 84, 94, 97, 122, 138 Force convective mass transfer, 112 Free water (FW), 2, 7, 8, 14, 18–21, 23, 30, 31, 33, 34, 42, 48, 49, 51, 52, 55, 57, 63, 69, 75–77, 87, 89, 94, 101–104, 109, 111, 119–121, 123, 125, 126, 132, 137 Freeze drying (FD), 88, 103, 104, 107, 108, 129 Frying, 1, 93, 94, 97–99, 119, 120, 138 G GAB equation, 35, 36, 40 Gibbs’ free energy, 18, 31 Glass transition, 20, 31, 32, 38, 40, 41, 129

H Heat of vaporization, 17, 22, 101 Heat sensitive materials, 103, 106, 107, 126 Heterogeneous, 1, 3, 15, 23, 35, 90, 137 Higher binding energy, 31 High-pressure frying, 99 Hot air drying, 88, 98, 102, 103, 108–110 Hot air freeze drying, 107, 108 Hybrid drying, 106–108, 110, 111 Hydrophilic, 13, 14, 33, 95, 129 Hygroscopic, 1, 2, 11, 18, 23, 89, 95, 96, 120, 137 I Indirect water content measurement, 49, 51, 138 Infrared drying, 52, 53, 102, 110 Initial water soluble carbohydrate (WSC), 12 Insoluble dietary fibre, 3 Intercellular water, 8, 15, 20, 22, 35, 48, 65–67, 70, 71, 73, 87, 90, 123, 124, 129 Intermediate moisture content, 9 Internal mass transfer, 93, 94, 99 Intracellular water, 8, 15, 23, 35, 48, 58, 65–67, 69, 71–73, 87, 88, 111, 129 K Karl Fischer (KF) method, 85, 86 Knudsen flow, 94, 96 L Latent heat of sublimation, 17 Liquid diffusion, 96 Loosely bound water (LBW), 14, 24, 48, 57, 69, 75–77, 101, 120, 121 Lower vapour pressure, 30, 34 M Maillard reaction, 52, 127 Metabolism activities, 119 Microbial growth, 1, 20, 38, 129 Microwave-assisted freeze drying, 107 Microwave-assisted vacuum drying (MAVD), 108 Microwave convective drying, 108–110 Microwaves, 33, 34, 53, 76, 87, 102, 103, 107–110, 128 Microwave–vacuum combined with infrared drying, 110 Migration of cellular water, 22, 23, 66, 123, 124

Index Moisture content, 1, 2, 7–13, 16, 19, 20, 23, 29, 30, 35, 36, 38–40, 49, 51–53, 55, 57, 64–66, 69, 74, 76, 84–87, 89, 91, 93, 94, 96–98, 101, 105, 106, 109, 120–122, 125–129 Moisture removal rate, 12, 66, 93 Moisture sorption isotherm (MSI), 10, 11, 13, 15 Molecular attraction forces, 97 Monolayer moisture content (MMC), 13, 15, 16, 22, 30, 33, 35, 36, 38, 40 Multilayer moisture content, 13, 22, 30 Mutual diffusion, 94 N Natural convection, 99 Nuclear magnetic resonance (NMR), 12, 17, 32, 41, 74–77, 83, 87, 90 Nutritional, 1, 122, 129 O Optimization of bound removal process, 131, 132 Osmotic dehydration, 110 Oxygen free drying, 107 P Phases of water, 17 Physical changes, 1, 39, 122 Physically bound water, 2, 8, 22 Plant based food, 3, 15, 21, 23, 30, 33, 35, 36, 47, 58, 70, 73, 77, 111 Poiseuille flow, 96 Pore formation, 1, 22, 23, 123 Porous media, 4, 12, 93, 97, 98 Processing times, 102, 107, 112, 119, 131, 132 Pure water, 2, 14, 29, 33, 37, 55, 62, 83, 87 Q Quality, 1, 4, 9, 14, 16, 17, 20, 23, 24, 29, 33, 35, 36, 38, 39, 59, 90, 94, 98, 100, 102, 104–110, 112, 119, 122, 131, 132, 137, 138 R Reaction rate, 41, 126–128 Real-time measurement, 88 Reflux distillation, 51 Rehydration, 9, 64, 97, 108–110

141 S Scanning electron microscope (SEM), 57, 59, 60, 83, 87, 88 Second falling rate, 101 Selection of standard, 85 Sensorial, 1 Shelf life, 2, 3, 9, 19, 93, 107, 129, 132, 138 Shrinkage, 2, 8, 20, 22, 31, 40, 41, 58, 59, 61, 62, 88, 108, 109, 122–124 Sigmoid isotherm, 13 Simultaneous heat and mass transfer, 2, 93, 98, 99, 120, 138 Slip flow, 94 Softness, 9, 108, 111, 123 Solid matrix, 2, 63, 74, 94, 119, 120, 138 Soluble dietary fibre, 3 Spatial distribution, 8, 15, 16, 20, 22, 35, 48, 76, 128, 129, 137 Spatial environments, 1 Specific heat, 17, 22, 29, 35, 40, 41, 137 Spectroscopy, 30, 52 Spray drying (SD), 105 Stability, 1, 3, 4, 13, 15, 16, 19, 29, 35, 38–41, 48, 84, 104, 120, 122, 126, 127, 129, 131, 132, 137, 138 Stefan diffusion, 96 Strongly bound water (SBW), 14, 24, 48, 49, 56–59, 62, 74, 76, 77 Structural aspects, 29 Sublimation, 17, 18, 103, 104 Surface diffusion, 93, 96 T Texture, 9, 38, 87, 107, 108, 122–124 Thermal blanching, 111 Thermal resistance, 103 Thermodynamic aspects, 7, 29 Thermogravimetric analysis (TGA), 67, 69, 83, 84, 87, 89 Tortuosity, 98 Transport mechanisms, 94, 100 Turgor pressure, 8, 15, 18, 35, 38, 48, 124 U Ultrasound pre-treatment, 111, 112 Unfrozen water, 18, 32 V Vacuoles, 15, 35, 94, 124 Vacuum drying (VD), 86, 103, 108, 110 Vacuum frying, 99

Index

142 Vaporization, 14, 17, 22, 31, 94, 98, 101 Viscoelastic, 122 Volatile components, 48, 84, 86, 128 Volumetric drying, 102 Volumetric Karl Fischer (VKF) method, 52 W Water activity, 2, 9–11, 13, 14, 16, 17, 19, 20, 30, 32, 36, 38–41, 123, 124, 126–130 Water and food appearance, 128, 129 Water concentration, 9

Water content, 1–4, 9–23, 30–36, 38–40, 50–58, 63–65, 67, 69, 70, 73, 76, 83–90, 93, 94, 97, 98, 101, 103, 104, 108, 109, 119, 122–124, 126–129, 137, 138 Water mobility, 2, 19, 32 Water potential, 18, 19 Water pressure, 7, 18, 30, 37, 103 Water retention capacity, 20 Z Zero energy, 110