moisture migration through chocolate-flavored …

The Pennsylvania State University

The Graduate School

Department of Food Science

MOISTURE MIGRATION THROUGH CHOCOLATE-FLAVORED COATINGS

A Thesis in

Food Science

by

Vikramaditya Ghosh

Submitted in Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

August 2003

The thesis of Vikramaditya has been reviewed and approved* by the following: Ramaswamy C. Anantheswaran Professor of Food Science Thesis Co-Advisor Chair of Committee Gregory R. Ziegler Associate Professor of Food Science Thesis Co-Advisor John N. Coupland Assistant Professor of Food Science John. L. Duda Professor of Chemical Engineering John. D. Floros Professor of Food Science Head of the Department of Food Science

*Signatures are on file in the Graduate School.

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ABSTRACT

The mechanism of moisture migration through chocolate-flavored coatings was

investigated in this study. An approach, developed by Weisz (1967) to understand

diffusion in heterogeneous materials, was used to elucidate the mechanism of moisture

migration through chocolate-flavored coatings.

In chocolate-flavored coatings, the sucrose and cocoa powder particles are

embedded in the continuous fat phase. It was hypothesized that the unsteady-state

diffusivity of moisture through the coatings could be estimated from the diffusivity of

water through the continuous fat phase and the partition coefficient of moisture between

the hydrophilic particles and the fat phase. Therefore, to predict moisture diffusivity

through a chocolate-flavored coating, the moisture sorption isotherms for all the

constituents of the coating and the diffusion coefficient through the fat phase are

required.

The moisture adsorption isotherms for sugar, cocoa powder, coconut oil, and

coconut oil + 0.5% lecithin were obtained by equilibration over saturated salt solutions.

It was found that up to an aw of 0.85, cocoa powder adsorbed more moisture than any

other component. The presence of lecithin increased the moisture adsorption capacity of

oil. The moisture adsorption isotherm for a coating made with 70% coconut oil + 0.5%

lecithin and 30% sugar was also determined. The equilibrium moisture content, for this

coating, at each water activity level was higher than the individual constituents, i.e., sugar

or coconut oil + 0.5% lecithin. One possible reason for this observation is that there is a

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layer of moisture present between the surface of the sugar particles and the polar regions

of lecithin.

The diffusion coefficient, of moisture diffusing through fat, was obtained by time-

lag experiments. The predicted diffusion coefficient through a coating containing fat and

varying amounts of sucrose and cocoa powder were determined. Experiments were

conducted with coatings containing different amounts of sucrose and cocoa powder to

validate the model. For coatings containing just coconut oil and cocoa powder, the

diffusion of moisture in the dispersed phase, i.e. cocoa powder, occurred through the

cocoa powder particle. When lecithin was added, the water molecules diffused through

the cocoa powder particles as well as along their surfaces. When coatings contained

sucrose and lecithin and the water activity was 0.85, the sucrose molecules dissolved in

the migrating moisture and were transported to surface, where upon evaporation of the

water sucrose crystals were deposited. With diffusion of moisture there were structural

changes that altered the diffusivity of moisture through the coating. The structural

changes occurred because the sucrose particles dissolved in the migrating moisture and

cocoa powder particles swelled in the presence of moisture.

The effect of sucrose, cocoa powder, emulsifier, fat type, and storage environment

on the water vapor permeability (WVP) of a chocolate-flavored coating was also studied.

In addition, optical microscope images of cocoa powder and SEM images of the structure

of two different coatings were obtained. A coating containing more than 20% cocoa

powder (w/w) significantly increased the WVP; a coating containing 60% sucrose

significantly decreased the WVP. There was an increase in the WVP with an increase in

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the absolute value of the relative humidity across the film. The increase in WVP at

higher absolute humidity was caused due to structural changes in the coatings. The WVP

of fat based films decreased with an increase in the solid fat content (SFC).

TABLE OF CONTENTS

LIST OF FIGURES ..................................................................................................... ix

LIST OF TABLES....................................................................................................... xiv

ACKNOWLEDGMENTS ........................................................................................... xx

Chapter 1 INTRODUCTION....................................................................................... 1

Chapter 2 LITERATURE REVIEW............................................................................ 3

2.1 Theoretical Aspects of Diffusion.................................................................... 5 2.1.1 Mathematical Analysis ......................................................................... 6 2.1.2 Diffusion Coefficient............................................................................ 10 2.1.3 Structural Effects .................................................................................. 12 2.1.4 Thermodynamic Interactions................................................................ 17 2.1.5 Diffusion Mechanism ........................................................................... 18

2.2 Moisture Migration......................................................................................... 20 2.2.1 Moisture Sorption Isotherm.................................................................. 21 2.2.2 Measurement Methods for Moisture Migration ................................... 29

2.2.2.1 Gravimetric Technique (ASTM E96) ........................................ 29 2.2.2.2 Infrared Sensor Technique (ASTM F-372) ................................ 32

2.2.3 Factors Affecting Moisture Migration Through Chocolates ................ 33 2.2.4 Mechanism of Moisture Migration....................................................... 40 2.2.5 Methods of Control for Moisture Migration ........................................ 46

2.3 Statement of the Problem................................................................................ 47 2.4 Hypothesis ...................................................................................................... 47 2.5 Objectives ....................................................................................................... 49 2.6 References....................................................................................................... 51

Chapter 3 MECHANISM OF MOISTURE MIGRATION THROUGH CHOCOLATE-FLAVORED COATINGS .......................................................... 60

3.1 Abstract........................................................................................................... 60 3.2 Introduction..................................................................................................... 61 3.3 Mathematical Analysis of Diffusion Through Dark Chocolate...................... 62 3.4 Materials and Methods ................................................................................... 68

3.4.1 Materials ............................................................................................... 68 3.4.1.1 Sucrose ....................................................................................... 68 3.4.1.2 Cocoa Powder ............................................................................ 69 3.4.1.3 Coconut Oil ................................................................................ 69

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3.4.1.4 Lecithin....................................................................................... 69 3.4.2 Experimental Design ............................................................................ 70 3.4.3 Measurement of Moisture Content ....................................................... 71 3.4.4 Moisture Sorption Isotherm.................................................................. 74 3.4.5 Film Preparation ................................................................................... 76 3.4.6 Measurement of Diffusion Coefficient................................................. 79

3.4.6.1 Controlled Environment Setup................................................... 79 3.4.6.2 Experimental Procedure ............................................................. 81

3.4.7 Water Vapor Permeability.................................................................... 83 3.5 Results and Discussion ................................................................................... 84

3.5.1 Moisture Sorption Isotherm.................................................................. 84 3.5.2 Diffusion Coefficients .......................................................................... 87

3.5.2.1 Diffusion of Moisture through Coatings Containing Cocoa Powder ............................................................................................ 88

3.5.2.2 Diffusion of Moisture through Coatings Containing Sucrose.... 93 3.5.3 Water Vapor Permeability.................................................................... 100

3.6 Conclusions..................................................................................................... 102 3.7 References....................................................................................................... 104

Chapter 4 EFFECT OF INGREDIENTS ON MOISTURE MIGRATION THROUGH CHOCOLATE-FLAVORED COATINGS...................................... 107

4.1 Abstract........................................................................................................... 107 4.2 Introduction..................................................................................................... 107 4.3 Materials and Methods ................................................................................... 111

4.3.1 Ingredients ............................................................................................ 111 4.3.1.1 Sugars ......................................................................................... 111 4.3.1.2 Cocoa Powder ............................................................................ 112 4.3.1.3 Fats ............................................................................................. 113 4.3.1.4 Emulsifiers ................................................................................. 114

4.3.2 Experimental Design ............................................................................ 115 4.3.3 Measurement of Moisture Content ....................................................... 117 4.3.4 Moisture Sorption Isotherm.................................................................. 119 4.3.5 Sample Preparation............................................................................... 120 4.3.6 Method for Measurement of Water Vapor Permeability...................... 122

4.3.6.1 Controlled Environment Setup................................................... 124 4.3.6.2 Experimental Setup .................................................................... 126

4.3.7 Scanning Electron Microscopy............................................................. 127 4.3.8 Structural Changes on Swelling ........................................................... 128 4.3.9 Water Transmission Studies at sub-Zero Temperatures....................... 129

4.4 Results and Discussion ................................................................................... 132 4.4.1 Effect of Coating Thickness ................................................................. 132 4.4.2 Effect of Ingredient Proportions on WVP of Coatings ........................ 135 4.4.3 Effect of Absolute Relative Humidity on WVP of Coatings ............... 144 4.4.4 Swelling of Cocoa Powder ................................................................... 152

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4.4.5 Structure of Coatings............................................................................ 154 4.4.6 Effect of Solid Fat Content................................................................... 156 4.4.7 Effect of Cocoa Powder type, Sugar, and Emulsifier........................... 157

4.5 Conclusions..................................................................................................... 159 4.6 References....................................................................................................... 160

Chapter 5 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH ......................................................................................................... 164

Appendix A DEVELOPMENT OF A METHOD TO MEASURE THE WATER VAPOR TRANSMISSION RATE....................................................................... 167

Appendix B MOISTURE SORPTION AND DIFFUSION COEFFICIENT DATA.. 172

B.1 Moisture Sorption Isotherms.......................................................................... 172 B.1.1 Coconut Oil.......................................................................................... 172 B.1.2 Coconut Oil + 0.5% lecithin ................................................................ 172 B.1.3 Sugar .................................................................................................... 173 B.1.4 Cocoa Powder ...................................................................................... 173 B.1.5 Coconut oil + 30% Sugar + 0.5% Lecithin.......................................... 174

B.2 Diffusion Coefficients.................................................................................... 174 B.2.1 Coconut Oil and Coconut Oil + 0.5% Lecithin ................................... 174 B.2.2 Coatings Containing Sugar .................................................................. 175 B.2.3 Coatings Containing Sugar and Lecithin ............................................. 176 B.2.4 Coatings Containing Cocoa Powder .................................................... 179 B.2.5 Coatings Containing Cocoa Powder and Lecithin............................... 182

Appendix C WATER VAPOR PERMEABILITY DATA.......................................... 188

C.1 Effect of Coating Thickness........................................................................... 188 C.2 Mixture Experiment ....................................................................................... 190 C.3 Effect of Ingredients....................................................................................... 200

C.3.1 Effect of Solid Fat Content on Water Vapor Permeability .................. 204 C.4 Effect of Humidity on Water Vapor Permeability ......................................... 208 C.5 Augmented Design Data ................................................................................ 209 C.6 Qualitative Study Pictures.............................................................................. 214

LIST OF FIGURES

Figure 2–1: Schematic of chocolate microstructure. Upper left – solid particles about to collide during fat crystallization. Insert – enlargement of space between one sugar crystal (gray) and cocoa particle (black). Reproduced with permission (Loisel et al., 1997) .................................................................... 14

Figure 2–2: Contour plot of Deff/Dc as a function of the dispersed phase volume, f, and the continuous phase diffusivity, Dc=10-a (dispersed phase diffusivity, Dd=10-10)............................................................................................................. 16

Figure 2–3: Relative flux for moisture as a function of continuous phase volume fraction (i.e. fat content). Calculated from the model of van der Zanden (2000) using Dd/Dc = 100. ................................................................................... 18

Figure 2–4: Moisture sorption isotherms for chocolate containing different sweeteners for two different lecithin contents: (a) Bournville chocolate, (b) sucrose, (c) b-D-fructose, (d) sorbitol, (e) L-sorbose, (f) maltose hydrate, ( ― ) 0.5% lecithin, (---). In each of these figures, the relative humidity is on the x-axis and the equilibrium moisture content is on the y-axis. Reproduced with permission (Ogunmoyela and Birch, 1984).......................................................... 24

Figure 2–5: Moisture sorption isotherm for an edible film and its components. Reprinted with permission from Morillon et al. (2000). Copyright (2000), American Chemical Society)................................................................................ 25

Figure 2–6: Moisture sorption isotherms for dark chocolate at 20°C. The upper curve is for desorption and the lower for adsorption. Reproduced with permission (Biquet and Labuza, 1988)................................................................. 26

Figure 2–7: Moisture sorption isotherms for chocolate at various temperatures. Reproduced with permission (Kim et al., 1999)................................................... 28

Figure 2–8: Schematic of the test cell for the ASTM E-96 method ............................ 30

Figure 2–9: Conceptual model for the diffusion of moisture through chocolate. The gradient in background shading is representative of the moisture content. The swelling of hydrophilic particles is suggested by the halos about the particles................................................................................................................. 41

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Figure 2–10: Kinetics of moisture sorption for dark chocolates at 20°C and varying relative humidity. Reproduced with permission (Biquet and Labuza, 1988). .................................................................................................................... 43

Figure 2–11: Modeling of moisture transfer through a chocolate film. Reprinted from (Rumsey and Krochta, 1994), with permission from Technomic Publishing Co., Inc., copyright [1994]) ................................................................ 45

Figure 3–1: Schematic of a structure of dark chocolate .............................................. 64

Figure 3–2: Setup for measuring moisture sorption isotherm ..................................... 76

Figure 3–3: Picture of the mold for making the chocolate coatings (hole diameter = 9 cm).................................................................................................................. 79

Figure 3–4: Temperature and relative humidity inside the plastic chamber................ 81

Figure 3–5: Setup for measuring the diffusivity of the fat coatings ............................ 82

Figure 3–6: Approach to steady state for a coating using the time lag method........... 83

Figure 3–7: Moisture sorption isotherm of sucrose and cocoa powder at 19oC .......... 85

Figure 3–8: Moisture sorption isotherm of sugar, coconut oil, and a mixture of coconut oil + 0.5% lecithin................................................................................... 86

Figure 3–9: Weight loss data for coatings containing 20% cocoa powder. The numbers in brackets are the thickness of the coatings in mm. (cp = cocoa powder, l = lecithin).............................................................................................. 89



Figure 3–12: Predicted versus the experimental data for coatings containing cocoa powder. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5. .......................................................... 92

Figure 3–13: Predicted versus the experimental data for coatings containing cocoa powder plus lecithin. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.......................................... 93

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Figure 3–14: Weight loss of coatings containing sucrose. The numbers in brackets denote the thickness in mm .................................................................... 94

Figure 3–15: Weight loss of coatings containing sucrose and lecithin. The numbers in brackets denote the thickness in mm ................................................. 95

Figure 3–16: Predicted versus the experimental data for coatings containing sucrose. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5. .......................................................... 96

Figure 3–17: Predicted versus the experimental data for coatings containing sucrose plus lecithin. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.......................................... 98

Figure 3–18: Moisture adsorption isotherm for sugar, coconut oil + 0.5% lecithin, and 30% sugar in coconut oil + 0.5% lecithin ...................................................... 99

Figure 3–19: Effect of ingredients and volume fraction on the water vapor permeability of coatings containing sucrose or cocoa powder............................. 101

Figure 3–20: Effect of ingredients and volume fraction on the water vapor permeability of coatings containing sucrose and lecithin or cocoa powder and lecithin. ................................................................................................................. 102

Figure 4–1: Structure of Citrem (Matissek, 2002)....................................................... 115

Figure 4–2: Picture of the mold for making the chocolate coatings (hole diameter = 9 cm).................................................................................................................. 121

Figure 4–3: Schematic of the test cell for the ASTM E-96 method ............................ 123

Figure 4–4: Picture of a test cell with the coating film................................................ 123

Figure 4–5: Picture of the setup for measuring the water vapor transmission rate of the coatings....................................................................................................... 124

Figure 4–6: Temperature and relative humidity inside the plastic chamber................ 126

Figure 4–7: Schematic diagram of the modified test cell used for measuring WVP at sub-zero temperatures ....................................................................................... 131

Figure 4–8: Schematic of the setup for measuring the water transmission through the coatings at negative temperature..................................................................... 131

Figure 4–9: Picture of the actual setup for measuring the water vapor transmission rate at –5oC........................................................................................................... 132

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Figure 4–10: Effect of coating thickness on the water vapor transmission rate at 20oC. The coating contains 15% cocoa powder and 85% coconut oil. The thickness of the coatings are given in mm............................................................ 135

Figure 4–11: Three dimensional surface plot of the fitted model for permeability data obtained from the mixture experiments ........................................................ 139

Figure 4–12: Three dimensional surface plot of the fitted model for permeability data obtained from the mixture experiments ........................................................ 143

Figure 4–13: Picture of the generic coating kept at humidity conditions of 75% on the outside and 33% in the bottom of the cell ...................................................... 145

Figure 4–14: Picture of the generic coating kept at humidity conditions of 75% on the outside and 54.5% in the bottom of the cell. .................................................. 146

Figure 4–15: Moisture sorption isotherms of sucrose and cocoa powder ................... 147

Figure 4–16: Weight gain versus storage time for a coating sample containing 50% cocoa powder with different relative humidity at the low humidity end. The numbers 1 and 2 at the end of the humidity in the legend signifies the sample numbers. The sample that did not cracked during the experiment is shown with an arrow as intact sample. The humidity at the high end was 75%. ...................................................................................................................... 148

Figure 4–17: Picture of a coating, containing 50% cocoa powder by weight, after 12 days with a humidity gradient of 75 – 3.5%.................................................... 149

Figure 4–18: Change in the voltage with storage time for coatings stored at –5 oC.... 151

Figure 4–19: Picture of the dry cocoa powder kept in shallow pans ........................... 153

Figure 4–20: Picture of hydrated cocoa powder .......................................................... 153

Figure 4–21: Structure of a section of dry generic coating seen using cryo SEM....... 155

Figure 4–22: Structure of a dry coating with 50% cocoa powder as seen under cryo SEM.............................................................................................................. 156

Figure 4–23: Effect of SFC on the WVP of fat coatings ............................................. 157

Figure 4–24: Picture of a cup with a generic coating that has a humidity of 85% inside the cup and 13% outside ............................................................................ 159

Figure A–1: Schematic of a test cell ............................................................................ 167

Figure A–2: Setup for measuring WVTR using desiccators ....................................... 168

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Figure A–3: Time to reach equilibrium humidity in a desiccator and the box fitted with a fan (convection) ......................................................................................... 169

Figure A–4: Setup for measuring the water vapor transmission rate at 20oC.............. 170

Figure B–1: Adsorption of lecithin on sugar at water activities > 0.8. Model proposed by Garbolino (2002).............................................................................. 187

Figure C–1: Coating with 50% cocoa powder after 14 weeks of storage at -5C......... 215




Figure C–5: Coating with 20% sucrose after 14 weeks of storage at -5C ................... 219

LIST OF TABLES

Table 2–1: Common terms used to describe barrier properties of edible films Reprinted from (Donhowe and Fennema, 1994), with permission from Technomic Publishing Co., Inc., copyright [1994]) ............................................. 10

Table 2–2: GAB constants at each aw value for dark chocolates at 20 oC (Biquet and Labuza, 1988) ................................................................................................ 27

Table 2–3: Effective water vapor permeability constants (keff) and water vapor transmission rate (WVTR) for a dark chocolate film at 20oC as determined by the cup method in two different configurations: (1) Drierite in the cup and salt solution outside, (2) saturated salt solution in the cup and Drierite outside (Biquet and Labuza, 1988) ................................................................................... 31

Table 2–4: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) of a dark chocolate at 20oC for three different thicknesses (Biquet and Labuza, 1988) ................................................................ 34

Table 2–5: Permeability data from (Landmann et al., 1960)...................................... 35

Table 2–6: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) as a function of the water vapor pressure gradient (Dp) for dark chocolate at 20oC (Biquet and Labuza, 1988) ............................... 37

Table 2–7: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) of a dark chocolate at 20oC for three different temperatures (Biquet and Labuza, 1988).............................................................. 39

Table 2–8: Effective Diffusion Coefficient of moisture through Dark Chocolate Film at 20oC (Biquet and Labuza, 1988) .............................................................. 44

Table 3–1: Experimental design for the moisture diffusion experiments.................... 70

Table 3–2: Partition coefficient of sucrose and cocoa powder with respect to oil or oil + 0.5% lecithin at different water activities..................................................... 87

Table 4–1: Solid Fat Contents of the Moisture Barrier Fats at 20oC........................... 114

Table 4–2: Mixture experimental design for studying the effect of sucrose, cocoa powder and lecithin............................................................................................... 116

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Table 4–3: Water Vapor Transmission Rate for the standard film (LIMA) * obtained using the External Cell........................................................................... 130

Table 4–4: Effect of coating thickness on the WVP of the generic coating at 17oC ... 134

Table 4–5: Effect of ingredients on the WVP of coatings ........................................... 137

Table 4–6: Summary of Data Analysis – Mixture Design........................................... 138

Table 4–7: Effect of ingredients on the WVP of coatings – Results from the augmented design ................................................................................................. 140

Table 4–8: Summary of Data Analysis – Augmented Mixture Design ....................... 141

Table 4–9: Effect of relative humidity difference on WVP of the generic coating at 18.5oC ............................................................................................................... 145

Table 4–10: Increase in the diameter of coatings stored at 75% RH........................... 154

Table 4–11: Effect of cocoa powder, sugar and emulsifier type on the WVP of coatings at 18.5oC ................................................................................................. 158

Table A–1: Comparison of WVTR (g m-2 day-1) data obtained with and without convection............................................................................................................. 171

Table A–2: Comparison of WVTR (g m-2 day-1) data obtained when one or two fans are used for creating convection inside the setup ......................................... 171

Table B–1: Equilibrium moisture content for the coconut oil samples ....................... 172

Table B–2: Equilibrium moisture content data obtained for coconut oil containing 0.5% lecithin ......................................................................................................... 173

Table B–3: Equilibrium moisture content for sugar at different water activities ........ 173

Table B–4: Equilibrium moisture content of cocoa powder at different water activities................................................................................................................ 174

Table B–5: Equilibrium moisture content for a coating containing 30% sugar in coconut oil + 0.5% lecithin................................................................................... 174

Table B–6: Weight loss data obtained for coatings made from coconut oil and coconut oil + 0.5% lecithin................................................................................... 175

Table B–7: Weight loss data obtained for coatings containing 30% and 40% sugar in coconut oil. These data were obtained when the relative humidity on the higher humidity side was 75%.............................................................................. 176

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Table B–8: Weight loss versus time data for coatings containing 30% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 85% ................................................... 177

Table B–9: Weight loss versus time data for coatings containing 40% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 85%. .................................................. 178

Table B–10: Weight loss versus time data for coatings containing 30% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 75% ................................................... 178

Table B–11: Weight loss versus time for coatings containing 2.5% cocoa powder in coconut oil ........................................................................................................ 179

Table B–12: Weight loss versus time for coatings containing 20% cocoa powder in coconut oil ........................................................................................................ 180



Table B–15: Weight loss versus time for coatings containing 20% cocoa powder in coconut oil and 0.5% lecithin ........................................................................... 183




Table C–1: Weight gain versus time data at 17oC for coatings with different thickness and its effect on water vapor permeability............................................ 189

Table C–2: Weight gain versus time obtained for coating made from trial 1 formulation ........................................................................................................... 190


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Table C–17: Weight gain versus time obtained for coating made from trial 1 formulation. Second replicate............................................................................... 197


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Table C–19: Weight gain versus time obtained for coating made from trial 3 formulation. Second replicate.............................................................................. 198



Table C–22: Weight gain versus time obtained for a generic coating.(Set 1) ............. 200




Table C–26: Weight gain versus time obtained for a coating made using Citrem ...... 202

Table C–27: Weight gain versus time obtained for a coating made using commercial natural cocoa powder ........................................................................ 202

Table C–28: Weight gain versus time obtained for a coating made using commercial alkalized cocoa powder..................................................................... 203

Table C–29: Weight gain versus time obtained for a coating made using lactose ...... 203

Table C–30: Weight gain versus time obtained for a coating made using dextrose.... 204

Table C–31: Weight gain versus time obtained for the AARHUSTM fat coating...... 204

Table C–32: Weight gain versus time obtained for the Victory-76 fat coating........... 205

Table C–33: Weight gain versus time obtained for the Karlshamns fat coating ......... 205

Table C–34: Weight gain versus time obtained for the DP1192 fat coating ............... 206



Table C–37: Weight gain versus time for a generic coating when the humidity at the bottom of the cell was 33%............................................................................. 208

Table C–38: Weight gain versus time for a generic coating when the humidity at the bottom of the cell was 54%............................................................................. 209

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Table C–39: Weight gain versus time for a coating made with 20% sugar and 80% coconut oil ............................................................................................................ 210

Table C–40: Weight gain versus time for a coating made with 30% sugar and 70% coconut oil ............................................................................................................ 210

Table C–41: Weight gain versus time for a coating made with 50% sugar, 49.5% coconut oil and 0.5% lecithin ............................................................................... 211

Table C–42: Weight gain versus time for a coating made with 60% sugar, 39.5% coconut oil and 0.5% lecithin ............................................................................... 211

Table C–43: Weight gain versus time for a coating made with 20% cocoa powder and 80% coconut oil ............................................................................................. 212

Table C–44: Weight gain versus time for a coating made with 30% cocoa powder and 70% coconut oil ............................................................................................. 212

Table C–45: Weight gain versus time for a coating made with 30% cocoa powder, 69.5% coconut oil, and 0.5% lecithin ................................................................... 213

Table C–46: Weight gain versus time for a coating made with 40% cocoa powder, 59.5% coconut oil, and 0.5% lecithin ................................................................... 213

Table C–47: Weight gain versus time for a coating made with 50% sugar, 10% cocoa powder, 39.5% coconut oil, and 0.5% lecithin........................................... 214

Table C–48: Weight gain versus time for a coating made with 30% sugar, 30% cocoa powder, 39.5% coconut oil, and 0.5% lecithin........................................... 214

ACKNOWLEDGMENTS

I would like to express my sincere thanks to Dr. Ramaswamy C. Anantheswaran

for allowing me to learn under his guidance. He has been responsible for my professional

growth. His dual role as a friend and advisor helped me immensely, and allowed me to

draw on his own experience to shape my own identity. I would like to thank Dr. Greg

Ziegler to allow me to learn under his guidance. His questions and valuable suggestions

during this study were very helpful. Words cannot describe the amount the patience both

Dr. Anantheswaran and Dr. Ziegler had shown at various stages of my study. To them I

owe a debt of gratitude that is difficult to quantify, and impossible to repay.

I am thankful to Dr. John Floros for serving on my committee and also for

allowing me the use of facilities within the department of food science and for providing

me with a research assistantship. I would also like to thank Dr. John Coupland for

serving on my committee and for his suggestions during this project.

I am very thankful to Dr. Larry Duda from the department of chemical

engineering for explaining me the diffusion models and also suggesting the possible

mechanisms of moisture diffusion through coatings.

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I would like to thank Missy Hazen from the electron microscopy laboratory for

helping me out with the cryo SEM. I am also thankful to Barry Dutrow from the machine

shop (physics department) for helping out in designing the diffusion cell.

I am thankful to Lynn Dalby for help with the optical microscope and Bob

Lumley-Sapanski for helping me out with the freezer. I am really grateful to Annette

Evans for translating many of the German papers and to Laura Nattress for showing me

how to prepare the chocolate coatings. Thanks also to Dinos Matsos, for allowing me to

use space in his laboratory for doing the sub-zero temperature experiments.

Special thanks to Johnny Casasnovas, who offered help whenever I needed. His

suggestions at different times on the project were very helpful. I feel lucky to have a

“senior” like him. Thanks also to my colleagues Liping Liang, Haiqiang Chen, Qingbin

Yuan, Li Xiong, and all the graduate students in the department of Food Science for

making my work at food science department more enjoyable. Also thanks to the radio

station 95.3 3WZ whose songs gave me company in the laboratory.

I would like to thank my friends Rajesh Potineni, Surajit Ray, and Davendra

Tolani, who made my life at Penn State worthwhile. Thanks to my parents and my

family who provided constant encouragement throughout my life and I will always be

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indebted to them. Finally, I would like to thank my wife Koel, who stood by me through

thick and thin. She showed tremendous patience during the thesis-writing period.

Chapter 1

INTRODUCTION

Ice-cream cones are one of the most popular desserts for people of all ages.

Initially one could purchase ice cream filled in cones at the ice-cream stores. Now a

consumer can buy readymade ice cream filled cones in the supermarket as a frozen

product. These cones can be kept in a home freezer and can be had anytime. This has

made frozen ice-cream cones one of the most popular ice-cream novelties in the

supermarket. Though it is a favorite product of many consumers, its shelf life is

restricted because the wafers turn soggy due to the migration of water from the ice cream.

To provide a moisture barrier between the ice cream and the wafer, a chocolate-flavored

coating is applied between the wafer and the ice cream. Even with the application of the

chocolate-flavored coating there are still some consumer complaints about the cone or the

wafer turning soggy due to moisture migration. Thus there is a need for obtaining a

better moisture barrier than that is currently being used. According to the FDA

compliance policy guide No. 515.800 a coating containing coconut oil, cocoa powder,

sucrose, and lecithin should be labeled as chocolate-flavored coatings. Therefore, in this

document the coatings will be referred to as chocolate-flavored coating.

There is a real lack of literature data and understanding on moisture migration

through chocolate-flavored coatings. Since chocolate-flavored is a composite product

consisting of several structural inclusions, there are several possible mechanisms for

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moisture uptake and transfer through it. A good understanding of the mechanism(s) of

moisture migration through chocolate-flavored coatings is needed in order to design a

superior barrier for the ice cream wafer. This can be done through selecting the

appropriate formulation for the chocolate-flavored coating and by using the best suitable

process to apply the coating that will result in moisture resistant ice cream wafers.

The objective of this work is to understand the mechanism of moisture migration

through chocolate-flavored coatings.

Chapter 2

LITERATURE REVIEW1

Migration of a component from one domain to another is a common

problem with chocolates, confectionery, bakery products and other multi-domain food

materials. Moisture migration occurs in chocolate-covered confectioneries that have high

water activity centers, often called ‘soft centers,’ and coated biscuits and wafers. The

migration of moisture will cause defects in the product, such as cracking of the coating

(Barron, 1977; Minson, 1990), staling of the wafer, and drying of the center. In ice-

cream cones moisture migration from ice cream to the sugar wafer makes the wafer

soggy, which makes the product unacceptable. Moisture migration can lead to sugar

bloom (Minifie, 1989; Rosenberger, 1994) and mold growth in chocolate (Larumbe et al.,

1991).

Migration of a molecular species occurs when there is a chemical potential

difference of that particular species between two points. The species will migrate until

the two points are in thermodynamic equilibrium. The movement of these molecules can

be described by the principles of diffusion. The rate of diffusion is proportional to the

difference in chemical potential (driving force) divided by the resistance to movement

encountered by the diffusing molecules. Since it is easier to determine the concentration

1 This chapter is taken from the paper “Fat, Moisture, and Ethanol Migration through Chocolates and Confectionery Coatings”. CRC Critical Reviews in Food Science and Nutrition, 2002, 42(6): 583 - 626

4

than the chemical potential, most diffusion equations are written in terms of

concentration. The relation between the concentration and chemical potential is

discussed in the section Theoretical Aspects of Diffusion. The resistance to diffusion is

dependent on the structure of the material the molecular species is migrating through and

the thermodynamic interaction between the material and the diffusing species.

In most situations the diffusion process can be described by Fick's law (to be

discussed later in the section on theoretical aspects). However, non-Fickian behavior is

observed in many cases of diffusion in foods (Landmann et al., 1960; Biquet and Labuza,

1988; Debeaufort et al., 1994; Peppas and Brannon-Peppas, 1994; Ozdemir and Floros,

2001) because diffusion is coupled with phase or structural changes that occur due to the

presence of the migrating molecule. The change in structure and phase will affect the

diffusion rate through the food material (Aguilera and Stanley, 1999; Aguilera et al.,

2000). Hence, when studying diffusion in food materials, it is important to couple the

diffusion process with the microstructure and the phase behavior.

To understand the migration process it is first necessary to know the principles of

diffusion, which are discussed immediately after this paragraph. Given the substantial

volume of literature devoted to diffusion in polymers, the theoretical aspects have been

adapted from the field of polymer science. For greater details on diffusion in food

systems see Gekas (1992).

5

2.1 Theoretical Aspects of Diffusion

Diffusion is the process by which matter is transported from one part of a system

to another as a result of random molecular motion (Crank, 1975). Diffusion of a species

occurs whenever a concentration gradient of that species exists between two points. The

molecules migrate from a higher concentration to a lower concentration until the system

reaches thermodynamic equilibrium. The phenomenon of diffusion due to a concentration

gradient is sometimes called ordinary diffusion in order to distinguish it from pressure,

thermal, and forced diffusion processes (Bird et al., 1960).

Diffusion across an interface between two domains takes place in two stages. In

the first stage the migrating molecule dissolves into the surface of the material it is

diffusing into. The solubility of the migrating molecule is defined as the maximum

volume (or mass) that dissolves in a unit volume of the material at equilibrium. The

second stage consists of the diffusion of the dissolved molecules through the material

under action of a concentration gradient. The solubility depends on the thermodynamic

compatibility between the penetrant molecule and the material the penetrant is migrating

through. The process of migration within a food matrix can be viewed as a series of

activated jumps from one vaguely defined "cavity" or void to another. Qualitatively, the

presence of a large of number of cavities, also defined as the free volume, increases the

diffusion rate (Chao and Rizvi, 1988). A molecule that increases the free volume of the

material is called a plasticizer, and hence the presence of a plasticizer in the food matrix

will increase the diffusion rate.

6

The factors affecting the diffusion process can be grouped into two categories.

One is thermodynamic, or the sorption/desorption behavior, describing the interaction

between the penetrant and the food. The other group of factors is non-thermodynamic,

and involves the composition and structure, including defect structures, of the food.

Defect structures can be macroscopic cracks or holes in the material. If defect structures

like cracks or holes are present in the matrix, movement will take place through these

preferentially. The dominant mechanism of mass transfer through the defect structures is

by capillary flow. It is difficult to characterize such defects, so diffusion through defects

cannot be easily quantified (Rogers, 1985).

Steady-state diffusion is achieved when the concentration profile is invariant with

time, i.e. ∂c/∂t at any geometric point equals zero. If the concentration gradient at a point

varies with time, as is typical in the early stages of migration, unsteady-state diffusion is

observed. In the case where oil or fat is migrating into a coating, the study of the

unsteady-state diffusion becomes more relevant.

2.1.1 Mathematical Analysis

The flux Ji for a species i diffusing in one-dimension is given by the equation 2.1:

i

ifii x

DJ∂µ∂

= ( 2.1 )

where Ji is the molar diffusion flux of the migrating molecules (mol cm-2 s-1), i.e. the rate

of flow per unit area, Df is the fundamental diffusion coefficient (cm2s-1), x is the

7

diffusion length (cm) and µ is the chemical potential of the species. The chemical

potential is defined by equation 2.2 (Vieth, 1991):

i0i

n,P,Ti

ii alnRT)P,T(

nG

j

+µ=

∂∂

=µ ( 2.2 )

where Gi is the Gibbs free energy, T is the temperature (K), P is the pressure, ai is the

activity, 0iµ is the potential for pure vapor, n is the number of molecules, and R is the

universal gas constant (8.314 J mol-1 K-1).

From the concepts of solution chemistry, the activity of the ith species can be

represented as the product of activity coefficient, γ, and the concentration c:

ai = γici ( 2.3 )

Applying Fick’s law, one may define the gradient in chemical potential in terms

of the experimentally defined concentration gradient in the following manner:

i

imi

i

ifii x

cDx

DJ∂∂

−=∂µ∂

−= ( 2.4 )

where Dmi is the mutual binary diffusion coefficient. From equation 2.4, one gets:

i

ifimi c

DD∂µ∂

= ( 2.5 )

From equations (2.2) and (2.3), one gets:

∂

γ∂+=

∂µ∂

i

i

iP,Ti

i

clnln1

cRT

c ( 2.6 )

So the mutual binary diffusion coefficient is related to the fundamental diffusion

coefficient Dfi by the equation:

8

∂

γ∂+=

i

i

ifimi cln

ln1cRTDD ( 2.7 )

For single component diffusion, the subscript can be dropped for the sake of

convenience. For steady-state diffusion in one-dimension, Fick's first law relates the

flux to the concentration gradient by the relation:

xcDJ

∂∂

−= ( 2.8 )

where ∂c/∂x is the concentration gradient, and c is the concentration (moles cm-3) and D

is the mutual diffusion coefficient (cm2 s-1). Fick’s equation expressed in different units

can be found in Bird et al. (1960). The diffusivity, D, can be thought of as the inverse of

the resistance to mass transfer.

Fick's second law, equation 2.10, from which the unsteady-state concentration

distribution may be calculated, is obtained from equation 2.8 and the equation of

continuity (Brown, 1988).

tc

xJ

∂∂

−=∂∂ ( 2.9 )

The equation of continuity is an expression of the conservation of mass. The flux

can be eliminated between equations 2.8 and 2.9 to give:

∂∂

∂∂

=∂∂

xcD

xtc ( 2.10 )

If the diffusivity is independent of concentration, which is often assumed but

generally not the case, this equation reduces to:

9

2

2

xcD

tc

∂∂

=∂∂ ( 2.11 )

For diffusion of gases and vapors, the solubility can be described by Henry’s law

and can be given by equation 2.12:

c = Sp ( 2.12 )

where c is the concentration expressed as volume (or mass) at Standard Temperature and

Pressure per unit volume of the material, S is the solubility coefficient, and p is the

applied pressure (mm Hg).

Combining equations 2.8 and 2.12 gives:

xpPJ

∂∂

−= ( 2.13 )

where P is the permeability g·cm·(cm2·s·mmHg)-1and ∂p/∂x is the pressure gradient (mm

Hg). Permeability is the product of diffusivity and solubility. Equations 2.9 and 2.11 can

also be written in terms of pressure instead of concentration.

Assuming that P is independent of pressure and that the sample is homogeneous,

when steady-state conditions have been achieved equation 2.13 can be integrated to give:

lpPJ ∆

= ( 2.14 )

where ∆p is the pressure drop across the sample and l is the sample thickness. If P

changes with pressure equation 2.14 will give a value of the permeability at the average

pressure across the film. Many different units have been used to describe permeability

and transmission rate. Table 2–1 gives the common terms and units used to describe the

barrier properties of edible films (Donhowe and Fennema, 1994).

10

2.1.2 Diffusion Coefficient

The diffusion coefficient, D is needed to quantitatively solve the diffusion

equation. As explained in the previous section, the value of D corresponding to the rate

at which the concentration gradients are dissipated is called the mutual diffusion

Table 2–1: Common terms used to describe barrier properties of edible films Reprinted from (Donhowe and Fennema, 1994), with permission from Technomic Publishing Co., Inc., copyright [1994])

Term Equationa Common Units Accepted SI units Permeability M·∆x·(A·t·∆p)-1

or M·∆x·(A·t·∆C)-1

g·mil·(100 in2·day·cmHg)-1 g·mil·(m2·day·cmHg)-1 cc·mil·(m2·day·atm)-1 g·cm·(cm2·s·mmHg)-1 cc·cm·(cm2·s·cmHg)-1 mg·cm·[cm2·s·(mg·ml-1)]-1

g·(Pa·s·m)-1 kg·(Pa·s·m)-1 gmol·(Pa·s·m)-1 m2·s-1

Permeability coefficient M·∆x·(A·t·∆p)-1 Same units as permeability Same units as permeability

Permeance M·(A·t·∆p)-1

or M·(A·t·∆C)-1d

g·(m2·day·mmHg)-1 g·(Pa·s·m2)-1 kg·(Pa·s·m2)-1 gmol·(Pa·s·m2)-1 m·s-1

Transmission rate M·(A·t)-1 or

M·∆x·(A·t)-1e

g·(m2·day)-1 g·(m2·h)-1 g·mil·(m2·day)-1 cc·mil·(100 in2·day)-1 cc·cm·(cm2·day)-1 g·cm·(cm2·h)-1

g·(m2·s)-1 gmol·(m2·s)-1 g·mm·(m2·s)-1

Resistance A·t·∆C·M-1 s·m-2 Activation energy P0exp(-Ep·R-1·T-1) kcal·mol-1

kJ·mol-1 kJ·mol-1

aM = mass of permeate; ∆x = thickness of film; A = area; t = time; ∆P = pressure gradient; ∆C = concentration gradient; P0 = permeability constant; Ep = activation energy; R = gas constant; T = temperature (K). b Units listed are found throughout in the literature on packaging films c SI = Systéme International (SI) units. These are preffered. d This expression for permeance is commonly used in describing the properties of barrier properties of plant lipids. e Transmission rate is often reported with a thickness term in the expression.

11

coefficient, which is also referred to as the effective diffusion coefficient (Chinachoti,

1998). Another diffusion coefficient that can be found in the literature is the self-

diffusion coefficient, which is a measure of the rate of diffusion of one component in

another of uniform chemical composition (Vrentas and Duda, 1986). As a rule of thumb,

mutual diffusion coefficients in gases are around 10-1 cm2s-1, for liquids in the range of

10-5 cm2s-1, and through solids around 10-10 cm2s-1. Hence, the molecules tend to move

much faster in liquids and gases as compared to solids (Cussler, 1997). The diffusion

coefficient can be measured by a number of methods: diaphragm cell, infinite couple

method, Taylor dispersion, capillary method, Gouy interferometer, light scattering, and

NMR (Crank and Park, 1968). Dunlap et al. (1986) have given a comprehensive

description of all the methods used for measuring the diffusion coefficient.

When measuring or reporting a diffusion coefficient one should ascertain whether

the method is measuring the self or the mutual diffusion coefficient. In recent years,

pulsed field gradient (PFG) NMR has been widely used to measure a diffusion

coefficient, but this method measures the self and not the mutual-diffusion coefficient.

This is why data from NMR does not match effective diffusion coefficients (McCarthy

and McCarthy, 1994; Labuza and Hyman, 1998) obtained by other methods. Data from

magnetic resonance imaging (MRI) however can be used to measure the effective

diffusion coefficient. MRI offers great promise in studying mass transport properties and

diffusion of moisture in foods (Cornillon and Salim, 2000; McCarthy et al., 2000;

Schmidt and Lai, 1991; Schmidt et al., 1996; Troutman, 1999).

12

Diffusion coefficients for liquids are generally modeled using the Stokes-Einstein

equation:

r6kTDπη

= ( 2.15 )

where D = diffusion coefficient (cm2s-1), k = Boltzmann constant (1.38 x 10-23 J K-1), T =

absolute temperature (K), η = viscosity of the medium (centipoise), and r = molecular

radius of the diffusing material (cm). The coefficient calculated using this equation is

accurate to only about 20%. Still this remains as a standard against which alternative

correlations are judged (Cussler, 1997).

2.1.3 Structural Effects

The situation is more complex when diffusion occurs through a heterogeneous,

multiphase material such as a food. The structure of the food material will have a major

impact on the diffusion rate. As has been pointed out by Aguilera et al. (2000), an

effective diffusion coefficient is often determined without separately considering the

effect of food microstructure. However, without this information food engineers can only

guess at the appropriate solution to migration problems. A brief overview of the effect of

the structure on diffusion is presented in this section. A more detailed analysis of the

effect of structure on the diffusion coefficient can be found in articles by Aguilera and

Stanley (1999) and Hallstrom and Skjoldebrand (1983).

Three important features influence the migration rate through a heterogeneous

material: the relative diffusivity in different domains, the relative volume fraction of

13

those domains, and the geometric distribution of the domains. The effective diffusion

coefficient, Deff, through a material consisting of impermeable porous solids with fluid

filled pores is given by Aguilera and Stanley (1999):

τε

= DDeff ( 2.16 )

where D is the diffusion coefficient within the pores, ε is the void fraction or porosity of

the solid, and τ is the tortuosity of the pores. Cussler et al. (1988) have proposed an

equation for membranes containing flakes or lamellae (Equation 2.17):

φ−φα

+=1

1DD 22

eff

0 ( 2.17 )

where α is the aspect ratio, which is defined as the ratio of half the second largest

dimension divided by the smallest dimension and φ is the volume fraction of the flakes or

the lamellae. It should be noted from equation 2.17 that the diffusion coefficient depends

only on the volume fraction and not on the size of the impermeable particles (Eitzman et

al., 1996). Equations 2.16 and 2.17 can be used to understand the migration of moisture

through chocolates.

Let us consider the structure of dark chocolate, where the sugar and cocoa solids

are embedded in a continuous fat phase (Figure 2–1). The continuous fat phase consists

of a fat crystal network. The pores between these fat crystals do not really have a

spherical shape and are either closed or completely (or partially) filled with the liquid

fraction of cocoa butter (Loisel et al., 1997). The water molecules diffuse mainly

through the oil that is present in between the fat crystals. When the moisture comes in

14

contact with a hydrophilic particle, e.g. sugar or cocoa powder, it diffuses through the

hydrophilic particle as well as along their surfaces. The combination of these two

mechanisms results in an effective diffusivity in the non-fat phase. The diffusion of

moisture in sugar will be different from the diffusion in the cocoa powder. There will be

more surface diffusion in the case of sugar as compared to that of cocoa powder. Since

the molecules will diffuse via the hydrophilic phase the diffusion coefficient in this phase

should be taken into account.

Figure 2–1: Schematic of chocolate microstructure. Upper left – solid particles about to collide during fat crystallization. Insert – enlargement of space between one sugar crystal (gray) and cocoa particle (black). Reproduced with permission (Loisel et al., 1997)

15

The effect of structure and architecture on the overall diffusion coefficient has

been studied for simple geometries. For a two-phase composite in which spherical

particles of a material are dispersed in a continuous phase, an effective diffusion

coefficient may be obtained from the expression (Aguilera and Stanley, 1999):

+−

φ=+−

cd

cd

ceff

ceff

D2DDD

D2DDD ( 2.18 )

in which Dc is the diffusion coefficient through the continuous phase, Dd is the diffusion

coefficient in the dispersed phase and φ is the volume fraction of the dispersed phase. A

graphical representation of equation 2.18 is presented in Figure 2–2. When Dc is two

orders of magnitude greater than Dd, say 10-8 and 10-10 respectively, then for dilute

systems, Deff is approximately equal to Dc(1-φ). However, as the diffusivity in the

continuous phase approaches that in the dispersed phase, then the dispersed phase volume

does not have any influence on the effective diffusivity (Figure 2–2). For the case of

moisture migration, the diffusion through the continuous phase will be the rate-limiting

step, since the diffusion time in the dispersed hydrophilic phase will be negligible when

compared to the diffusion time through the hydrophobic lipid.

16

J0 0.2 0.4 0.6 0.8 1

8

8.5

9

9.5

10

0.902

0.803

0.803

0.803

0.705

0.705

0.705

0.607

0.607

0.607

0.508

0.508

0.508

0.41

0.41

0.41

0.312

0.312

0.213

0.213 0.115

Figure 2–2: Contour plot of Deff/Dc as a function of the dispersed phase volume, φ, and the continuous phase diffusivity, Dc=10-α (dispersed phase diffusivity, Dd=10-10).

Deff/Dc

Dd=10-10 (cm2/s)

Dc (

cm2 /s

)

Dispersed phase volume (φ)

10-

10-

10-

10-

10-

17

2.1.4 Thermodynamic Interactions

Many processes, e.g. diffusion through porous media or fat and moisture

migration through chocolates, involve a coupling of diffusion and phase equilibrium

thermodynamics. In addition to describing equilibrium at the interface between two

phases, thermodynamic information can facilitate the description of mobility in solutions.

As described in the mathematical analysis section, the diffusive flux is conventionally

related to a gradient in the concentration, but the fundamental driving force for molecular

diffusion is a gradient in the chemical potential or free energy of a species. The

conventional mutual binary diffusion coefficient is a product of a more fundamental

mobility parameter times a thermodynamic term that indicates how the chemical potential

changes with concentration. It has been clearly shown for many polymer systems that

when a conventional mutual binary diffusion coefficient is corrected with such a

thermodynamic term, the resulting coefficient has a simpler relationship in terms of its

concentration dependency (Duda, 1999).

van der Zanden (2000) proposed a more general model of mass transfer in a

heterogeneous media that incorporated a partition coefficient, K’, equal to the ratio of the

dispersed phase concentration and the continuous phase concentration, cd/cc , in

equilibrium at the interface. For simplicity, let us consider the case when the

concentration of moisture at the interface is the same in the dispersed and the continuous

phase. In this situation, K’=1 and using their relations to calculate the relative mass flux

(J/Jmax x 100) we obtained Figure 2–3. The migration of moisture increases rapidly as the

non-fat particulate phase approaches close-packed density (φ > 0.5). The precise

18

relationship between the lipid phase volume and the flux of moisture or fat will depend

on the equilibrium relationships, here modeled with a simple partition coefficient. It

would be valuable to extend this type of modeling to include more complex equilibrium

behavior such as sorption isotherms and eutectic interactions. Furthermore, we could

reasonably expect Deff to be a function of the phase structure (e.g., aspect ratio of the

dispersed phase particles) and not simply the phase volume.

2.1.5 Diffusion Mechanism

Diffusion behavior and transport process are classified according to the relative

rates of mobility of the penetrant. There are three basic categories of behavior described

as follows (Marom, 1985):

Figure 2–3: Relative flux for moisture as a function of continuous phase volume fraction (i.e. fat content). Calculated from the model of van der Zanden (2000) using Dd/Dc = 100.

19

(i) Case I or Fickian diffusion, in which the rate of diffusion is much less than that of

the polymer segment mobility. Sorption equilibrium is rapidly established,

leading to time-independent boundary conditions that exhibit no dependence on

swelling kinetics.

(ii) Case II (or Super Case II), the other extreme in which diffusion and penetrant

mobility are much greater compared with other relaxation processes. Sorption

processes are dependent on swelling kinetics.

(iii) Non-Fickian or anomalous diffusion that occurs when the penetrant mobility and

polymer segment relaxation are comparable.

If Mt is the mass diffused at time t and M∞ is the mass diffused at infinite time,

then the type of diffusion process occurring is obtained by plotting time verses Mt/M∞,

and then fitting the equation Mt/M∞ = k’tn. If the exponent n is 0.5, the diffusion is

Fickian. Non-Fickian diffusion is observed for 0.5 < n <1.0 (Peppas and Brannon-

Peppas, 1994). Anomalous behavior is seen when phase changes occur during mass

transfer, e.g. when there are components in the system that change from an amorphous to

a crystalline state due to the presence of a plasticizer (the effect of water on amorphous

milk powder). This phenomenon, seen quite often in polymers, is called solvent-induced

crystallization (Neogi, 1996). Anomalous behavior has also been seen in systems that do

not undergo morphological changes associated with the formation of crystals. In this

case the anomalous behavior is associated with slow relaxation of the polymer chains

compared to the migration of the solvent (Duda, 1999).

20

Case I is observed when there are no changes, such as swelling or phase transition

that are associated with diffusion. Non-Fickian diffusion occurs in systems where there

are such physical changes associated with diffusion. In fat, moisture and ethanol

migration through chocolates, one expects either physical changes such as swelling or

phase transition to occur, and would expect non-Fickian diffusion behavior.

2.2 Moisture Migration

Moisture migration will occur in chocolates when regions of different moisture

content are brought in contact with each other. The direction of moisture migration is not

necessarily from higher moisture content to lower moisture content (Cakebread, 1972),

but in the direction of higher to lower water activity. Water activity difference is the

driving force for moisture migration. The water activity (aw) is related to the

thermodynamic chemical potential by equation 2.2 (van den Berg and Bruin, 1981). In

this case, µ is the chemical potential in the sample water vapor; 0iµ is the chemical

potential for pure water vapor.

Due to the strong interaction between the water and components such as sugar

and cocoa solids in chocolate, it is reasonable to assume that the moisture equilibrium

sorption will have a significant impact on the observed molecular diffusion properties.

Some of the water molecules will adsorb to the surface and a second class of water

molecules can be considered to be a mobile fraction of the species. Also, it is reasonable

to assume that there exists an equilibrium between these two species at every location in

chocolate during the diffusion process (Duda, 1999).

21

Consider a system consisting of a concentration c of the mobile species and c1 of

the adsorbed molecules. Fick’s equation for this system in one dimension can be written

as (Weisz, 1967):

2

2

xcD

t'c

∂∂

=∂∂ ( 2.19 )

where c' = c + c1. Since equation 2.19 contains one more variable than equation 2.11, a

solution is not attainable without additional information. For all processes where

equilibrium between mobile and adsorbed species is rapid compared to the overall rate of

the sorption process, an isotherm c1 = f(c) will provide the information necessary to

define the system. In the case of food systems, moisture sorption isotherms are used to

obtain an equilibrium relationship between the water activity and the moisture content.

2.2.1 Moisture Sorption Isotherm

A moisture sorption isotherm is the plot of water content (expressed as mass of

water per unit mass of dry material) of a food versus the water activity (aw) at constant

temperature (Fennema, 1996). The moisture absorption isotherms (MSI) for most foods

follow a sigmoidal curve.

The equation that had been most used to model the moisture isotherm of foods is

the Braunaur, Emmet and Taylor (BET) equation. The BET is equation is given in the

form:

22

)Caa1)(a1(Camm

www

wo

+−−= ( 2.20 )

where m is the moisture content, aw is the water activity, mo is the BET monolayer

moisture value, and C is a constant. The limitation of the BET equation is that it is

applicable only between aw values of 0 and 0.5 (Bell and Labuza, 2000). At the

International Symposium on the Properties of Water (ISOPOW) held in 1983, it was

agreed that the Guggenheim-Anderson and DeBoer (GAB) equation, is the best equation

for modeling moisture sorption isotherms (van den Berg, 1985; Wolf et al., 1985). The

GAB equation is given below:

)CKaKa1)(Ka1(CKamm

www

wo

+−−= ( 2.21 )

where K is the GAB multilayer constant. The GAB model gives a better fit than the

BET, but this is due more to the additional fitting parameter than any improved physical

understanding of sorption. The GAB reduces to the BET equation, when K=1 (Coupland

et al., 2000). The GAB equation gives a good fit from water activity values between 0

and 0.8 (Peleg, 1993). It is interesting to note that most researchers have reported the

moisture sorption isotherm only to water activity values of 0.8. The MSI for chocolates

with different sugars obtained by Ogunmoyela and Birch (1984) are given in Figure 2–4.

The amount of moisture adsorbed by fructose and L-sorbose is significantly higher than

the moisture adsorbed by other sweeteners.

The sorption isotherm for sucrose, cocoa powder and an edible film is given in

Figure 2–5. It can be seen that for water activities greater than 0.75, the amount of

moisture adsorbed by sucrose increases significantly; hence, for aw values greater than

23

0.75 the rate of migration through chocolate containing sucrose should be significantly

higher than at lower water activities.

24

Figure 2–4: Moisture sorption isotherms for chocolate containing different sweeteners for two different lecithin contents: (a) Bournville chocolate, (b) sucrose, (c) β-D-fructose, (d) sorbitol, (e) L-sorbose, (f) maltose hydrate, ( ― ) 0.5% lecithin, (---). In each of these figures, the relative humidity is on the x-axis and the equilibrium moisture content is on the y-axis. Reproduced with permission (Ogunmoyela and Birch, 1984).

25

The MSI for dark chocolate obtained by Biquet and Labuza (1988) is given in

Figure 2–6. Biquet and Labuza (1988) found that equilibrium times for moisture sorption

and desorption ranged between 40 and 60 days as opposed to Ogunmoyela and Birch

(1984) who reported that constant weights were usually observed after 14 days. Biquet

and Labuza (1988) used the GAB equation to model the sorption isotherm of dark

chocolate. The GAB constants for dark chocolate are given in Table 2–2.

Figure 2–5: Moisture sorption isotherm for an edible film and its components. Reprinted with permission from Morillon et al. (2000). Copyright (2000), American Chemical Society)

26

Figure 2–6: Moisture sorption isotherms for dark chocolate at 20°C. The upper curve is for desorption and the lower for adsorption. Reproduced with permission (Biquet and Labuza, 1988)

27

The sorption isotherm obtained for a particular food product is dependent on the

temperature at which it is measured and so the temperature must be specified. Molecular

motion increases with temperature and hence the amount of water adsorbed is less for the

same aw with increasing temperatures. The effect of temperature on the moisture sorption

isotherm seems to follow the Clausius-Clapeyron equation (Bell and Labuza, 2000):

−=

21

s

1w

2w

T1

T1

RQ

)a()a(ln

…(24)

where R is the universal gas constant (1.987 cal mol-1 K-1), Qs is the heat of sorption (cal

mol-1), (aw)1 is the water activity at temperature T1 (K) and (aw)2 is the water activity at

temperature T2 (K). The moisture sorption isotherm of chocolates at different

temperature is given in Figure 2–7 (Kim et al., 1999), and it can be seen that the

maximum amount of water adsorbed at a particular water activity is at the lowest

temperature (20oC). However, there is not a large variation in the amount of moisture

adsorbed in this temperature range because the equilibrium moisture content is very low

for chocolate.

Table 2–2: GAB constants at each aw value for dark chocolates at 20 oC (Biquet and Labuza, 1988)

Mode of soprtion

Initial aw aw range of test

Mo gH2O/100g

solids

K C

Adsorption 0.01 0.01 – 0.808 0.545 1.024 103.857

Desorption 0.81 0.754 – 0.112 1.067 0.785 34.262

28

Figure 2–7: Moisture sorption isotherms for chocolate at various temperatures. Reproduced with permission (Kim et al., 1999)

29

2.2.2 Measurement Methods for Moisture Migration

The rate of moisture migration through chocolate is very slow compared to most

food products due to the hydrophobicity of the continuous lipid phase. This is similar to

the situation for plastic films. Hence, the rate of moisture migration thorough chocolate

coatings can be measured using the methods used for plastic films. The methods

commonly used for measuring the water transmission rates through plastic and edible

films are discussed below.

2.2.2.1 Gravimetric Technique (ASTM E96)

The most commonly used method for determining the permeability or water vapor

transmission rate (WVTR) of edible films is the ‘cup’ method (ASTM, 1995). The setup

consists of a test cell covered with the test film (Figure 2–8), which is placed in a

chamber with controlled temperature and humidity. The test can be done in two ways,

the desiccant method and the water method. In the desiccant method, the desiccant is kept

in the test cell and the cell is kept in a controlled humidity chamber, while in the water

method the cell contains water or saturated salt solution and the cell is kept in a

controlled humidity chamber. In both cases, the weight of the cell is taken at definite

time intervals until steady state is reached. The cell will gain weight if desiccant is kept

inside the cell and will lose weight if water or salt solution is kept inside the cell. The

weight gain or loss is plotted verses time. After a certain time, the rate of weight gain or

loss becomes constant, meaning steady state has been reached. Once steady state is

30

reached, at least eight measurements should be taken to determine the steady-state

transmission rate (ASTM, 1995). This rate divided by the area of the film gives the

WVTR.

Major sources of error in this method are: (1) film support and sealing, and (2) the

effect of intervening air spaces in determining the exact vapor pressure differences across

the film (Stannett and Yasuda, 1965). The sealing suggested by the ASTM method is

quite cumbersome. Gennadios et al. (1994) in their work with edible films found that

using an O – ring and silicon grease gave a good seal.

The gravimetric method assumes that the time for water to diffuse through the

stagnant air is negligible compared to the time taken for it to diffuse through the film,

which should be a reasonable assumption with chocolate. One way to maintain proper

humidity is to maintain an air velocity over the specimen to be at least ten times the

permeance of the specimen expressed in perms, but not to exceed 600 ft/min (3.05 m/s)

(McHugh et al., 1993). Erroneous results may be obtained if the proper humidity

gradient is not maintained during the test. Biquet and Labuza (1988) found that keeping

the desiccant on different sides of the film changed the WVTR values (Table 2–3). When

Figure 2–8: Schematic of the test cell for the ASTM E-96 method

31

the desiccant is kept inside the cell, the air gap between the desiccant and the film is very

small, so the humidity at the interface of the film and the air in the cell side is 0%, on the

other hand when the desiccant is kept outside in the desiccator, the air gap between the

film and the desiccant is large and the humidity at the interface of the film is not 0%, as is

assumed during the test. Thus the actual vapor pressure difference in the two

configurations is different and therefore their results varied.

The test cell should be made of non-corroding material and should be

impermeable to water vapor. A large and shallow dish is normally preferred, but the size

and weight is limited when an analytical balance is chosen to detect small weight

changes. The mouth of the cell should be as large as possible and should not be less than

4.65 in2 (3000 mm2) (ASTM, 1995).

The gravimetric technique seems to be the method used in most laboratories for

testing the water vapor transmission rate for edible films. The major advantage of this

method is its simplicity, even though it takes a long time to obtain results.

Table 2–3: Effective water vapor permeability constants (keff) and water vapor transmission rate (WVTR) for a dark chocolate film at 20oC as determined by the cup method in two different configurations: (1) Drierite in the cup and salt solution outside, (2) saturated salt solution in the cup and Drierite outside (Biquet and Labuza, 1988) Configuration Thickness (mm) WVTR (g·m-2·day-1) keff (g·mil·m-2·day-1·mmHg-1)1 1.114 3.21 10.73 2 1.020 0.30 0.86

32

2.2.2.2 Infrared Sensor Technique (ASTM F-372)

This is a rapid method for determining the WVTR of materials. The cell consists

of two chambers, which are separated by the test film. On one side of the cell a pad

saturated with water or salt solution is kept and on the other side dry air is circulated.

The circulating dry air picks up the moisture permeating through the film and this air is

then passed through an infrared detector. The infrared detector gives a voltage based on

the amount of moisture present. To convert this voltage into WVTR values, the voltage

for an unknown sample is compared with the voltage of a standard reference material

(ASTM, 1995). Instruments for measuring the WVTR using this method are

manufactured by Modern Controls Inc. (MOCON, Minneapolis, MN), and are called the

Permatran – W series. The range of WVTR is from 1 g m-2 day-1 to 100 g m-2 day-1

(MOCON, 1984). Using aluminum foil to reduce the effective area of diffusion can

extend this range on the high end. Work in our laboratory with plastic films has shown

that masking is not effective and can give erroneous results, although Kester and

Fennema (1989) have found masking to be effective in their work.

The advantage of this method is that it gives results in much shorter time than the

traditional ‘cup’ method, and the results are quite repeatable. However, high cost and

possible problems with moisture condensation in the sensor present a major disadvantage.

The condensation problems occur mainly with highly permeable films, and if caution is

not observed, this will give erroneous results for films that are tested subsequently.

However, this should not be the case with chocolate. In terms of the pressure differential,

similar disadvantages as the gravimetric method exists. Other methods, such as

33

coulometric and spectrophotometric methods also exist and have been discussed by

McHugh and Krochta (1994).

2.2.3 Factors Affecting Moisture Migration Through Chocolates

The factors influencing moisture migration through chocolates are vapor pressure

differential (or water activity difference), temperature, composition, thickness, solid fat

content and structure. The work done by Biquet and Labuza (1988) and Landmann et al.

(1960) are the main works in this area, and most of the following discussion is taken from

these two studies.

Thickness: Fick’s law (equation 2.8) suggests that with increasing thickness, the water

vapor transmission rate (WVTR) should decrease. This behavior has been observed

when the thickness of a dark chocolate film was increased from 0.612 to 0.926 mm, but

not when the thickness was increased from 0.926 to 1.192 mm (Table 2–4). Increasing

the thickness of a cocoa butter film from 1.59 to 2.92 mm (Table 2–5) did not change its

WVTR (Landmann et al., 1960). One possible explanation for a deviation from Fick’s

law is that the researchers did not study the steady state diffusion and all the data they

had gathered were in the sorption regime. In the sorption regime the rate of moisture

gain will be the same initially irrespective of the thickness. Assuming a diffusion

coefficient of 1 x 10-13 m2 s-1 (Biquet and Labuza, 1988), it will take about 90 days for a

1.5 mm sample and 350 days for a 3mm sample to reach equilibrium with moisture.

Landmann et al. (1960) have taken data for only 19 days, which means that they were

34

likely still in the sorption region, and hence they did not see any difference in the water

vapor transmission rates.

Table 2–4: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) of a dark chocolate at 20oC for three different thicknesses (Biquet and Labuza, 1988) RH (%) ∆P (mmHg) Thickness

(mm) WVTR (g·m-

2·day-1) keff (g·mil·m-2·day-

1·mmHg-1) 0 - 80.8 0 - 14.13 0.612 3.21 5.56

0.926 2.01 5.28

1.192 2.30 7.74

35

Table 2–5: Permeability data from (Landmann et al., 1960)

Product Temperature (oC)

Vapor pressure gradient (mmHg)

Film thickness

(mm)

Moisture transferred x

103

(mg·cm-2·h-1)

Permeability constant x 1012

(g·cm·s·cm-

2·mmHg-1)

Cocoa butter 3.0 5.7 - 0 1.57 4.37 33.3

5.7 - 0 1.58 4.86 37.2

26.7 11.5 - 0 1.58 3.84 14.8

19.8 - 0 1.60 2.61 5.8

19.8 - 0 1.60 2.95 6.6

26.3 - 0 1.59 21.6 35.7

26.3 - 0 1.63 18.0 31.0

26.3 - 0 1.96 17.7 36.7

26.3 - 0 2.14 26.9 60.5

26.3 - 0 2.16 16.3 36.7

26.3 - 0 2.91 25.7 78.7

26.3 - 0 2.92 26.3 81.6

Cocoa butter in lower melting polymorphic

form

3.0

5.7 - 0

1.54

41.6

310

4.3 - 0 1.51 41.7 410

4.3 - 0 1.52 58.8 580

Chocolate liquor

26.7 11.5 - 0 1.71 1.94 8.0

19.8 - 0 1.70 3.82 9.1

19.8 - 0 1.69 5.60 13.3

26.3 - 0 1.63 32.3 556

26.3 - 0 1.61 30.9 526

Sweet milk chocolate,

coating type

26.7

19.8 - 5.9

1.78

4.03

14.3

19.8 - 5.9 1.75 4.29 15.0

26.3 - 0 2.08 49.2 1080

26.3 - 0 1.73 64.9 1190

36

Structure: No systematic study has been made on the effect of structure on moisture

migration through chocolates. Normally it is expected that a close-packed fat crystal

network will allow less moisture to pass through the interstices (Kempf, 1967). One

might expect that the α polymorph should have the higher permeability than the β’

polymorph, which in turn will have a higher permeability than the β polymorph. In

contrast, Kester and Fennema (1989) in their studies with a mixture of hydrogenated

rapeseed oil and soybean oil found that the α-polymorph (the lowest melting polymorph)

was the most resistant to moisture transmission compared to other polymorphic forms

(β and β’). There are a couple possible reasons for this behavior. First, the shape of the

crystal may be different which might cause a change in tortuosity and thus change the

water diffusion rate. Secondly, the method used also can lead to differences in the

observed data. Kester and Fennema (1989) used a filter paper as a supporting matrix for

the fat when measuring the WVP. The interaction of the different polymorphs with the

filter paper may also lead to differences in the observed water diffusion behavior.

Preliminary experiments in our laboratory had shown that there is a difference in the

water vapor transmission rate data with and without a support. Landmann et al. (1960)

obtained a lower melting polymorph by quickly chilling melted cocoa butter from 60oC

to –18oC. The WVTR of the low melting polymorph was much greater than that

obtained by proper tempering (Table 2–5), but they did not confirm the type of

polymorph experimentally.

37

Vapor pressure: From the data in Table 2–5 and Table 2–6, it can be seen that increasing

the vapor pressure gradient increases the WVTR. With about the same vapor pressure

gradient 0 - 5.9 vs. 9.5 - 14.1 mm Hg, but the latter with higher absolute pressure, the

WVTR almost doubles. Similar results are seen when the pressure difference was 9 mm

Hg. This shows that the diffusion coefficient is dependent on the concentration of water.

Similar results have been seen with fatty acid films (Kamper and Fennema, 1984;

Fennema et al., 1994) and in hydrophilic polymers (Myers et al., 1961; Morillon et al.,

1999; Morillon et al., 2000).

When the relative humidity difference is increased to 100%, there is a large

change in the permeability constant (Table 2–5). As can be seen from the isotherms of

sucrose and cocoa powder (Figure 2–4), they tend to adsorb a significant amount of

moisture at high humidity, which will tend to swell the coating and change its structure.

Swelling and structural changes will affect the permeability behavior of the film (Rogers,

Table 2–6: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) as a function of the water vapor pressure gradient (∆p) for dark chocolate at 20oC (Biquet and Labuza, 1988) RH (%) ∆P (mmHg) Thickness

(mm) WVTR (g·m-

2·day-1) keff (g·mil·m-2·day-

1·mmHg-1) 0 - 33.0 0 - 5.79 0.594 1.20 4.94

0 - 54.4 0 - 9.54 0.598 1.34 3.38

0 - 64.8 0 - 11.40 0.608 2.74 5.85

0 - 80.8 0 - 14.13 0.612 3.21 5.56

33.0 - 80.8 5.79 - 14.13 0.590 2.35 6.65

54.4 - 80.8 9.54 - 14.13 0.597 2.12 11.02

38

1985), which may be the reason for the observed permeability behavior of chocolates and

coatings at high humidity.

In a coating film containing a fat, cocoa solids and sugar, Morillon et al. (2000)

found that the film permeability was much higher when liquid water was used as

compared to water vapor. This may be because the actual vapor pressure at the interface

of the coating was not 100% when water vapor was used. The existence of such

problems was pointed out in the section on the methods of measurement.

Composition: The presence of non-fat particles does not influence the WVTR or the

permeability constant at low vapor pressures, but at high relative humidity it drastically

changes the WVTR (Table 2–5). This behavior can again be explained in terms of the

moisture sorption isotherm. The data thus suggests that most of the diffusion takes place

through the hydrophilic particles.

The solid fat content is another important factor. Studies by Landmann et al.

(1960) and Talbot (1994) show that fats with high solid fat content are very good

moisture barriers. The fat present in the solid state is more tightly packed than in the

liquid state and hence will allow less amount of moisture to migrate.

Temperature: In theory, the WVTR should have an Arrhenius-type relationship with

temperature, but this is not observed (Table 2–7). For chocolates or cocoa butter, a

change in temperature is accompanied by phase and structural changes. Hence, there are

other factors, apart from the mobility of the penetrating molecule, that change with

temperature. Increasing the temperature increases the diffusivity of the penetrant

molecule and also decreases the solid fat content, both of which increase migration rate.

39

In judging the effect of temperature on WVTR, not only the temperature at which the

data is recorded will matter, but also the thermal history of the chocolate film will make a

difference, since the thermal history will determine the structure of the film.

Landmann et al. (1960) found that on reducing the temperature from 26.7 to 3oC,

there was no significant change in the permeability coefficient of hydrogenated

cottonseed oil films. Biquet and Labuza (1988) in their studies with dark chocolate found

that changing the temperature from 10 to 20oC did not change the WVTR, but increasing

it to 26oC almost tripled the rates (Table 2–7). SFC at 10 and 20°C is comparable and the

change in diffusivity through the liquid is small. Hence, the WVTRs obtained at these

two temperatures are comparable. On the other hand, changing the temperature from 20

to 26oC reduces the SFC from 90% to 80%, which may account for the large increase in

WVTR at 26oC. It can be seen here that the doubling the liquid fat content from 10 to

20%, almost doubles the peremeablity, which further suggests that the diffusion occurs

mainly through the liquid phase.

Table 2–7: Effective water vapor permeability constant (keff) and water vapor transmission rate (WVTR) of a dark chocolate at 20oC for three different temperatures (Biquet and Labuza, 1988)

RH (%)

∆P (mmHg)

Thickness (mm)

Temperature (oC)

WVTR (g·m-2·day-

1)

keff (g·mil·m-2·day-

1·mmHg-1) 0 - 82.1 0 - 7.58 0.584 10 3.34 5.56

0- 80.6 0 - 14.13 0.612 20 3.21 5.56

0 – 80.4 0 - 20.27 0.593 26 10.38 12.14

40

2.2.4 Mechanism of Moisture Migration

Chocolate contains hydrophilic particles embedded in a continuous fat phase.

Water molecules must diffuse through the fat phase (represented by the gradient in

background shading in Figure 2–9) to come in contact with the hydrophilic particles.

Hence, the rate of migration will depend on the structure of fat phase and the presence of

liquid fat. It was pointed out before that the moisture would have less resistance

migrating through the liquid lipid phase than the solid lipid phase. When the moisture

comes in contact with the hydrophilic particle, it gets adsorbed there. The amount of

moisture that will be adsorbed by the hydrophilic particle can be obtained from the

moisture sorption isotherm. Swelling of these particles may occur (shown in Figure 2–9

as a halo around the particles), which may cause cracking and accelerate moisture

migration. In addition, water adsorption may result in surface dissolution and accretion of

sugar, and crystallization of amorphous milk powder, which could lead to sugar bloom.

Once the hydrophilic particle is saturated, the moisture moves in the direction of lower

water activity.

41

Figure 2–9: Conceptual model for the diffusion of moisture through chocolate. The gradient in background shading is representative of the moisture content. The swelling of hydrophilic particles is suggested by the halos about the particles

42

The diffusion rate of moisture in the hydrophilic phase is much faster than that

through the hydrophobic phase. Evidence of this is presented in Table 2–5 where the

permeability is much greater through sweet milk chocolate containing non-fat particles as

compared to cocoa butter alone. Hence the rate-limiting step is diffusion through the

hydrophobic, lipid phase. From equation 2.8, the flux is inversely proportional to the

diffusion path length. Therefore, the interparticle distance, or packing density of the non-

fat particles will be an important factor in moisture migration rate.

The kinetics of moisture absorption for dark chocolates is shown in Figure 2–10.

From this data, it is evident that the moisture adsorption by dark chocolate demonstrates

anomalous non-Fickian behavior. Biquet and Labuza (1988) suggested that this

anomalous behavior is seen probably because of the crystallization of sugars. However,

in dark chocolate the sugar is in the crystalline form, hence the abnormal behavior would

probably be due some relaxation effects (Duda, 1999), such as swelling of the cocoa

particles as suggested in Figure 2–9.

43

Biquet and Labuza (1988) used sorption methods to determine the diffusion

coefficient of moisture through dark chocolates (Table 2–8). The data shows that the

effective diffusion coefficient is independent of the surrounding relative humidity. The

data obtained by Biquet and Labuza (1988) was used by (Rumsey and Krochta, 1994) to

numerically solve for the moisture diffusion through a model gel covered with a

chocolate film with poor results (Figure 2–11). They used a constant diffusion

coefficient for solving the equations, while from the above discussion one can conclude

that the diffusion coefficient is a function of the moisture content or water activity. Using

diffusion coefficients as a function of moisture content might have given better results.

Figure 2–10: Kinetics of moisture sorption for dark chocolates at 20°C and varying relative humidity. Reproduced with permission (Biquet and Labuza, 1988).

44

Table 2–8: Effective Diffusion Coefficient of moisture through Dark Chocolate Film at 20oC (Biquet and Labuza, 1988)

Sorption mode Initial aw RH (%) Thickness (mm) Deff·1013 (m2s-1)

Adsorption 0.01 75.4 0.607 (0.05) 1.08 (0.22)

64.8 0.594 (0.04) 0.82 (0.31)

Desorption 0.81 54.4 0.607 (0.04) 0.87 (0.09)

33.0 0.599 (0.04) 1.33 (0.13)

45

Antunes and Antunes (2000) modeled the kinetics of moisture sorption using a

non-linear diffusion equation. The diffusivity term used in their model was a linear

combination of two terms. The first term was a constant and the second term contained a

factor that was a function of moisture concentration. They fitted the kinetics of moisture

absorption data by Biquet and Labuza (1988) and found a good fit between the predicted

data and experimental values. Their attempt seemed more like a curve fitting effort

rather than modeling the diffusion behavior with an understanding of the mechanism.

Figure 2–11: Modeling of moisture transfer through a chocolate film. Reprinted from (Rumsey and Krochta, 1994), with permission from Technomic Publishing Co., Inc., copyright [1994])

46

The model however showed that using a diffusivity that is dependent on concentration

gives a better prediction of the actual data.

Moisture does not have much effect on either the cocoa solids or sugar in the

humidity range for which the diffusion coefficients have been determined by Biquet and

Labuza (1988). A more accurate determination of the diffusivity across a wider range of

relative humidity is required. Biquet and Labuza (1988) studied the migration

characteristics through dark chocolates. Studies need to be done for milk chocolates and

other coatings (coatings made with fats other than cocoa butter). The effect of different

ingredients such as emulsifiers in the coating on migration also needs to be studied to

understand the migration mechanism.

2.2.5 Methods of Control for Moisture Migration

Tempering the coating properly will reduce the moisture migration to a great

extent. Since tempering will give rise to a packed structure (i.e. less porous) (Loisel et

al., 1997), it will increase the diffusion resistance and decrease the migration rate. The

others aspects are selection of proper emulsifiers and sweeteners. Linke (1998) has

shown that using 0.3% lecithin and 0.1% PGPR tends to reduce ethanol migration. This

concept should be applicable for moisture migration as well. Non-fat particles tend to

adsorb a large quantity of moisture and swell the chocolate. This destruction of the

structure leads to a faster migration rate. Sweeteners that adsorb less moisture could be

used to delay migration. Research needs to be done on the optimal sweetener and

emulsifier to minimize migration.

47

Moisture migration is affected by the structure and the presence of hydrophilic

particles. Moisture will be adsorbed by the hydrophilic particles in the matrix and will

change the structure. The rate-limiting step for moisture migration will be the migration

through the lipid phase. The moisture migration studies should thus include the effect of

the lipid structure and the influence of moisture on the structural change of chocolate.

2.3 Statement of the Problem

Some of the specific questions that need to be answered are:

a) What are the possible mechanisms for moisture migration through chocolate

coatings?

b) How does composition affect moisture migration (What are the effects of

different types of fats, non-fat particles e.g. sugar and cocoa powder, and the

effect of emulsifiers on moisture migration?

c) How does the presence of water affect the structure of the chocolate coatings and

its functional behavior on wafers?

2.4 Hypothesis

A dark chocolate coating can be considered as a composite with the lipid being

the continuous phase, and the sugar and the cocoa particles, being the discontinuous

phase embedded in the continuous phase. Moisture migrates mainly through the lipid

phase and when comes in contact with the hydrophilic particles (sugar or cocoa powder),

48

it get adsorbed very quickly into the hydrophilic particle and it can be assumed that the

time taken for the moisture to get adsorbed into the hydrophilic particle is negligible

compared to the time taken for the moisture to diffuse through the continuous lipid phase.

The diffusion behavior of moisture in chocolate coatings in one-dimension can be given

by equation 21. If K’ is the partition coefficient between the hydrophilic particles and the

continuous lipid phase and if we assume K’ to be constant at each relative humidity, then

equation (21) can be written as:

2

2

'ss

xc

K1D

tc

∂∂

+=

∂∂

…(25)

When the hydrophilic particles are saturated with water, the diffusion of water

through the coatings can be thought of as the diffusion of gas molecules through a

membrane containing particles. If we assume that the particles are spherical in shape

then the steady-state diffusion coefficient Dss for a dilute system can be given by the

equation (26) (Cussler, 1997):

−φ++

−φ−+

=

0s0s

0s0s

0

ss

D1

D1

D1

D2

D1

D12

D1

D2

DD

…(26)

where D0 is the diffusion coefficient through the continuous media, Ds is the diffusion

coefficient through the spheres, and φ is the volume fraction of the spheres in the

composite material. If the particles does not take part in the diffusion process after

absorbing moisture i.e. Ds is zero then, we have:

49

φ+φ−

=2

)1(2DD

0

ss …(27)

If we consider the other limit in which the diffusion through the spheres is

extremely rapid i.e. Ds ∞, then we have:

φ−φ+

=1

21DD

0

ss …(28)

For this study, the diffusion coefficients obtained should be closer to the value

predicted by equation (27).

The rate of migration will be different for water vapor and liquid water. In the

presence of liquid water some sugar can solubalize with the moisture and diffuse into the

liquid water. The migration of sugar from the chocolate will change the structure of

coating and there will be higher free volume for the moisture to migrate through the

coating. The increase in the free volume will cause an increase in the rate of moisture

migration through the chocolate. Hence, the rate of moisture migration through the

coating in contact with liquid water will be higher than the migration rate when the

coating is in contact with the water vapor.

2.5 Objectives

The overall objective of this study was to understand the mechanisms of moisture

migration through a chocolate-flavored coating. The specific objectives were:

50

1. Understand the mechanism of moisture migration in chocolate coatings.

a. Measure the diffusion through coconut oil.

b. Construct the moisture sorption isotherm for each of the ingredients,

coconut oil, sugar, and cocoa powder, and also for a mixture of coconut oil

+ 0.5% lecithin.

c. Based on the diffusion model, predict the diffusion behavior through a

coating containing either sugar or cocoa powder.

d. Validate the model with data obtained using water vapor to study

diffusion.

e. Determine the effect of coating thickness on the water vapor transmission

rate

2. Study the effect of different ingredients and ingredient proportions on the rate of

moisture migration

a. Determine the effect of the proportion of coconut oil, cocoa powder, sugar

and lecithin.

b. Determine the effect of fat type (the SFC of the fat will influence the rate

of moisture migration)

c. Determine the effect of the sugar type (sucrose, glucose, lactose)

d. Effect of emulsifier type (lecithin, Citrem).

51

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58

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59

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Chapter 3

MECHANISM OF MOISTURE MIGRATION THROUGH CHOCOLATE-FLAVORED COATINGS

3.1 Abstract


investigated. Approaches used by researchers in chemical engineering to study diffusion

through heterogeneous systems were applied to study moisture diffusion in chocolate-

flavored coatings. It was found that the unsteady-state diffusivity of moisture through the

coatings could be estimated from the diffusivity of water through the continuous fat

phase and the partition coefficient of moisture between sucrose and cocoa powder and the

fat phase. The partition coefficient is the local equilibrium between the concentration in

the continuous phase, c, and the concentration in the dispersed phase, cp (K = cp/c). For

coatings containing just oil and cocoa powder, the diffusion of moisture in the dispersed

phase, i.e. cocoa powder, occurred through the cocoa powder particle. When lecithin was

added, the water molecules diffused through the cocoa powder particles as well as along

their surfaces. With the diffusion of moisture, there were structural changes that altered

the diffusivity of moisture through the coating. These structural changes occurred

because the sucrose particles dissolved in the migrating moisture and the swelling of

cocoa powder in the presence of moisture.

61

3.2 Introduction

The diffusion of moisture in food materials is of fundamental importance for

processing and storage (Saravacos and Maroulis, 2001). The transport of moisture into or

from food materials is an important factor in controlling food quality, chemical reactions

and microbial growth during storage (Labuza and Hyman, 1998; Saravacos and Maroulis,

2001). One way to slow down moisture transport is to use an edible barrier between the

two domains of a food material. Chocolate and chocolate-flavored coatings are used as

an edible film in many applications (Morillon et al., 2000; Ghosh et al., 2002). An

understanding of the diffusion mechanism through chocolate coatings can help in

formulating more moisture resistant barriers.

The transport of water, between different domains of a food material or into the

environment, occurs mainly by diffusion (Labuza and Hyman, 1998; Saravacos and

Maroulis, 2001). The diffusion process is modeled using Fick’s law and the term that is

generally used to compare diffusion rates in different systems is the diffusion coefficient,

D. Most diffusivity data available for food systems is the apparent diffusivity and there is

no understanding of the actual diffusion mechanism. “The word ‘apparent’ reaffirms that

we do not know exactly the mechanism of transport, which in most cases can be quite

complex” (Aguilera and Stanley, 1999).

Most food materials are heterogeneous in nature and the moisture sorption

process involves the adsorption of water molecules into the food material. In such

situations, the water molecule diffusing through the food material will exist as two

distinct species. Some of the water molecules will be adsorbed to the food material and

62

can be considered to be bound or immobile, while a second class of molecules can be

considered to be the mobile fraction. Similar phenomena, i.e., interaction of a mobile and

immobile species, can be found in other systems, such as diffusion of dyes in fiber,

diffusion of solvents in two-phase block copolymers, and diffusion in porous media

(Duda, 1999). Weisz (1967) had proposed a solution for the diffusion problem where

sorption and diffusion are taking place in heterogeneous systems. This solution was

verified using experimental data from the diffusion of dye molecules through a porous

substrate (Weisz, 1967; Weisz and Hicks, 1967; Weisz and Zollinger, 1967; Weisz and

Zollinger, 1968). van der Zanden (2000) used an approach similar to Weisz (1967) for

modeling heat and mass transfer in heterogeneous media with phase transition.

As with many other food systems, only the effective diffusion coefficient data for

chocolate is available in literature (Biquet and Labuza, 1988). Chocolate or chocolate-

flavored coatings are heterogeneous food systems and consist of a continuous fat phase

with the cocoa powder and sucrose as a dispersed phase. The objective of this paper is to

understand the mechanism of moisture migration through chocolate-flavored coatings

and to assess the applicability of the Weisz (1967) model to predict diffusion through a

chocolate-flavored coating.

3.3 Mathematical Analysis of Diffusion Through Dark Chocolate

Let us first consider the structure of dark chocolate (Figure 3–1). It has a

continuous fat phase with sucrose and cocoa powder particles as the dispersed phase. In

63

the following paragraphs, an approach for obtaining the diffusivity of such systems from

the diffusivity data of the continuous phase is outlined.

The diffusion through a homogeneous material, e.g. a homogeneous fat, is given

by Fick’s law (equation 3.1):

∂∂

∂∂

=∂∂

xcD

xtc ( 3.1 )

where D is the fundamental diffusion coefficient (m2 s-1), x is the diffusion length (m),

and c is the concentration (moles m-3). If the diffusivity is independent of concentration,

this equation reduces to equation 3.2:

2

2

xcD

tc

∂∂

=∂∂ ( 3.2 )

64

Addition of any particles to this homogenous material will change the diffusion

coefficient. The change in the diffusion coefficient will depend on the geometry of the

particle and volume fraction of the particles added. The effect of structure and

architecture on the overall diffusion coefficient has been studied for simple geometries.

For a two-phase composite in which spherical particles are dispersed in a continuous

phase, the diffusion can occur in the continuous region between the spheres and through

the spheres themselves. An effective diffusion coefficient (D’) for such a system can be

obtained from the equation 3.3 (Cussler, 1997):

Figure 3–1: Schematic of a structure of dark chocolate

Sucrose Cocoa

65

−φ++

−φ−+

=

0dd

0d

0dd

0d

0

'

D1

D1

D1

D2

D1

D12

D1

D2

DD ( 3.3 )

where D0 is the diffusion coefficient through the continuous media, Dd is the diffusion

coefficient through the spheres, and φd is the volume fraction of the spheres in the

composite material. There can be two extreme cases, in the first case the diffusion

coefficient of the dispersed phase is very small compared to the diffusion coefficient in

the continuous phase, i.e. Dd → 0. In the second situation, the diffusion coefficient

through the dispersed phase is very large compared to the diffusion coefficient in the

continuous phase, i.e. Dd → ∞. If the particles do not take part in the diffusion process

i.e. Dd → 0, then, equation 3.3 reduces to:

d

d

0

'

2)1(2

DD

φ+φ−

= ( 3.4 )

In the case when Dd → ∞, equation 3.3 reduces to equation 3.5.

d

d

0

'

121

DD

φ−φ+

= ( 3.5 )

An interesting aspect that comes out from equations 3.3, 3.4, and 3.5 is that the

diffusion in a composite system is controlled by the diffusion through the continuous

phase (Cussler, 1997).

66

Let us consider the case when the spherical particles in the dispersed phase adsorb

more moisture than the continuous phase. Fick’s equation for such a system in one

dimension can be written as (Weisz, 1967):

2

2'

xcD

t'c

∂∂

=∂∂ ( 3.6 )

where c is the concentration of the moisture in the continuous phase (g cm-3) and c' is the

effective moisture concentration (g cm-3) in the sample and is given by equation 3.7.

ddd' cc)1(c φ+φ−= ( 3.7 )

where cd is the concentration of moisture in the dispersed particles. Since equation 3.6

contains one more variable than equation 3.1, a solution is not attainable without

additional information. For all processes where equilibrium between the mobile and

dispersed phases is rapid compared to the overall rate of the diffusion process, an

isotherm cd = f(c) will provide the information necessary to define the system. In the case

of food systems, moisture sorption isotherms are used to obtain an equilibrium

relationship between the water activity and the moisture content. Assuming that the local

equilibrium between the concentration in the continuous phase, c, and the concentration

in the dispersed phase, cd, is described with a partition coefficient, K (van der Zanden,

2000) as:

mm

ccK ddd

ρρ

== ( 3.8 )

where ρ is the density of the continuous phase, ρd is the density of the spherical particles,

m is the moisture content in the continuous phase, and md is the moisture content in the

spherical particles. Since the equilibrium moisture content of different food constituents

67

differ with water activity, the partition coefficient, K, will vary with water activity.

However, if we assume that the partition coefficient is constant, then, from equations 3.4,

3.6, 3.7 and 3.8, we get:

2

2

dd

'

xc

K)1(D

tc

∂∂

φ+φ−=

∂∂ ( 3.9 )

Equation 3.9 can be used to describe the unsteady state diffusion and the effective

diffusion coefficient can be given by equation 3.10.

K)1('DD

ddeff φ+φ−

= ( 3.10 )

where D’ is given by equation 3.3. Equation 3.10 can also be used to understand

the mechanism of moisture diffusion through the dispersed particles. If the diffusion

coefficient, D’ in equation 3.10 follows equation 3.4, then the diffusion occurs through

the continuous phase, while if it follows equation 3.5, the diffusion occurs through the

spheres. The unsteady state diffusion can be described by equation 3.10 and the steady

state diffusion can be described using equation 3.3.

In the above derivation, it was assumed that the partition coefficient is constant

over the whole range of water activities; the extension towards a non-linear distribution

of K has been described by Smith and Keller (1985). While deriving the unsteady state

diffusivity, it was also assumed that there are no structural changes associated with the

diffusion process. In the case of chocolate-flavored coatings structural changes can occur

due to swelling of cocoa powder or the dissolution of sugar at high relative humidities.

These structural changes in the coating can change the rate of diffusion of moisture

through the coating.

68

3.4 Materials and Methods

3.4.1 Materials

The ingredients needed to perform this study were sucrose, cocoa powder,

lecithin, and coconut oil.

3.4.1.1 Sucrose

Crystalline sucrose (pure cane extra fine granulated sucrose with purity ~ 100%)

was obtained from Florida Crystals (Palm Beach, FL). The particle size of granulated

sucrose was larger than 100 µm and needed to be ground into a particle size range that is

typically present in chocolate-flavored coatings (average size 24-28 µm). The sucrose

crystals were therefore ground using a jet mill (Model 0101-C6 (S), Jet-O-Mizer, Fluid

Enery Aljet, Plumsteadville, PA) to obtain the desired particle size.

When operating the jet mill, the inlet air pressure was 120 psi, the pusher nozzle

was set at 100 psi, the first grinder nozzle was set at 100 psi, and the second grinder

nozzle was set at 90 psi. The flow rate dial was set at 25. An air compressor (Model #

C1071080VMSA, Campbell Hausfeld, Harrison, OH) supplied the high-pressure air at

the inlet of the jet mill.

To avoid clumping, the sucrose was dried immediately after grinding in a vacuum

oven (National Appliance Co., Portland, OR), maintained at 60oC and a vacuum of 20”

Hg, for 10-12 hours. The dried sucrose was transferred into airtight containers, and was

stored in a desiccator cabinet at 18oC.

69

3.4.1.2 Cocoa Powder

Defatted cocoa powder in pellet form was obtained from Comet Specialty

Ingredients Co. (Freeport, TX). The cocoa powder pellets were ground using a jet mill

(Model 0101-C6 (S), Jet-O-Mizer, Fluid Enery Aljet, Plumsteadville, PA) to obtain cocoa

powder particles with particle size less than 20 µm. The operating conditions were the

same as that for sucrose, except for the feed rate, which was set at 35. The cocoa powder

was dried in a vacuum oven (National Appliance Co., Portland, OR), maintained at 60oC

and a vacuum of 20” Hg, for 10-12 hours. The dried cocoa powder transferred into

airtight containers, and was stored in a desiccator cabinet at 18oC.

3.4.1.3 Coconut Oil

Coconut oil (Victory 76, Lot# 49815, Formula # F00880) was obtained from

ACH Humko (Cordova, TN). The melting point of the coconut oil was 24oC, and the

solid fat content at 18.5oC was 54%.

3.4.1.4 Lecithin

Granular lecithin (purity ~ 97%) was obtained from Acros Organics (Fisher

Scientific, Pittsburgh, PA). Lecithin is made of a mixture of phospholipids with

phosphatidyl-choline, phosphatidyl-ethanolamine and phosphatidyl-inositol as its main

components. The lecithin had a moisture content of less than 0.1%.

70

3.4.2 Experimental Design

To validate the model proposed is section 3.3, and to determine the mechanism of

moisture migration through chocolate-flavored coatings, two series of experiments were

performed. In the first series, coatings were prepared using three levels of cocoa powder

(20, 30 & 40% w/w) and two levels of sucrose (30 & 40% w/w) in coconut oil. In the

second series, the coconut oil from the first series was replaced by coconut oil + 0.5%

lecithin. The unsteady-state moisture diffusivity was determined for the above-

mentioned coatings and also for coconut oil and coconut oil + 0.5% lecithin. The

experimental design is shown in Table 3–1. The diffusivity at each composition was

determined in duplicates.

From equation 3.10, it can be seen that the unsteady-state diffusivity depends on

two variables, the partition coefficient K, and the volume fraction of the dispersed phase

φd. Four different values of partition coefficient were obtained, two values for sucrose

and cocoa powder with coconut oil, and another two values of K with coconut oil + 0.5%

Table 3–1: Experimental design for the moisture diffusion experiments

Coconut Oil Coconut Oil + 0.5% lecithin Sucrose (w/w) Cocoa Powder

(w/w) Sucrose (w/w) Cocoa Powder

(w/w) 0 0 0 0 0 2.5 0 2.5 30 0 30 0 40 0 40 0 0 20 0 20 0 30 0 30 0 40 0 40

71

lecithin. The difference in the two series of experiments will also provide an

understanding of the role of lecithin on moisture migration. Varying levels of φd was

obtained by varying the levels of sucrose and cocoa powder. The weight fraction was

converted into volume fraction by using equation 3.11.

c

c

d

d

d

d

d mm

m

ρ+

ρ

ρ=φ ( 3.11 )

where md is the weight fraction of the dispersed phase, mc is the weight fraction of the

continuous phase, ρd is the density of the dispersed phase, and ρc is the density of the

continuous phase. For calculations, the density values used for each component were:

sucrose 1.5 g cm-3, cocoa powder 1.3 g cm-3, and coconut oil and coconut oil +0.5%

lecithin 0.9 g cm-3.

3.4.3 Measurement of Moisture Content

The moisture analysis of sucrose and cocoa powder was performed according to

the method suggested by Troutman (1999). About 1g of the sample (i.e., sucrose or cocoa

powder) was placed in a Kimble Kimax culture tube with approximately 10 g of a 1:1

formamide (Fisher Scientific, Pittsburgh, PA): methanol (Karl Fischer grade, anhydrous,

VWR Scientific, Pittsburgh, PA) solvent. The weights of the sample and the solvent

were recorded to obtain the dilution factor. A layer of Teflon tape was put around the

threads of the culture bottle. The culture bottles were closed using the cap of the tube.

Application of Teflon tape ensures airtight seal in the culture bottles. The culture

72

bottles were stored in an oven at 50oC for 12 hours. Simultaneously, two culture tubes

containing the solvent and sealed in the same manner as the samples were kept in the

oven at 50oC for 12 hours.

The moisture content for sucrose and cocoa powder was measured using a Karl

Fischer titrator (Model DL 31, Mettler-Toledo GmBH, Switzerland). Duplicate

measurements were done for each sample. Approximately 40 ml of methanol solvent

(Karl Fischer grade, anhydrous, VWR Scientific, Pittsburgh, PA) was added to the titrator

vessel and neutralized using a pyridine-free Karl Fischer reagent (Hydranal-composite 5,

Riedel-de Haën, GmBH. Seelze, Germany). The pyridine-free reagent contained

imidazole, sulfer dioxide and iodine. The modified Karl Fischer reaction, due to change

in the components of the Karl Fischer reagent, is given by equation 3.12

I)RNH(2RSO)RNH(OHIRSO)RNH(RSO)RNH(RNROH

4223

3

+→++→+

( 3.12 )

Triplicate measurement of the reagent concentration was made using a water

standard (Hydranal-water standard 10.0, Riedel-de Haën, GmBH. Seelze, Germany) to

determine the concentration of the reagent. For moisture content determination of cocoa

powder and sucrose, about 1 ml of the solvent was drawn from the culture tube and added

to the titration vessel. The weight of the sample was determined by weighing the syringe

before and after delivery of solvent using an Ohaus Galaxy 200 balance (Ohaus

Corporation, Florham Park, NJ), with an accuracy of ±0.0001g. The amount of moisture

73

in the sample was determined automatically by the Karl Fischer instrument using

equation 3.13.

% moisture = strength of the Karl Fischer reagent (mg water/ml reagent) x ml Karl Fischer reagent added/mg sample

( 3.13 )

The moisture content of the solvent was also determined. The moisture content of the

original sample was determined using equation 3.14.

2

11

2

1

ff

x100

xff

xmoisture% −

+= ( 3.14 )

where x is the moisture content of the solvent with the sample, x1 is the moisture content

of the solvent, f1 is the weight of the solvent, and f2 is the weight of the sample.

The moisture content of the fat samples was in the range of the moisture content

of the solvent being used. Due to dilution factor a small variation in the moisture content

value reported by Karl Fischer method gave a large variation in the moisture content.

Therefore, the moisture content of coconut oil and coconut oil + 0.5% lecithin was

determined using vacuum oven method. The moisture in the samples was removed by

keeping the samples in a vacuum oven (National Appliance Co., Portland, OR),

maintained at 60oC and a vacuum of 20” Hg, for 24 hours. The weight of the samples

before and after drying was taken using an Ohaus Galaxy 200 balance (Ohaus

Corporation, Florham Park, NJ), with an accuracy of ±0.0001g. The moisture content of

the samples were determined using equation 3.15.

100xm

mmmoisture%d

di −= ( 3.15 )

where mi is the initial weight, md is the weight of the dried sample.

74


The moisture sorption isotherm for ground sucrose, cocoa powder, coconut oil,

and coconut oil + 0.5% lecithin was determined at 20oC. Saturated salt solutions of

lithium chloride (LiCl), magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2),

potassium iodide (KI), sodium chloride (NaCl), ammonium chloride (NH4Cl), and

potassium chloride (KCl) were used to obtain water activity values of 0.113 ± 0.003,

0.331 ± 0.002, 0.544 ± 0.002, 0.699 ± 0.003, 0.755 ± 0.004, 0.792 ± 0.004, and 0.851 ±

0.003, respectively (Bell and Labuza, 2000). Duplicate measurements of equilibrium

moisture content were done at each water activity. The saturated salt solutions were put

in the bottom of Mason jars (~ 473 ml), to a depth of about 1 cm. A square support of

size approximately 3 cm x 3 cm x 6 cm (high) was made from steel wire mesh and placed

in the Mason jar. For sucrose and cocoa powder, approximately 5g of sample was put in

an aluminum weighing dish and placed on the wire mesh support (Figure 3–2). The

Mason jars were kept in a temperature-controlled chamber (Model 310, Imperial III

Incubator, Labline, Inc., Melrose Park, IL) at 20±1oC. The weight of the samples was

taken every day using an Ohaus Galaxy 200 balance (Ohaus Corporation, Florham Park,

NJ), with an accuracy of ±0.0001g, until there was no change in weight (±0.001g) for 3

days. Both cocoa powder and sucrose reached equilibrium with 2-3 days of storage. The

moisture contents of the samples were determined using the Karl Fischer method as

previously described. For the desorption isotherm, the sucrose and cocoa powder

samples were allowed to equilibrate over potassium chloride solution for one week.

Desorption isotherms using these hydrated samples were obtained as explained above.

75

For the sorption isotherm of coconut oil and coconut oil + 0.5% lecithin, the fat

samples was melted at 40oC and poured into aluminum weighing dish and was allowed to

solidify in a room maintained at 18oC. A mark was made in each weighing dish at a

height of 3 mm and the melted fat was poured up to that mark. The weight of the

samples was approximately 6g. The aluminum weighing dishes were put in Mason jars

containing saturated salt solutions. The weight was taken once in every three days using

an Ohaus Galaxy 200 balance (Ohaus Corporation, Florham Park, NJ), with an accuracy

of ±0.0001g. Since the moisture content of coconut oil or coconut oil + 0.5% lecithin

was very low (<0.5%) at any water activity, no noticeable weight change was apparent

after one week. To estimate the time needed for the fat samples to reach equilibrium, the

diffusion of moisture through the fat kept in the pan was assumed to be similar to

diffusion in plane sheet. The time needed to reach equilibrium for diffusion in plane

sheet can be estimated to be l2/D, where l is the thickness of the sheet and D is the

diffusion coefficient. From preliminary experiments, the diffusion coefficient of

moisture in coconut oil was found to be of the order of 10-11 m2s-1. Assuming the

thickness of the fat to be 3 mm, the time needed to reach equilibrium is approximately 11

days. To account for any variation in the thickness of the fat samples, the samples were

allowed to equilibrate for three weeks, after which the moisture content was determined

using the vacuum oven method.

76

3.4.5 Film Preparation

For making the coating, the ingredients were mixed at the ‘blend’ setting in a

blender (Pulse Matic, Oster Corporation, Milwaukee, WI) for two minutes. The total

weight of the ingredients per batch was 250g and two batches were made. To ensure that

Figure 3–2: Setup for measuring moisture sorption isotherm

77

the fat was in the liquid phase, the blender was kept inside a chamber (Model 680A,

Labline Instruments, Inc., Melrose Park, IL) maintained at 40oC. The two batches were

mixed after blending to get the total weight of the coating to be approximately 500g.

During the mixing process, numerous air bubbles were incorporated into the coating mix.

Hence, the coating mix was kept in a vacuum oven, maintained with a vacuum of 20” Hg

and a temperature of 70oC, for 24 hours to remove the air bubbles. When the coating

samples were removed from the oven, settling of the particulate phase was observed. The

melted test samples (approximately 500g) were mixed at the lowest setting, being careful

that no air was incorporated during this process, in a mixer (Model C-100T, Hobart

Corporation, Troy, OH) at 50oC for three hours.

To prepare films for diffusion studies, parchment paper was placed on a

marble slab that had a flat surface. The purpose of the marble slab was to absorb the heat

from the coating during the solidification process. For coatings containing cocoa powder,

a stainless steel sheet (0.8 mm thick) containing four 9 cm diameter holes was placed on

top of the parchment paper (Figure 3–3). Melted coating was poured into the holes and

allowed to solidify for about 30 minutes. The temperature of the room was maintained at

around 16oC. After the coating had solidified, as observed visually, excess material was

removed using a hot spatula to obtain a film of about 1 mm thickness. After removing

the excess material, the whole system was kept in a refrigerator (Model TBX18SLB,

General Electric Co., Louisville, KY) for 10 minutes to solidify the film completely,

which eased the removal of the coating from the parchment paper. The test film was

removed from the mold by cutting the coating around the edges with a hot knife. The

78

thickness of each coating was measured at four different points using a micrometer

(Craftsman, Sears, Roebuck & Co, Chicago, IL). Preliminary experiments had shown

that the films made out of coconut oil or coconut oil + 0.5% lecithin and the films

containing just sugar particles in oil had much lower diffusivity than films containing

cocoa powder. Therefore, to obtain a measurable lag time (see section 3.4.6.2), the films

for coconut oil, coconut oil + 0.5% lecithin, and the coatings containing sugar were made

using a 1.8 mm thick mold. The process was similar to that used for the 1 mm thick film,

except that for the thick films, the time allowed for solidification was about 90 minutes.

The films were stored in a desiccator cabinet, in a room maintained at 18oC. The

coatings with sugar were stored in the desiccator cabinet for at least one week, and the

coatings with cocoa powder for at least two weeks, prior to being used for diffusion

experiments.

79

3.4.6 Measurement of Diffusion Coefficient

3.4.6.1 Controlled Environment Setup

A system was setup to provide the desired temperature and relative humidity

(Figure 3–5). A chamber made using 0.635 cm thick acrylic plastic sheets with an inside

volume of 31400 cm3 (dimension 43.18 cm x 27.94 cm x 26.04 cm) was used for this

study. The chamber had an O-ring gasket and a lid made with 1.27cm thick plastic.

Figure 3–3: Picture of the mold for making the chocolate coatings (hole diameter = 9 cm)

80

The relative humidity inside the chamber was maintained 13 ± 2 % using

saturated potassium hydroxide solution. A fan (type U920IB, Tobishi Kosan Co. Ltd.,

Japan) was used to create convection inside the chamber, which allowed the humidity

inside the chamber to reach equilibrium quickly after any disturbance. The plastic

chamber was kept inside an environmental chamber (Model 680A, Labline Instruments,

Inc., Melrose Park, IL) maintained at 16.5oC. The fan used for convection and the

absorption of moisture by KOH solution, generated heat and so the temperature inside the

plastic chamber was maintained at 18.5 ± 0.5oC.

The temperature and relative humidity within the chamber was monitored every

five minutes using a temperature and relative humidity probe (Model HMP35C,

Campbell Scientific, Inc., Logan, UT) and a data logger (21X Micrologger, Campbell

Scientific, Inc., Logan, UT). The temperature and relative humidity inside the chamber

during a typical experiment is shown in Figure 3–4. The spikes in the relative humidity

line show the times when the chamber was opened for weighing the samples. The extent

of the change in the humidity depended on the humidity of the atmosphere outside.

81

3.4.6.2 Experimental Procedure

For measuring the diffusion coefficient, saturated salt solution (NaCl or KCl) was

put at the bottom of Thwing Albert cups (Thwing Albert, Philadelphia, PA) to obtain a

relative humidity of approximately 75% (NaCl) or 85% (KCl). The test sample was then

carefully placed in the Thwing Albert cups (outer diameter 9.2 cm, inner diameter 7.65

cm, and depth 1 cm). The exposed area of the coating was 46 cm2. The edges of the

sample were sealed using an excess amount of melted sample material. The cups were

immediately put in the plastic chambers. The weight of the samples was measured at

0

5

10

15

20

25

30

0 50 100 150 200

Time (h)

Tem

pera

ture

(C) o

r Rel

ativ

e H

umid

ity(%

)

TemperatureRelative Humidity

Figure 3–4: Temperature and relative humidity inside the plastic chamber

82

regular intervals using an analytical balance (AB105, Mettler Toledo, Switzerland) with

an accuracy of 0.0001g.

The unsteady state diffusivity of moisture through the test films was

obtained using the time lag method (Vieth, 1991). The unsteady state diffusivity (Deff)

through a sample of thickness l, can be determined from the x-intercept of the straight

line portion of the weight loss versus time curve (Figure 3–6) using the following relation

(Equation 3.16):

Figure 3–5: Setup for measuring the diffusivity of the fat coatings

83

lag

2

eff t6lD = ( 3.16 )

3.4.7 Water Vapor Permeability

The water vapor permeability (WVP) was calculated using equation 3.17:

mmHgdaymmilg

pAlslopeWVP 2∆•

•= ( 3.17 )

Time

Wei

ght l

oss

tlag

Curve from Experimental Data

Figure 3–6: Approach to steady state for a coating using the time lag method

84

where “slope” is the slope of the straight line portion of the time-weight loss curve, A is

the area of the film (m2), l is the thickness of the coating (mil) and ∆p is the vapor

pressure difference (mm Hg).

3.5 Results and Discussion


The moisture adsorption and desorption isotherm for cocoa powder and sucrose

are shown in Figure 3–7. It can be clearly seen that the cocoa powder adsorbs more

moisture than sucrose at water activity levels below 0.85. The moisture content at each

water activity represents the average value of two replications. The sorption isotherms of

cocoa powder have a sigmoidal shape, which is typical of most food materials (Iglesias

and Chirife, 1982). The sorption isotherms of cocoa powder also show that the

adsorption and desorption curves exhibited hysteresis, i.e., the moisture content at a

particular water activity was higher for the desorption curve than for the adsorption

curve. However, no hysteresis was observed in the sorption isotherm of sucrose. Cocoa

powder has a very porous structure (Garbolino, 2002) and the moisture goes into these

pores during adsorption. The moisture present in these pores can remain trapped during

the desorption process. On the other hand, crystalline sucrose has a very ordered

structure with close molecular packing, therefore, no hysteresis is observed for sucrose.

85

The moisture adsorption isotherm of coconut oil, sucrose, and a mixture of

coconut oil +0.5% lecithin is shown in Figure 3–8. Oils are hydrophobic materials and

therefore the moisture adsorbed by the oils at any water activity is very low (<0.1%).

Since lecithin is a hydrophilic substance and can adsorb large quantities of moisture

(Elworthy, 1961), the presence of lecithin increases the moisture sorption capacity of oil.

0

2

4

6

8

10

12

14

16

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water Activity (aw)

Moi

stur

e co

nten

t (%

d.b

.)

sucrose-adsorptioncocoa powdersucrose - desorptioncocoa powder - desorption

Figure 3–7: Moisture sorption isotherm of sucrose and cocoa powder at 19oC

86

The partition coefficient for sucrose and cocoa powder with respect to coconut oil

or coconut oil + 0.5% lecithin was calculated using equation 3.8. The partition

coefficient values at different water activity levels are shown in Table 3–2. From Figure

3–8, it can be seen that at a water activity of 0.75, the moisture content of sucrose is less

than that of coconut oil + 0.5% lecithin. However, the partition coefficient, which is the

ratio of equilibrium concentrations is greater than 1. The reason for the apparent

discrepancy is because the moisture content ratio is multiplied by the ratio of the

densities. The density ratio for sucrose and coconut oil + 0.5% lecithin is 1.67 and the

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water Activity

Moi

stur

e co

nten

t (%

db)

sucrosecoconut oilcoconut oil + 0.5% lecithin

Figure 3–8: Moisture sorption isotherm of sugar, coconut oil, and a mixture of coconut oil + 0.5% lecithin

87

equilibrium moisture content ratio at an aw of 0.75 is 0.63, the product of these two

numbers gave a K value of 1.06.

The partition coefficient for cocoa powder is much higher than sucrose at any

given water activity. Thus there will be a larger change in the diffusivity values of a

coating when cocoa powder is added. The partition coefficient is highest for cocoa

powder at a water activity level of 0.75, and for sucrose at a water activity of 0.85. Thus

for getting a large change in the diffusivity values, the diffusion of coatings containing

sucrose were measured at a water activity of 0.85 and the coatings containing cocoa

powder were measured at 0.75.

3.5.2 Diffusion Coefficients

The diffusivity of moisture through coconut oil at 18.5oC was 3.96 x 10-11 m2s-1

and that of coconut oil + 0.5% lecithin is 3.4 x 10-11 m2s-1. From these diffusion

coefficients and the partition coefficient values one can calculate the diffusivity of

coatings containing sucrose or cocoa powder using equation 3.10. The partition

coefficient values for both sucrose and cocoa powder vary with water activity. Thus a

Table 3–2: Partition coefficient of sucrose and cocoa powder with respect to oil or oil + 0.5% lecithin at different water activities

Coconut oil Coconut oil + 0.5% lecithin Water Activity Sucrose Cocoa Powder Sucrose Cocoa Powder

0.11 3.87 135.68 2.83 99.280.75 2.68 202.2 1.06 80.280.85 11.98 189.3 4.45 70.26

88

constant partition coefficient cannot be assumed for a coating system containing sucrose

or cocoa powder. However, for simplicity of calculations, an average value of the

partition coefficients were assumed and substituted in equation 3.10. The D’ value in

equation 3.10 was calculated at both the extremes using equations 3.4 and 3.5.

3.5.2.1 Diffusion of Moisture through Coatings Containing Cocoa Powder

The weight loss data for the coatings made with cocoa powder are shown in

Figure 3–9, Figure 3–10, and Figure 3–11. The diffusion coefficient predicted by

equation 3.10 is compared to experimental data in Figures 3–12 and 3–13. From these

figures it can be seen that the greatest change in diffusivity occurs on addition of the first

10% of volume of cocoa powder. Since the partition coefficient K is large in this case, the

addition of cocoa powder has a large effect on the unsteady state diffusivity through it

ability to adsorb moisture. On the other hand, the diffusion coefficient of moisture

through the dispersed phase does not have a great effect on overall moisture transport

(compare the lines for equation 3.4 with that of equation 3.5 in figures 3.4 and 3.5).

For the coatings containing cocoa powder (Figure 3–12), the experimental values

were closer to the values obtained by using equation 3.5 for D’ in equation 3.10. This

suggests that diffusion of moisture occurred through the cocoa particles. When lecithin

was added to the coatings, the experimental diffusivity values fell between the values

predicted by substituting either equation 3.4 or 3.5 into equation 3.10 (Figure 3–13). This

is likely due to a decrease in K resulting from the greater affinity of the oil phase for

water in the presence of lecithin.

89

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100 120 140 160

Time (h)

Wei

ght l

oss (

g)

20%cp (0.723)20%cp (0.750)20% cp + 0.5% l (0.770)20% cp + 0.5% l (0.952)

Figure 3–9: Weight loss data for coatings containing 20% cocoa powder. The numbers in brackets are the thickness of the coatings in mm. (cp = cocoa powder, l = lecithin)

90

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 20 40 60 80 100 120

Time (h)

Wei

ght l

oss (

g)

30%cp (0.763)30%cp (0.968)30% cp + 0.5 l (0.874)30% cp + 0.5%l (0.785)


91

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120 140 160 180

Time (h)

Wei

ght l

oss (

g

40% cp (0.775)40% cp (1.022)40% cp + l (1.005)40% cp + l (0.842)40% cp + l (1.063)


92

0

5E-12

1E-11

1.5E-11

2E-11

2.5E-11

3E-11

3.5E-11

4E-11

4.5E-11

0 0.1 0.2 0.3 0.4 0.5

Volume fraction (φ)

Diff

usio

n C

oeff

icie

nt (m

2 s-1)

equation 3.4

equation 3.5

experimental

Figure 3–12: Predicted versus the experimental data for coatings containing cocoa powder. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.

93

3.5.2.2 Diffusion of Moisture through Coatings Containing Sucrose

The weight loss data for sucrose are given in Figures 3–14 and 3–15. The

partition coefficient, K, in this case is much lower than for cocoa powder, so there is

much less change to the unsteady-state diffusivity with the addition of sugar.

The moisture sorption isotherm for sucrose shows that the moisture adsorption

rate increases sharply for water activities above 0.8. Therefore, the moisture diffusivity

through coatings containing sucrose was studied at two different water activities 75% and

85%. The diffusivity data for coating containing sucrose and coconut oil, when relative

humidity is 75%, show that the experimental diffusivity follows the values predicted by

0

5E-12

1E-11

1.5E-11

2E-11

2.5E-11

3E-11

3.5E-11

4E-11

0 0.1 0.2 0.3 0.4 0.5


Diff

usiv

ity (m

2 s-1)

equation 3.4equation 3.5experimental

Figure 3–13: Predicted versus the experimental data for coatings containing cocoa powder plus lecithin. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.

94

equation 3.4 (Figure 3–16). This suggests that the diffusion through the sucrose particles

is much lower than that through coconut oil. This is expected because the sucrose

particles are crystalline in nature and the diffusion through the crystals is much slower

than through the semi-solid coconut oil.

0

0.005

0.01

0.015

0.02

0.025

0.03

0 20 40 60 80 100 120 140

Time (h)

Wei

ght l

oss (

g)

30% sucrose (1.803)30% sucrose (2.047)40% sucrose (1.910)40% sucrose (1.743)

Figure 3–14: Weight loss of coatings containing sucrose. The numbers in brackets denote the thickness in mm

95

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 50 100 150 200

Time (h)

Wei

ght l

oss (

g)

30% sucrose + lecithin (2.092)


40% sucrose+ lecithin (2.002)


Figure 3–15: Weight loss of coatings containing sucrose and lecithin. The numbers in brackets denote the thickness in mm

96

When lecithin was present along with sucrose, the experimental diffusivity values

were much lower than the predicted values (Figure 3–17). The diffusion data with the

coatings containing lecithin suggests that there is an increase in the partition coefficient K

in the presence of lecithin. To test the above hypothesis, an isotherm for the coating

containing 30% sugar in coconut oil + 0.5% lecithin was also determined (Figure 3–18).

The predicted equilibrium moisture content data was obtained by using the following

relation:

cocoss mymym += ( 14 )

0

1E-11

2E-11

3E-11

4E-11

5E-11

6E-11

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


Diff

usio

n ce

offic

ient

(m2 s-1

)

equation 3.4equation 3.5experimental data

Figure 3–16: Predicted versus the experimental data for coatings containing sucrose. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.

97

where m is the equilibrium moisture content of the coating containing 30% sugar in

coconut oil + 0.5% lecithin, ys is the weight fraction of sugar (here 0.3), yco is the weight

fraction of coconut oil + 0.5% lecithin (here 0.7), ms is the equilibrium moisture content

of sugar, and mco is the equilibrium moisture content of coconut oil + 0.5% lecithin. It

can be seen in Figure 3–18 that the actual equilibrium moisture content for a coating

containing 30% sugar in coconut oil + 0.5% lecithin is always higher than the predicted

value. Garbolino (2002) suggested that there is a multilayer of moisture present at the

interface of sugar and lecithin, and the data obtained in this study is consistent with that

model. In the coating, the polar head groups of the emulsifier are directed towards the

sucrose crystals and the hydrocarbon chains are directed towards the oil phase (Johansson

and Bergenstahl, 1992). Lecithin tends to absorb much more moisture than sucrose at

water activity levels less than 0.85 (Elworthy, 1961; Iglesias and Chirife, 1982) and when

the hydrophilic end is aligned with sucrose, the tendency to retain moisture at the

interface increases. The diffusion coefficient of a coating containing 30% sucrose in

coconut oil + 0.5% lecithin was recalculated by estimating the new K from the sorption

isotherm. The new predicted value of diffusion coefficient was 7.36 x 10-12 m2 s-1. This

value is closer to the actual value of average diffusivity of 5.8 x 10-12 m2 s-1 obtained

from experiments.

98

0

1E-11

2E-11

3E-11

4E-11

5E-11

6E-11

7E-11

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


Diff

usio

n co

effic

ient

(m2 s-1

)

equation 3.4equation 3.5experimental

Figure 3–17: Predicted versus the experimental data for coatings containing sucrose plus lecithin. Lines marked equation 3.4 or 3.5 are the effective diffusivity calculated using either equation 3.4 or 3.5.

99

When the relative humidity inside the cup was 85%, sucrose migrated to the

surface and crystallized on the top of the coatings containing lecithin. The volume

fraction of sucrose used in this study was less than 0.3, which suggests that the fraction of

the dispersed phase is below the percolation limit. However, when observed carefully, it

was seen that the crystallization of sugar at the surface was seen only at two to three spots

of the whole coating surface. There may have been some connectivity of the dispersed

phase in these particular regions. Due to the presence of the moisture layer at the

interface of the emulsifier and sugar particles there might have been accretion of the

sugar particles. The moisture layer would dissolve some of the sugar, which can then

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water Activity (aw)

% m

oist

ure

(db)

sugar

oil + 0.5% lecithin

(oil + 0.5% lecithin) + 30% sugar

predicted (oil + 0.5% lecithin+ 30% sugar)

Figure 3–18: Moisture adsorption isotherm for sugar, coconut oil + 0.5% lecithin, and 30% sugar in coconut oil + 0.5% lecithin

100

percolate to the surface of the coating through these connected regions, finally

crystallizing at the surface as it dries out.

When coatings containing sucrose and lecithin were kept for more than a week,

cracks appeared in the coatings, while the coatings containing just the sucrose particles

were still intact. Comparing the data for coatings containing lecithin versus the ones

without, it can be concluded that there are structural changes occurring in the coating

containing lecithin. Robinson (1971) in his studies on ethanol migration through

chocolates had found similar results with lecithin; the swelling of chocolate in ethanol

increases with increase in the lecithin content.

3.5.3 Water Vapor Permeability

The WVP, as a function of volume fraction, for the coatings containing sugar and

cocoa powder is shown in Figure 3–19. The WVP decreased with an increase in the

volume fraction of sucrose while it increased with increase in the cocoa powder content.

In the case of sucrose, diffusion occurs along the surface of the particles, which increased

the tortuosity of the diffusion path and slowed down the diffusion rate. While, in the case

of cocoa powder, the moisture diffused through the cocoa particles. Therefore, the WVP

increased with increase in cocoa powder content.

101

The WVP, as function of volume fraction, for the coatings with sugar & lecithin

and cocoa powder & lecithin is shown in Figure 3–20. It can be seen that the WVP

increases with the addition of particles in both the cases. It has been noted above that in

the presence of lecithin structural changes occur that increased the diffusion rate.

Therefore, in the presence of lecithin the WVP increased. A detailed study on the effect

of various ingredients and storage conditions on the WVP of chocolate-flavored coatings

can be found in the second part of this study.

0

2

4

6

8

10

12

14

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


Perm

eabi

lity

(g m

il m-2

day

-1 m

mH

g-1)

cpsucrose

Figure 3–19: Effect of ingredients and volume fraction on the water vapor permeability of coatings containing sucrose or cocoa powder

102

3.6 Conclusions

The observations for sucrose and cocoa powder confirm the Weisz (1967)

approach for modeling moisture diffusion through chocolate-flavored coatings. From the

results obtained, it can be seen that the unsteady-state or transient diffusivity decreased

with an increase in the partition coefficient.

There are structural changes associated with the diffusion of moisture into the

coating. These structural changes can occur due to the swelling of the cocoa particles or

dissolving of the sucrose particles in the moisture and subsequent migration to the

0

10

20

30

40

50

60

70

80

90

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


Perm

eabi

lity

(g m

il m-2

day

-1 m

mH

g-1)

cp + lsucrose + l

Figure 3–20: Effect of ingredients and volume fraction on the water vapor permeability of coatings containing sucrose and lecithin or cocoa powder and lecithin.

103

surface. Robinson (1971) and Zurcher (1971) had found that the presence of lecithin

increased the swelling of chocolates. This study confirmed their findings that the

structural changes during migration are aided by the presence of lecithin, which probably

tends to increase localized moisture concentrations at the interface of the hydrophilic

particles.

The diffusion coefficient of moisture through the dispersed phase can be

estimated from equation 3. From the estimated diffusion coefficient, the mechanism of

moisture migration through the dispersed phase can be understood. In the case of

chocolate-flavored coatings, the mechanism of diffusion through the two dispersed

phases sugar and cocoa powder is completely different. The diffusion of moisture

through cocoa powder, which is porous in nature, occurs through the particles. The rate

of diffusion of moisture through the cocoa powder particles is much faster than that

through coconut oil. For sugar, which has a crystalline structure, the moisture cannot

diffuse through the sugar crystal and, therefore moisture diffuses along the surface of the

sugar particles.

The presence of lecithin has a great effect on moisture migration through

chocolate-flavored coatings. Garbolino (2002) suggested that there is a layer of water at

the interface of the sugar and emulsifier. The model proposed by Garbolino (2002) was

validated from the moisture isotherm data of a coating containing 30% sugar in coconut

oil + 0.5% lecithin. This layer moisture can dissolve the sugar and the sugar can percolate

to the surface of the coating to cause sugar bloom.

104

The mechanism of diffusion through the disperse phase of the chocolate-flavored

coating suggests that the presence of sugar will decrease the WVP. On the other hand, the

presence of cocoa powder particles and lecithin will increase the WVP. The WVP data

obtained in this study confirmed the above predictions.

3.7 References

Aguilera, J. M. and D. W. Stanley (1999). Microstructure and mass transfer: Solid liquid

extraction. Microstructural Principles of Food Processing and Engineering.

Gaithersburg, MD, Aspen Publishers: 325 - 372.

Bell, L. N. and T. P. Labuza (2000). Moisture Sorption: Practical Aspects of Isotherm


Biquet, B. and T. P. Labuza (1988). Evaluation of the moisture permeability


989 - 998.

Cussler, E. L. (1997). Diffusion Mass Transfer in Fluid Systems. New York, NY,

Cambridge University Press.

Duda, J. L. (1999). Theoretical aspects of molecular mobility. Water Management in the

Design and Distribution of Quality Foods. Y. H. Roos, R. B. Leslie and P. J.

Lillford. Lancaster, PA, Technomic Publishing Company, Inc.: 237-253.

Elworthy, P. H. (1961). The adsorption of water vapour by lecithin and lysolecithin, and

the hydration of lysolecithin micelles. J.Chem. Soc.: 5385 - 5389.

105

Garbolino, C. (2002). The influence of surfactants and moisture on the colloidal and

rheological properties of model chocolate dispersions. Ph.D. thesis. Department

of Food Science. Pennsylvania State Univerisity, University Park, PA

Ghosh, V., Ziegler, G. R. and Anantheswaran, R. C. (2002). Fat, moisture, and ethanol

migration through chocolates and confectionery Coatings. Crit. Rev. Food Sci.

Nutr. 42(6): 583 - 626.

Iglesias, H. A. and J. Chirife (1982). Handbook of Food Isotherms: Water Sorption

Parameters for Food and Food Components. New York, NY, Academic Press.

Johansson, D. and B. Bergenstahl (1992). The influence of food emulsifiers on fat and

sugar dispersions in oils .1. Adsorption, sedimentation. J. Am. Oil Chem. Soc.

69(8): 705-717.

Labuza, T. P. and C. R. Hyman (1998). Moisture migration and control in multi-domain


Morillon, V., F. Debeaufort, M. Capelle, G. Blond and A. Voilley (2000). Influence of



Robinson, L. (1971). Veranderungen in krustenlosen pralinen mit alkoholhaltiger

flussiger Fullung I. Fette Seifen Anstrmittel 73(8): 521 - 526.

Saravacos, G. D. and Z. B. Maroulis (2001). Transport Properties of Foods. New York,

NY, Marcel Dekker.

Smith, D. M. and J. F. Keller (1985). Nonlinear sorption effects on the determination of

diffusion-sorption parameters. Ind. Eng. Chem. Fundamentals 24: 497 - 499.

106

Troutman, M. Y. (1999). Moisture migration and textural changes during manufacture of

soft-panned confections. M. S. thesis, The Pennsylvania State University,

University Park, PA.

van der Zanden, A. J. J. (2000). Heat and mass transfer in heterogeneous media where a

phase transition takes place. Chem. Eng. Sci. 55: 6235 - 6241.

Vieth, W. R. (1991). Diffusion In And Through Polymers. New York, Hanser Publishers.

Weisz, P. B. (1967). Sorption - diffusion in heterogenous systems. Part 1. General

sorption behavior and criteria. Trans. Faraday Soc. 63: 1801 - 1807.

Weisz, P. B. and J. S. Hicks (1967). Sorption-diffusion in heterogeneous systems. Part 2

- Quantitative solutions for uptake rates. Trans. Faraday Soc. 63: 1807 - 1814.

Weisz, P. B. and H. Zollinger (1967). Sorption-diffusion in heterogeneous systems. Part 3

- Experimental models of dye sorption. Trans. Faraday Soc. 63: 1815-1823.

Weisz, P. B. and H. Zollinger (1968). Sorption-diffusion in heterogeneous systems. Part 4

- Dyeing rates in organic fibers. Trans. Faraday Soc. 64: 1693-1700.

Chapter 4

EFFECT OF INGREDIENTS ON MOISTURE MIGRATION THROUGH CHOCOLATE-FLAVORED COATINGS

4.1 Abstract

The effect of sucrose, lactose, dextrose, cocoa powder, emulsifier, fat type, and

storage environment on the water vapor permeability (WVP) of a chocolate-flavored

coating was studied. The WVP of fat films decreased with increase in the solid fat

content (SFC). Cocoa powder and lecithin increased the WVP while sugar decreased the

WVP through the coatings. There was an increase in the WVP with an increase in the

absolute value of the relative humidity across coating containing about 12.5% cocoa

powder, 33% sucrose, 0.5% lecithin in coconut oil. This increase was due to swelling of

the cocoa powder particles that changed the structure of the coatings. Replacing sucrose

with dextrose increased the WVP of the coatings. In addition to permeability data, optical

microscope images of cocoa powder and SEM images of the structure of two different

coatings were obtained.

4.2 Introduction

Moisture migration is a common problem in multi-domain foods that have regions

of differing water activities (Labuza and Hyman, 1998). Moisture migrates from the

domain with higher water activity to the domain with lower water activity or into the

108

environment until there is thermodynamic equilibrium. Moisture migration can be

prevented either by matching the water activity of the different domains of the food or by

using an edible moisture barrier between the two domains. The former solution is not

always practical as it is not always possible to match water activities of each domain. An

edible film on the other hand is a more practical alternative to prevent moisture migration

in multi-domain foods (Biquet and Labuza, 1988).

The properties and applications of various edible films can be found in articles by

Kester and Fennema (1986), Koelsch (1994), Guilbert and Biquet (1995), Krochta and

Mulder-Johnston (1997), and Debeaufort et al. (1998). Lipid-based edible films are most

resistant to moisture migration due to their hydrophobic nature (Kester and Fennema,

1986; Callegarin et al., 1997; Morillon et al., 2002). Some of the commonly used lipid-

based edible films or coatings are chocolate or chocolate-flavored confectionary coatings

(Morillon et al., 2000; Ghosh et al., 2002).

Chocolate-flavored coatings consist of sucrose, cocoa powder, emulsifier, and

milk solids (in the case of milk chocolate) that are embedded in a continuous lipid phase.

The continuous fat phase consists of a fat crystal network. The pores between these fat

crystals do not really have a spherical shape and are either closed or completely (or

partially) filled with the liquid fraction of cocoa butter (Loisel et al., 1997). Water

molecules diffuse mainly through the oil that is present between the fat crystals. When

the moisture comes in contact with a hydrophilic particle, e.g. sucrose or cocoa powder, it

will first get adsorbed onto these particles and then diffuse through the hydrophilic

particles as well as along their surfaces. Moisture sorption behavior for each of the

109

constituent materials in the coating is different and, therefore, the moisture migration

through such coatings is complex due to their heterogeneous nature. The moisture

transfer through such coatings will be dependent on the type and the physical state of fat

used, the amount and the type of hydrophilic constituents (sugar, cocoa solids, emulsifier)

and the storage conditions (temperature and relative humidity).

The physical state (i.e., solid or liquid) of the lipid component has a strong

influence on the water vapor permeability (WVP) of the film; water is less soluble in

solid lipid than in liquid lipids (Kamper and Fennema, 1984). Landmann et al. (1960)

found a 300-fold increase in the permeability of a hydrogenated cottonseed oil when the

liquid oil content was increased from 0 to 40%. Other researchers (Kamper and

Fennema, 1984; Talbot, 1994) have shown that an increase in solid fat content from 0 to

30% decreases the WVP of the films. Such behavior is observed because the structure of

the solid fat is denser than liquid lipids and therefore solid lipids are more resistant to

moisture diffusion. Martin-Polo et al. (1992) observed that for a mixture of paraffin wax

and oil blends the WVP decreased with an increase in SFC. However, they also found

that for mixture of pure alkanes (C16H34 and C28H58) the WVP increased at solid fat

contents higher than 50% due to a very porous structure at high SFC.

The water activity or relative humidity gradient is the driving force for moisture

transfer though an edible film. In an ideal situation increasing the relative humidity

gradient should increase the water vapor transfer through a coating, but the permeability

should remain constant. However, for lipid and chocolate films it has been found that the

WVP depends on both the relative humidity (RH) gradients and the absolute humidity

110

values. In other words, for the same relative humidity gradient, the WVP is higher when

the vapor pressure values are higher. Biquet and Labuza (1988) found that the WVP

increased from 4.9 g mil m-2 day-1 mmHg-1 to 11.0 g mil m-2 day-1 mmHg-1 when the

relative humidity difference was changed from 0-33% to 54.4 – 80.4%. Similar results

were reported by Kamper and Fennema (1984) and Fennema et al. (1994). Kamper and

Fennema (1984) found that the permeability increased from 0.69 g mil m-2 day-1 mmHg-1

to 10.1 g mil m-2 day-1 mmHg-1 when the relative humidity range was changed from 65 –

33% to 97 – 65%. The increase in the WVP at higher relative humidity may be due to

structural changes resulting from the sorption of moisture by the hydrophilic support of

their film. Landmann et al. (1960) found a large increase in the WVP of milk chocolate

when the humidity gradient was varied from 22 – 75% to 0 to 100%. This behavior may

be due to presence of hydrophilic particles in the milk chocolate which behave differently

at different humidity regions.

Most of the papers dealing with chocolate-flavored coatings have studied the

effect of storage conditions on the water vapor permeability. Very little work has been

done to study the effect of hydrophilic ingredients, such as sucrose, cocoa powder, and

emulsifiers, on the water vapor permeability of lipid-based coatings. The objective of

this study was to understand the effect of sucrose, sugar type, cocoa powder, emulsifier,

fat type, and storage environment on the water vapor permeability of a chocolate-flavored

coating.

111

4.3 Materials and Methods

4.3.1 Ingredients

The ingredients needed to perform this study were different types of sugars, cocoa

powders, emulsifiers, and barrier fats. These details of these ingredients are discussed

below.

4.3.1.1 Sugars

The sugars used in this study were sucrose, lactose, and dextrose. Crystalline

sucrose (pure cane extra fine granulated sugar with purity ~ 100%) was obtained from

Florida Crystals (Palm Beach, FL). The particle size of granulated sucrose was larger

than 100 µm and needed to be ground into a particle size range that is typically present in

chocolate-flavored coatings (average size 24-28 µm). The sucrose crystals were

therefore ground using a jet mill (Model 0101-C6 (S), Jet-O-Mizer, Fluid Enery Aljet,

Plumsteadville, PA) to obtain the desired particle size.

When operating the jet mill, the inlet air pressure was 120 psi, the pusher nozzle

was set at 100 psi, the first grinder nozzle was set at 100 psi, and the second grinder

nozzle was set at 90 psi. The flow rate dial was set at 25. An air compressor (Model #

C1071080VMSA, Campbell Hausfeld, Harrison, OH) supplied the high-pressure air at

the inlet of the jet mill.

To avoid clumping, the sucrose was dried immediately after grinding in a vacuum


112

Hg, for 10-12 hours. The dried sucrose was transferred into airtight containers, and was

stored in a desiccator cabinet at 18oC.

Granular dextrose (SD99, Lot # NAD4508) was obtained from Cargill, Inc.

(Eddyville, IA). The dextrose granules were ground using a jet mill (Model 0101-C6 (S),

Jet-O-Mizer, Fluid Enery Aljet, Plumsteadville, PA) using the same operating conditions

as that for sugar. The dextrose powder was dried in a vacuum oven (National Appliance

Co., Portland, OR), maintained at 60oC and a vacuum of 20” Hg, for 10-12 hours and was

used immediately after drying. Crystalline lactose (particle size~66 µm, Edible Grade,

Lot # 28452, plant #19-34) was dried in a vacuum oven (National Appliance Co.,

Portland, OR), maintained at 60oC and a vacuum of 20” Hg, for 10-12 hours before use.

4.3.1.2 Cocoa Powder

To avoid any confounding of the results due to the presence of the fat in the cocoa

powder, defatted cocoa powder was used in this study. A separate experiment was done

to study the effect of two different commercial cocoa powders.

Defatted cocoa powder in pellet form was obtained from Comet Specialty

Ingredients Co. (Freeport, TX). The cocoa powder pellets were ground using a jet mill

(Model 0101-C6 (S), Jet-O-Mizer, Fluid Enery Aljet, Plumsteadville, PA) to obtain cocoa

powder particles with particle size less than 20 µm. The operating conditions were the

same as that for sugar, except for the feed rate, which was set at 35. The cocoa powder

was dried in a vacuum oven (National Appliance Co., Portland, OR), maintained at 60oC

113

and a vacuum of 20” Hg, for 10-12 hours. The dried cocoa powder was transferred into

airtight containers, and was stored in a desiccator cabinet at 18oC.

Natural and alkalized cocoa powder (particle size < 20 µm, 10-12% fat), were

obtained from Hershey Foods (Hershey, PA). The cocoa powder was dried in a vacuum


Hg, for 10-12 hours before use.

4.3.1.3 Fats

Coconut oil was obtained from Aarhus, Inc (Lot # 209, AI# 2578-1) and ACH

Humko (Victory 76, Lot# 49815, Formula # F00880). Five other moisture barrier fats

were obtained from Loders Croklaan, Aarhus, and Karlshamns (Table 4–1).

For determining the solid fat content, the fats were subjected to thermal

treatments similar to the thermal treatment received during coating preparation. The fats

were melted overnight in an oven maintained at 70oC. The melted oils were transferred

into NMR tubes and the tubes were kept in a water bath (Model 1157, VWR Scientific,

Philadelphia, PA) maintained at 20oC for 24 hours. The solid fat contents (SFC) of the

fats were determined by the direct method (Waddington 1995) using a Minispec mq20

NMR (Bruker Analytik, GmBH, Germany). The solid fat content of the fats used in this

study are shown in Table 4–1.

114

Fat Type Solid Fat Content (%)

ISAO 43-82 (Aarhus Olie, Denmark) 25.2 Coconut oil (Aarhus, Inc, Newark, NJ) 43.0 Coconut oil (Lot# 49815, Formula # F00880, Victory 76, ACH Humko, TN)

35.1

Akoice C (Karlshamns, Sweden) 44.1 DP 1192 (Loders Croklaan, IL) 55.3 DP 1193(Loders Croklaan, IL) 67.5 DP 1194 (Loders Croklaan, IL) 75.9

In the WVP experiments, the average temperature in the chamber varied during

different experiment. The SFC of the fats were also determined at the temperature of

measurement.

4.3.1.4 Emulsifiers

Granular lecithin (purity ~ 97%) was obtained from Acros Organics (Fisher

Scientific, Pittsburgh, PA). Lecithin is made of a mixture of phospholipids with

phosphatidyl-choline, phosphatidyl-ethanolamine and phosphatidyl-inositol as its main

components. The lecithin had moisture content less than 0.1%.

Another emulsifier Citrem (Batch # 11225, Material # 17636), obtained from

Danisco (Denmark) was used in this study. “Citrem stands for a group of citric acid

esters of the mono- and diglycerides of edible fatty acids containing 1-2 molecules of

edible fatty acids and 1-2 molecules of citric acid, wherein the citric acid as a tribasic

acid may also be esterified with several glycerides, and as a hydroxy acid may also be

esterified with fatty acids (Figure 4–1). Equally usual are names such as citroglyceride,

Table 4–1: Solid Fat Contents of the Moisture Barrier Fats at 20oC

115

citric acid glyceride ester, monoglyceride citrate, etc. The name Citrem derives from the

term 'citric acid ester of mono- and diglycerides” (Matissek, 2002).

4.3.2 Experimental Design

From theoretical consideration, it was hypothesized that the relative proportions

of each ingredient will have an effect on the permeability of the coatings and thus the

effect of each ingredient can be best understood by using a mixture experiment (Cornell,

2002). Hence, to understand the effect of each ingredient and their proportions in the

coating a mixture experimental design was made using the following constraints:

Sucrose ≤ 0.4, ----- (i)

Cocoa powder (defatted) ≤ 0.2 -----(ii)

0.003 ≤ Lecithin ≤ 0.01 -----(iii)

Figure 4–1: Structure of Citrem (Matissek, 2002)

116

The above constraints were used to keep the level of each ingredient within the

weight range that is commercially used for chocolate-flavored coatings. The experimental

design was obtained using ECHIP’s experimental design software (ECHIP Inc.,

Hockession, DE). The experimental design is shown in Table 4–2.

The effects of replacing individual ingredients in a generic coating were studied

using single factor experiment (Montgomery, 1997). The generic coating, which was

used as a control, contained approximately 33% sucrose, 12.5% cocoa powder (defatted),

and 0.5% lecithin in coconut oil. In this series, two sugars (lactose and dextrose), two

Table 4–2: Mixture experimental design for studying the effect of sucrose, cocoa powder and lecithin

Trial # Coconut oil Sucrose Cocoa powder Lecithin 4 0.597 0.2 0.2 0.0033 0.797 0 0.2 0.003

14 0.97 0.02 0 0.018 0.79 0 0.2 0.016 0.68 0.21 0.1 0.012 0.597 0.4 0 0.0035 0.497 0.4 0.1 0.0035 0.497 0.4 0.1 0.003

12 0.397 0.4 0.2 0.0037 0.97 0.027 0 0.003

13 0.8835 0 0.1135 0.0034 0.597 0.2 0.2 0.0033 0.797 0 0.2 0.0031 0.39 0.4 0.2 0.01

15 0.59 0.4 0 0.012 0.597 0.4 0 0.0039 0.3935 0.4 0.2 0.00651 0.39 0.4 0.2 0.01

11 0.7835 0.21 0 0.006510 0.8835 0 0.11 0.0065

117

different types of commercial cocoa powder (natural and alkalized), six different

moisture barrier fats, and one emulsifier (Citrem) were studied.

The mixture experiments were performed using the coconut oil from Aarhus, Inc.,

while the other experiments were performed using the coconut oil from ACH Humko.

The mixture experiments were done at 20oC while the experiments on the effect of

ingredients and solid fat content were carried out at 18.5oC.

4.3.3 Measurement of Moisture Content

The moisture analysis of sucrose and cocoa powder was performed according to

the method suggested by Troutman (1999). About 1g of the sample (i.e., sucrose or cocoa

powder) was placed in a Kimble Kimax culture tube with approximately 10 g of a 1:1

formamide (Fisher Scientific, Pittsburgh, PA): methanol (Karl Fischer grade, anhydrous,

VWR Scientific, Pittsburgh, PA) solvent. The weights of the sample and the solvent

were recorded to obtain the dilution factor. A layer of Teflon tape was put around the

threads of the culture bottle. The culture bottles were closed using the cap of the tube.

Application of Teflon tape ensures airtight seal in the culture bottles. The culture

bottles were stored in an oven at 50oC for 12 hours. Simultaneously, two culture tube

containing the solvent and sealed in the same manner as the samples were kept in the

oven at 50oC for 12 hours.

The moisture content for sugar and cocoa powder was measured using a Karl

Fischer titrator (Model DL 31, Mettler-Toledo GmBH, Switzerland). Duplicate

measurements were done for each sample. Approximately 40 ml of methanol solvent

118

(Karl Fischer grade, anhydrous, VWR Scientific, Pittsburgh, PA) was added to the titrator

vessel and neutralized using a pyridine-free Karl Fischer reagent (Hydranal-composite 5,

Riedel-de Haën, GmBH. Seelze, Germany). The pyridine-free reagent contained

imidazole, sulfer dioxide and iodine. The modified Karl Fischer reaction, due to change

in the components of the Karl Fischer reagent, is given by equation 4.1

I)RNH(2RSO)RNH(OHIRSO)RNH(RSO)RNH(RNROH

4223

3

+→++→+

( 4.1 )

Triplicate measurement of the reagent concentration was made using a water

standard (Hydranal-water standard 10.0, Riedel-de Haën, GmBH. Seelze, Germany) to

determine the concentration of the reagent. For moisture content determination of cocoa

powder and sugar, about 1 ml of the solvent was drawn from the culture tube and added

to the titration vessel. The weight of the sample was determined by weighing the syringe

before and after the experiment using an Ohaus Galaxy 200 balance (Ohaus Corporation,

Florham Park, NJ), with an accuracy of ±0.0001g. The amount of moisture in the sample

was determined automatically by the Karl Fischer instrument using equation 4.2.

% moisture = strength of the Karl Fischer reagent (mg water/ml reagent) x ml Karl Fischer reagent added/mg sample

( 4.2 )

The moisture content of the solvent was also determined. The moisture content of

the original sample was determined using equation 4.3.

2

11

2

1

ffx

100x

ffxmoisture% −

+= ( 4.3 )

119

where x is the moisture content of the solvent with the sample, x1 is the moisture content

of the solvent, f1 is the weight of the solvent, and f2 is the weight of the sample.


The moisture sorption isotherm for ground sugar, cocoa powder, coconut oil, and

coconut oil + 0.5% lecithin was determined at 20oC. Saturated salt solutions of lithium

chloride (LiCl), magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2), potassium

iodide (KI), sodium chloride (NaCl), ammonium chloride (NH4Cl), and potassium

chloride (KCl) were used to obtain water activity values of 0.113 ± 0.003, 0.331 ± 0.002,

0.544 ± 0.002, 0.699 ± 0.003, 0.755 ± 0.004, 0.792 ± 0.004, and 0.851 ± 0.003,

respectively (Bell and Labuza, 2000). Duplicate measurements of equilibrium moisture

content were done at each water activity. The saturated salt solutions were put in the

bottom of Mason jars (~ 473 ml), to a depth of about 1 cm. A square support of size

approximately 3 cm x 3 cm x 6 cm (high) was made from steel wire mesh and placed in

the Mason jar. For sucrose and cocoa powder, approximately 5g of sample was put in an

aluminum weighing dish and placed on the wire mesh support. The Mason jars were

kept in a temperature-controlled chamber (Model 310, Imperial III Incubator, Labline,

Inc., Melrose Park, IL) at 20±1oC. The weight of the samples was taken every day using

an Ohaus Galaxy 200 balance (Ohaus Corporation, Florham Park, NJ), with an accuracy

of ±0.0001g, until there was no change in weight (±0.001g) for 3 days. Both cocoa

powder and sucrose reached equilibrium with 2-3 days of storage. The moisture contents

of the samples were determined using the Karl Fischer method as previously described.

120

4.3.5 Sample Preparation

For making the coating, the ingredients were mixed at the ‘blend’ setting in a

blender (Pulse Matic, Oster Corporation, Milwaukee, WI) for two minutes. The total

weight of the ingredients per batch was 250g and two batches were made. To ensure that

the fat was in the liquid phase, the blender was kept inside a chamber (Model 680A,

Labline Instruments, Inc., Melrose Park, IL) maintained at 40oC. The two batches were

mixed after blending to get the total weight of the coating to be approximately 500g.

During the mixing process, numerous air bubbles were incorporated into the coating mix.

Hence, the coating mix was kept in a vacuum oven, maintained with a vacuum of 20” Hg

and a temperature of 70oC, for 24 hours to remove the air bubbles. When the coating

samples were removed from the oven, settling of the particulate phase was observed. The

melted test samples (approximately 500g) were mixed at the lowest setting, being careful

that no air was incorporated during this process, in a mixer (Model C-100T, Hobart

Corporation, Troy, OH) at 50oC for three hours.

To prepare films for diffusion studies, parchment paper was placed on a

marble slab that had a flat surface. The purpose of the marble slab was to absorb the heat

from the coating during the solidification process. For coatings containing cocoa powder,

a stainless steel sheet (0.8 mm thick) containing four 9 cm diameter holes was placed on

top of the parchment paper (Figure 4–2). Melted coating was poured into the holes and

allowed to solidify for about 30 minutes. The temperature of the room was maintained at

121

around 16oC. After the coating had solidified, as observed visually, excess material was

removed using a hot spatula to obtain a film of about 1 mm thickness. After removing

the excess material, the whole system was kept in a refrigerator (Model TBX18SLB,

General Electric Co., Louisville, KY) for 10 minutes to solidify the film completely,

which eased the removal of the coating from the parchment paper. The test film was

removed from the mold by cutting the coating around the edges with a hot knife. The

thickness of each coating was measured at four different points using a micrometer

(Craftsman, Sears, Roebuck & Co, Chicago, IL).

The method had to be modified for fats with solid fat contents greater than 60%.

These fats were melted by keeping them overnight in a chamber maintained at 70oC. For

Figure 4–2: Picture of the mold for making the chocolate coatings (hole diameter = 9 cm)

122

preparing the coating, the fats were poured in the mold as described above, but in this

case the excess fat was removed immediately using a spatula. Immediately after

removing the excess fat, the fat coating was cut around the edges of the mold using a

knife. The mold was removed after cutting the edges and the coatings solidified within

ten to fifteen minutes. The fat coatings cracked if they were allowed to solidify within

the mold. The thickness of the coatings was measured very carefully as these coatings

were very brittle and had a tendency to crack while taking the measurement.

4.3.6 Method for Measurement of Water Vapor Permeability

The water vapor permeability (WVP) through the coatings was measured using

the ASTM E-96 method. A schematic of the test cell is shown in Figure 4–3. The

dimensions of the cup were, outer diameter 9.2 cm, inner diameter 7.65 cm, and depth 1

cm. The exposed area of the coating was 46 cm2. Calcium chloride or saturated salt

solution was put in the bottom of the cell and then the coating was carefully placed in the

Thwing Albert cups (Thwing Albert, Philadelphia, PA). The edges of the cups were

sealed using excess amount of the melted coating material. A picture of the Thwing

Albert cup with a coating sample is shown in Figure 4–4.

123

Figure 4–3: Schematic of the test cell for the ASTM E-96 method

Figure 4–4: Picture of a test cell with the coating film

124

4.3.6.1 Controlled Environment Setup

A system was setup to provide the desired temperature and relative humidity

(Figure 4–5). A chamber made using 0.635 cm thick acrylic plastic sheets with an inside

volume of 31400 cm3 (dimension 43.18 cm x 27.94 cm x 26.04 cm) was used for this

study. The chamber had an O-ring gasket and a lid made with 1.27cm thick plastic.

The chamber was maintained at a relative humidity of 73 ± 2 % using saturated

sodium chloride solution. The relative humidity over saturated NaCl at 20oC is about

75.5%, but because of some leakage due to the presence of a one cm hole in the side of

the chamber, the actual humidity inside the chamber was slightly lower. A fan (type

Figure 4–5: Picture of the setup for measuring the water vapor transmission rate of the coatings

125

U920IB, Tobishi Kosan Co. Ltd., Japan) was used to create convection inside the

chamber, which allowed the humidity inside the chamber to reach equilibrium quickly

after any disturbance. The plastic chamber was kept inside an environmental chamber

(Model 680A, Labline Instruments, Inc., Melrose Park, IL), which was maintained at

18.5oC for the mixture experiments and at 17.5oC for the single factor experiment. The

fan used for convection, generated heat and so the temperature inside the plastic box was

maintained at 20 ± 0.5oC for the mixture experiments and at 18.5 ± 0.5oC for the single

factor experiments.

The temperature and relative humidity within the chamber was monitored every

five minutes using a temperature and relative humidity probe (Model HMP35C,

Campbell Scientific, Inc., Logan, UT) and a data logger (21X Micrologger, Campbell

Scientific, Inc., Logan, UT). The temperature and relative humidity inside the chamber

during a typical experiment is shown in Figure 4–6. The spikes in the relative humidity

line show the times when the chamber was opened, for weighing the samples.

126

4.3.6.2 Experimental Setup

The Thwing Albert cups were kept in the controlled atmosphere chamber

described above. The weight of the samples was measured at regular intervals using an

analytical balance (AB105, Mettler Toledo, Switzerland) with an accuracy of 0.0001g.

The weight gain versus time was plotted to obtain the WVTR of the coating. The ASTM

(1995) suggests that at least eight points in the straight line be taken for obtaining

WVTR. During the preliminary experiments, it was found that there was no significant

difference upon taking more than four or five points in the straight-line portion of the

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120 140

Time (h)

Tem

pera

ture

(C) o

r Rel

ativ

e H

umid

ity(R

H)

Temperature (C)Relative Humidity

Figure 4–6: Temperature and relative humidity inside the plastic chamber

127

curve. Hence, the weight gain with time was monitored until at least four points in the

plot of weight gain versus time were in a straight line and the R2 was greater than 0.99.

The water vapor transmission rate (WVTR) through the coatings was calculated

using equation 4.4:

daymg

AslopeWVTR 2= ( 4.4 )

where A is the area of the coating (m2) and slope is the slope of the straight line portion

of the plot of weight gain versus time. The water vapor permeability (WVP) was

calculated using equation 4.5:

mmHgdaymmilg

pxWVTRWVP 2∆

∆•= ( 4.5 )

where ∆x is the thickness of the coating (mil) and ∆p is the vapor pressure difference

(mm Hg). The equilibrium relative humidity above the calcium chloride particles was

found to be 3.5%. Hence, in the calculations the relative humidity at the low humidity

end was taken as 3.5%.

4.3.7 Scanning Electron Microscopy

A small piece of the coating sample was cut using a razor blade and glued to the

cryo SEM sample stub using OCT/colloidal graphite. The sample was then plunge frozen

128

in liquid nitrogen and transferred rapidly to the SEM cryo chamber (Model C1500C,

Gatan, Warrendale, PA). The coating sample was fractured inside the chamber using a

probe. After fracturing, the sample was transferred to the SEM stage under vacuum (-

196°C). The samples were then etched to sublimate the ice at the surface of the coating

using an accelerating voltage of 2.5kV and a stage temperature of -90°C. During the

etching process, the samples were viewed using the lowest magnification possible to

maximize the view area. When there were no ice crystals that could be seen in the image,

the samples were removed to the cryo stage for sputter coating (BALTEC SCD050,

Techno Trade, Manchester, NH) with 10nm Au. After coating with gold, the sample was

put back into the SEM stage maintained at -150°C. SEM images were generated at

2.5kV or 5kV on a JOEL 5400 SEM (Peabody, MA) and transferred to Princeton-Gamma

Tech’s Integrated Micro-analyzer for Imaging (IMIX v.8, Princeton, NJ). The cryo SEM

was performed at the Electron Microscopy Facility, Huck Institute of Life Sciences, The

Pennsylvania State University.

4.3.8 Structural Changes on Swelling

Cocoa powder particles were dried in a vacuum oven (National Appliance Co.,

Portland, OR), maintained at 60oC and a vacuum of 20” Hg, for 24 hours and then put on

a microscope slide. The slide was observed using an Olympus BX40 light microscope

equipped with a Sony Power HAD DXC-970MD CCD color video camera used for

image capture through Pax-It (Version 4.2) software (MIS, Franklin Park, IL). After

obtaining the image, the slide was kept in a desiccator containing saturated sodium

129

chloride solution for two days and then the particles were observed again under the

microscope using the procedure described above.

Cocoa powder was kept in tow shallow pans (diameter 6.35cm, height 0.5 cm)

and the surface of the pan was flattened. These pans were stored for one week at room

temperature (around 18oC) in a desiccator containing saturated sodium chloride solution.

Another study was done to determine the change in the diameter of films made with

coating material containing different levels of cocoa powder. The films were kept in a

desiccator cabinet for three weeks at room temperature (18o) to remove any moisture

present in the film. The diameters of dry film samples were measured using calipers.

The change in the diameter after moisture adsorption by the films was determined by

measuring the diameter of the film samples after equilibrating them in a desiccator

containing saturated sodium chloride solution for three weeks in room temperature

(18oC)

4.3.9 Water Transmission Studies at sub-Zero Temperatures

The water vapor transmission through the coatings at sub-zero temperature was

measured using a Permatran W (MOCON, Minneapolis, MN) and ASTM method F-

1249. The Permatran available in our laboratory has only a heating unit. So an external

cell was used to obtain data at temperatures below room temperature. The external cell

was kept in a temperature-controlled chamber (Model Z16, Cincinnati Sub-Zero Products

Inc., Cincinnati, OH) maintained at –5oC. The schematic of the external cell is given in

Figure 4–7. The concept of the permeability cell is similar to that used by Martin-Polo et

130

al. (1992) with some modifications. The schematic of the setup is shown in Figure 4–8

and a picture of the actual setup is shown in Figure 4–9.

To check the effectiveness of the setup using an external cell, the WVTR of a

standard film LIMA (MOCON, Minneapolis, MN) was determined at four different

temperatures (Table 4–3). There was a good agreement (error <5%) between the data

measured by the external cell setup and the values suggested by MOCON.

Table 4–3: Water Vapor Transmission Rate for the standard film (LIMA) * obtained using the External Cell

Temperature (oC) Experimental (g m-2 day-1)

Value provided by MOCON

(g m-2 day-1)

% Error

5.3 3.958 3.797 4.24

9.0 4.982 5.020 -0.76

15.6 7.380 7.418 -0.51

24.5 12.899 12.614 2.26 * There can be up to 10% variation WVTR of the film from the suggested values

131

Figure 4–7: Schematic diagram of the modified test cell used for measuring WVP at sub-zero temperatures

Figure 4–8: Schematic of the setup for measuring the water transmission through the coatings at negative temperature

132

4.4 Results and Discussion

4.4.1 Effect of Coating Thickness

The WVP of the generic coating at two different thicknesses is shown in Table 4–

4. The relative humidity gradient across the film during the experiment was 75 – 3.5%,

i.e. the relative humidity inside the chamber was 75% (NaCl) and the relative humidity

Figure 4–9: Picture of the actual setup for measuring the water vapor transmission rate at –5oC

133

inside the cup was 3.5% (CaCl2). It can be seen from the data that there is no significant

change in WVP of the coatings (p>0.05) with thickness while the WVTR decreases with

increase in the coating thickness. These results contradict the behavior observed by

Biquet and Labuza (1988) and Landmann et al. (1960). Biquet and Labuza (1988)

observed that when the thickness of a dark chocolate film was increased from 0.612 to

0.926 mm, the WVTR decreased, but when the thickness was increased from 0.926 to

1.192 mm, there was no change in the WVTR. Landmann et al. (1960) observed that

increasing the thickness of a cocoa butter film from 1.59 to 2.92 mm did not change its

WVTR. The results obtained by Landmann et al. (1960) and Biquet and Labuza (1988)

deviate from Fick’s law, which suggests that the WVTR should decrease with an increase

in the thickness of the coating. One possible explanation for a deviation from Fick’s law

is that the researchers did not study the steady-state diffusion and all the data they had

gathered were in the sorption or the unsteady-state regime. In the sorption regime the

rate of moisture gain will be the same initially irrespective of the thickness. Hence, they

did not see any difference in the water vapor transmission rates. To illustrate this point,

another study was done with a coating containing higher amount of cocoa powder than

the generic coating.

134

Figure 4–10 shows the weight gain versus time data for coatings with different

thickness under similar storage conditions. It can be seen that during the initial stages

(time < 100 hours) there is no difference in the weight gain data for different coatings.

This is the region of unsteady-state diffusion. During the unsteady-state, the diffusion

through the coatings is analogous to diffusion in an infinite sheet; therefore, the thickness

of the coating will not have any effect on the weight gain. Thus, it can be concluded that

the values at unsteady-state have been presented in the literature, and have been mistaken

for the steady-state data. Once the diffusion process reaches the steady state, the

thickness will have an effect on the rate of weight gain as is observed in data shown in

Figure 4–10.

Table 4–4: Effect of coating thickness on the WVP of the generic coating at 17oC Sample Thickness (mm) WVTR (g m-2 day-1) Permeability [g mil m-2 day-

1 (mmHg)-1]

0.81 2.464 7.95

0.88 2.236 8.05

2.011 1.134 9.2

2.014 0.957 7.79

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4.4.2 Effect of Ingredient Proportions on WVP of Coatings

The experimental design and results are shown in Table 4–5. The permeability

data given in the table is an average of two readings. The relative humidity gradient

across the film in all the experiments was 75 – 3.5%, i.e. the relative humidity inside the

chamber was 75% (NaCl) and the relative humidity inside the cup was 3.5% (CaCl2).

The permeability data obtained was analyzed using ECHIP software (ECHIP Inc.,

Hockession, DE) and a summary of the analysis is shown in Table 4–6. A three

dimensional surface plot of the fitted data was obtained (Figure 4–11). In the figure, the

shaded area in the axis shows the region that was used in the current experimental design.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 100 200 300 400 500 600

Time (h)

Wei

ght G

ain

(g)

1.222.572.570.790.82

Figure 4–10: Effect of coating thickness on the water vapor transmission rate at 20oC. The coating contains 15% cocoa powder and 85% coconut oil. The thickness of the coatings are given in mm.

136

It is predicted that at higher amounts of cocoa powder the WVP will increase, while

increasing the amount of sugar will decrease the WVP. The trend of the plot agrees with

the theoretical prediction (Ghosh et al., 2002). The plot of mass transfer rate versus the

volume fraction, obtained from theoretical considerations (Ghosh et al., 2002) suggested

that there would be very small change in the WVP, with addition of either sucrose or

cocoa powder, at the chosen experimental region. This small change in the WVP with

the addition of sucrose or cocoa powder is masked by the experimental error and

therefore there seems to be no significant effect on changing either the fat, cocoa powder,

or sucrose (p>0.05).

Let us now consider the effect of lecithin. Table 4–6 shows that in the

experimental region, lecithin does not have a significant effect (p>0.05) on the WVP of

the coating. Studies done by Garblino (2002) showed that less than 0.05% lecithin is

needed to get a monolayer coating of lecithin around the sucrose or cocoa powder

particles. Therefore, adding lecithin beyond 0.05% will not have a great effect on the

WVP. The amount of lecithin used in this study ranged from 0.3 – 1% and therefore, no

significant difference in the WVP were observed with change in the lecithin content. The

part of this study also suggested that addition of lecithin would increase WVP and the

WVP of coatings without lecithin will be lower than the coatings with lecithin.

137

Table 4–5: Effect of ingredients on the WVP of coatings

Trial # Coconut oil Sucrose Cocoa powder LecithinPermeability [g mil m-2 day-1 (mmHg)-1]

4 0.597 0.2 0.2 0.003 13.99 ± 0.03 3 0.797 0 0.2 0.003 12.77 ± 0.19

14 0.97 0.02 0 0.01 8.81 ± 0.028 0.79 0 0.2 0.01 10.11 ± 2.256 0.68 0.21 0.1 0.01 10.60 ± 0.052 0.597 0.4 0 0.003 10.89 ± 0.745 0.497 0.4 0.1 0.003 8.91 ± 0.115 0.497 0.4 0.1 0.003 11.72 ± 0.45

12 0.397 0.4 0.2 0.003 9.36 ± 0.117 0.97 0.027 0 0.003 12.69 ± 1.51

13 0.8835 0 0.1135 0.003 11.48 ± 0.544 0.597 0.2 0.2 0.003 12.96 ± 1.793 0.797 0 0.2 0.003 12.89 ± 4.221 0.39 0.4 0.2 0.01 11.09 ± 0.20

15 0.59 0.4 0 0.01 9.19 ± 1.452 0.597 0.4 0 0.003 9.84 ± 0.489 0.3935 0.4 0.2 0.0065 14.80 ± 8.511 0.39 0.4 0.2 0.01 10.25 ± 0.01

11 0.7835 0.21 0 0.0065 9.03 ±0.7010 0.8835 0 0.11 0.0065 11.32 ± 0.42

138

Table 4–6: Summary of Data Analysis – Mixture Design

Coefficients for response 'Permeability' Coefficient scaling is in terms of the stretched experimental region. COEFFICIENTS SD P CONDITION TERM -298.343 0 CONSTANT 309.635 195.409 0.1442 0.007 1 coil 306.598 196.227 0.1492 0.008 2 sucrose 329.559 199.686 0.1299 0.016 3 cpowder -62157.5 39060.6 0.1426 0.002 4 lecithin 2.80854 11.2703 0.8083- 0.461 5 coil*sucrose -22.3102 39.9113 0.5885- 0.139 6 coil*cpowder 62896.2 39706.3 0.1443 0.004 7 coil*lecithin -17.3019 42.9715 0.6957- 0.124 8 sucrose*cpowder 63220.2 39712 0.1425 0.004 9 sucrose*lecithin 63084.1 39692.6 0.1431 0.008 10 cpowder*lecithin N trials = 20 N terms - mixture constraints = 10 Residual SD = 2.034599, Lack-Of-Fit P=0.0252 * Residual DF = 10 Residual SD used for tests Replicate SD = 1.010104 Replicate DF = 5 Cross val RMS = 3.554036 R Squared = 0.526, P=0.3718 Adj R Squared = 0.100

139

To test the hypothesis developed above that an increase in the sucrose content will

decrease the WVP, while an increase in the cocoa powder content will increase the WVP,

the mixture design was augmented, to include coatings containing up to 40% cocoa

Figure 4–11: Three dimensional surface plot of the fitted model for permeability data obtained from the mixture experiments

140

powder and 60% sucrose. The augmented design also included coatings made without

any lecithin. The augmented design and the WVP data obtained are shown in Table 4–7,

and the three dimensional surface plot is shown in Figure 4–12. A summary of data

analysis of the augmented design is shown in Table 4–8. As predicted by Figure 4–11,

there is a significant increase (p<0.05) in the WVP upon increasing the cocoa powder

content, while increasing the sucrose content significantly decreases (p<0.05) the WVP.

Also, there is a significant effect (p<0.05) on addition of lecithin on the WVP of the

coatings. These results obtained from the augmented design confirm the hypothesis that

the addition of cocoa powder or lecithin increases WVP of the coatings while addition of

sucrose decreases the WVP. In mixture design, the change in the amount of one

ingredient affects the quantity of another ingredient; therefore the effect of change in one

ingredient is relative to the change in another ingredient. In the above, all the changes

have been discussed relative to the amount of fat in the coating.

Table 4–7: Effect of ingredients on the WVP of coatings – Results from the augmented design

Trial # Coconut oil Sucrose Cocoa powder LecithinPermeability [g mil m-2 day-1 (mmHg)-1]

16 0.8 0.2 0 0 10.10 ± 0.07 17 0.7 0.3 0 0 9.73 ± 0.3118 0.8 0 0.2 0 11.64 ± 0.2119 0.7 0 0.3 0 12.44 ± 0.3220 0.695 0 0.3 0.005 22.36 ± 0.8121 0.595 0 0.4 0.005 28.79 ± 1.2022 0.495 0.5 0 0.005 7.21 ± 0.2123 0.395 0.6 0 0.005 6.06 ± 0.1524 0.395 0.5 0.1 0.005 9.86 ± 0.1725 0.395 0.3 0.3 0.005 19.44 ± 1.54

141

The analysis in Table 4–8 also shows that the model is significant (p<0.05) while

the model was not significant in the initial mixture design (Table 4–6). The first part of

this study had shown that lecithin has an interaction with hydrophilic particles, and the

results from this study also confirmed that finding. The interaction term sucrose*lecithin,

Table 4–8: Summary of Data Analysis – Augmented Mixture Design

Coefficients for response 'Permeability' Log e transformation used Coefficient scaling is in terms of the stretched experimental region. COEFFICIENTS SD P CONDITION TERM -7.48948 0 CONSTANT 9.26802 2.86309 0.0041 0.025 1 Sucrose 12.0727 2.86957 0.0004 0.043 2 Cocoa 9.95564 2.9241 0.0028 0.026 3 Fat -3377.78 959.519 0.0022 0.005 4 lecithin -1.36858 0.702493 0.0656 0.393 5 Sucrose*Cocoa 1.15739 0.395636 0.0084 0.652 6 Sucrose*Fat 3421.12 974.867 0.0022 0.009 7 Sucrose*lecithin -2.6312 0.796507 0.0035 0.308 8 Cocoa*Fat 3445.34 970.451 0.0020 0.017 9 Cocoa*lecithin 3393.65 967.89 0.0022 0.008 10 Fat*lecithin N trials = 30 N terms - mixture constraints = 10 Residual SD = 0.128416 Residual DF = 20 Residual SD used for tests Replicate SD = 0.098774 Replicate DF = 5 Cross val RMS = 0.169422 R Squared = 0.881, P=0.0000 *** Adj R Squared = 0.827

142

and cocoa*lecithin significantly affects (p<0.05) the WVP. On the other hand, it can be

expected that there is no interaction between the sucrose, cocoa powder and fat. The

results also show that the interaction terms between the sucrose, cocoa powder, and fat

does not have a significant effect on WVP.

Comparing Figure 4–11 and Figure 4–12, it can be seen that the trend is similar in

both the figures. However, the permeability values with change in cocoa powder content

is much higher in Figure 4–12. Cocoa powder tends to swell upon adsorbing moisture.

The swelling of the cocoa powder particles can change the structure of the coating.

Higher the cocoa powder content, greater will be its impact on the structural changes of

the coatings. The effect of 50% cocoa on WVP was also studied and the results have

been discussed below.

143

In all the above-mentioned studies the humidity gradient across the film was

constant (75-3.5%). Biquet and Labuza (1988) had found that the WVTR of a dark

chocolate coating is not only dependent on the humidity gradient but also on the absolute

Figure 4–12: Three dimensional surface plot of the fitted model for permeability data obtained from the mixture experiments

144

humidity at each side of the test cell. Hence, the effect of absolute humidity on WVP of

coatings was also studied and the results are discussed in the next section.

4.4.3 Effect of Absolute Relative Humidity on WVP of Coatings

The effect of absolute humidity on the WVP was measured using the generic

coating formulation. In one setup, the humidity on the high and low end was 75% and

33% (average 54%) respectively, for the other study it was 75% and 54.3% (average

64.65%). The results obtained from the study are shown in Table 4–9. It can be seen

from the data below that the samples kept at a higher average humidity gave a higher

WVP. These data are in agreement with the results obtained by Landmann et al. (1960),

who found a large increase in the WVP of milk chocolate when the humidity gradient

was varied from 22 – 75% to 0 to 100% and Biquet and Labuza (1988), who found that

the WVP increased from 4.9 g mil m-2 day-1 mmHg-1 to 11.0 g mil m-2 day-1 mmHg-1

when the relative humidity difference was changed from 0-33% to 54.4 – 80.4% even

though the relative humidity gradient was constant (33%).

Figure 4–13 shows a picture of the generic coating which was kept at humidity

conditions of 75% on the outside and 33% in the bottom of the cell. A picture of the

generic coating kept at humidity conditions of 75% on the outside and 54.5% in the

bottom of the cell (average 64.75%) is shown in Figure 4–14, and a picture of the generic

coating sample taken before the water vapor transmission studies is shown in Figure 4–4.

It can be clearly seen that the coating kept at 75 – 54.3% condition swelled more than the

sample kept at 75 – 33 % condition. This implies that with an increase in the absolute

145

humidity, the coating absorbs more moisture and changes the structure of the coating.

The change in the structure of the coating causes an increase in the WVP.

Table 4–9: Effect of relative humidity difference on WVP of the generic coating at 18.5oC Relative Humidity Range Water Vapor Permeability (g mil m-2 day-1 mmHg-1)

3.5 - 75 10.67 ± 1.06 33 - 75 12.38 ± 0.70 54.5 - 75 61.96 ± 21.07

Figure 4–13: Picture of the generic coating kept at humidity conditions of 75% on the outside and 33% in the bottom of the cell

146

An increase in the WVP with changes in the absolute humidity has been seen in

other food and polymer systems. Kamper and Fennema (1984) and Fennema et al.

(1994) have found that the WVP of fatty acids increases with an increase in the absolute

relative humidity. Myers et al. (1961) found that the WVP of certain polymers, such as

cellulose, polyamide and polyvinyl alcohol, which swell in the presence of water,

increased with increase in the absolute value of the relative humidity.

The WVP of a plastic film or a coating is a product of the diffusivity, D, and

solubility coefficient, S. The effect of relative humidity on the solubility can be best

shown by means of an adsorption isotherm. The adsorption isotherm of the hydrophilic

particles, i.e., sucrose and cocoa powder is shown in Figure 4–15. The adsorption

Figure 4–14: Picture of the generic coating kept at humidity conditions of 75% on the outside and 54.5% in the bottom of the cell.

147

isotherm for cocoa powder is sigmoidal in shape, which is typical for most food materials

(Bell and Labuza, 2000). Sucrose on the other hand does not absorb much moisture until

a relative humidity of 80%, but upon increasing the humidity above 80%, there is a large

increase in the moisture sorption. The sorption of moisture by these hydrophilic particles

effectively plasticizes the coating and thus facilitates the diffusion process, i.e., the

diffusivity increases with increasing water in the coating. Since both the solubility and

the diffusivity increases, the net effect is a considerable increase in the permeability with

increase in the relative humidity.

In the above studies, it was found that an increase in the cocoa powder content

and the average humidity increased the WVP of the coatings. So a coating was made

with maximum possible cocoa powder content (50%) and its WVP was studied at

0

2

4

6

8

10

12

14

16

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water Activity (aw)

Moi

stur

e co

nten

t (%

d.b

.)

sucrosecocoa powder

Figure 4–15: Moisture sorption isotherms of sucrose and cocoa powder

148

different average humidities. Of the six samples tested, all but one coating cracked

during the test. Figure 4–16 shows the weight gain versus storage time of the coating.

The coating that did not have a crack during the test is marked as no cracks in the figure.

As evident from Figure 4–16, the cracking of the coating causes a large increase in the

weight gain of the samples. A picture of a coating after cracking is shown in Figure 4–

17. An attempt was also made to study the effect of storing the samples at 100%

humidity, but the samples cracked within two to three days of storage.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300

Time (h)

Wei

ght g

ain

(g)

33% - 133% - 260%-160%-23.5%-13.5% - 2

No Cracks

Figure 4–16: Weight gain versus storage time for a coating sample containing 50% cocoa powder with different relative humidity at the low humidity end. The numbers 1 and 2 at the end of the humidity in the legend signifies the sample numbers. The sample that did not cracked during the experiment is shown with an arrow as intact sample. The humidity at the high end was 75%.

149

Chocolate-flavored coatings are used in many frozen desserts, however, no data

has been reported in the literature on their behavior at sub-zero temperatures. To

understand the behavior of coating samples at sub-zero temperatures, the water vapor

transmission rates of coatings were measured. Since the coatings containing high

amounts of cocoa powder had the maximum increase in the WVP those coatings were

Figure 4–17: Picture of a coating, containing 50% cocoa powder by weight, after 12 days with a humidity gradient of 75 – 3.5%

150

tested for the sub-zero temperatures. The water vapor transmission rate for the coatings,

obtained as a function of time, at –5oC is shown in Figure 4–18. The voltage is directly

proportional to the water transfer rate. The thickness of the coating containing 40%

cocoa powder was 0.8mm, while the thickness of the coating containing 50% cocoa

powder was 1.2mm. The time required to reach steady state is l2/2D (Saravacos and

Maroulis, 2001), where l is the thickness of the coating and D is the unsteady-state

diffusion coefficient. Previous studies have shown that the unsteady-state diffusion

coefficient of a coating containing 50% cocoa powder should be lower than that of a

coating containing 40% cocoa powder. So, with a thicker film and lower diffusivity, the

time needed for the coating with 50% cocoa powder to crack should be more than the

time required for the coating with 40% cocoa powder to crack.

As expected, time required at -5oC was longer than the time needed for the

samples to crack at 18.5oC. There are two changes that are occurring with decrease in

temperature that reduces the diffusion rate. First, the mobility of moisture decreases with

a decrease in the temperature. Second, with a decrease in the temperature, there is an

increase in the SFC of the coating, which increases the resistance to moisture diffusion.

The two effects combine to give a large decrease in the moisture transfer rates.

151

Another qualitative study was done at –5oC to look the time taken for coatings to

crack during storage. In this study, the coating samples were cut into the size of a regular

Mason jar lid. The Mason jars were filled with water and were closed on top by using the

coatings as lids. The Mason jars were then put in a large desiccator containing saturated

sodium chloride solution. Thus there was a humidity gradient across the coatings. The

desiccators were put in a chamber maintained at –5oC. In this study, five coating samples

containing cocoa powder levels of 10, 20, 30, 40, 50% (w/w) and one coating with 40%

sucrose were used. The lecithin content in each of the samples was 0.5%. Each sample

was prepared in duplicate. The thickness of each of the sample was 0.9 ± 0.1 mm. The

samples were stored for 14 weeks (98 days). Both samples containing 50% cocoa

powder cracked after 42 days, one of the samples containing 40% sugar cracked after 77

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500 600 700 800 900

Time (h)

Vol

tage

(V)

40% cp50%cp20% sugar

Figure 4–18: Change in the voltage with storage time for coatings stored at –5 oC

152

days, the second sample containing 40% cocoa powder cracked after 81 days, and one of

the samples containing 20% sucrose cracked after 98 days. The pictures of the coatings

after 98 days of storage are shown in Appendix B. This study further suggests that high

amounts of cocoa powder in the coating leads to a change in the structure in the presence

of moisture.

4.4.4 Swelling of Cocoa Powder

From the sorption isotherm of sucrose and cocoa powder (Figure 4–15), it can be

seen that cocoa powder absorbs more moisture than sucrose at water activities lower than

0.85. Combining the moisture absorption data with the results obtained from the studies

on the effect of humidity, one can infer that the swelling in the coating may be due to the

cocoa powder particles.

Two studies were done to test this hypothesis. First was a qualitative study where

the dried cocoa powder was and put on two shallow pans and the surfaces were flattened

(Figure 4–19). These pans were then kept in a desiccator containing saturated NaCl

solution (75% RH). The desiccator was kept in a room maintained at 18oC. It was found

that within two days of storage, the surface had changed showing cracks in the surface

and the height had also increased above the level of the pan (Figure 4–20). This study

suggested that the change in the surface is due to the swelling of the cocoa powder. No

visual changes were observed from the data obtained using light microscopy.

153

Coatings containing varying amounts of cocoa powder were kept for a week at

75%RH in a room maintained at about 18oC. The percent increase in diameter of the

coatings increased with an increase in the cocoa powder content in the coating (Table 4–

10). These studies supported our hypothesis that the cocoa powder swells in the presence

Figure 4–19: Picture of the dry cocoa powder kept in shallow pans

Figure 4–20: Picture of hydrated cocoa powder

154

of moisture. Cocoa powder contains about 15.5 – 16 % starch (Schmieder and Keeney,

1980; Zaan, 1993) and about 32-34% fibers (Zaan, 1993). Starch molecules can swell up

to 30% in the presence of moisture (Full, 2003). Fibers are hygroscopic (Cadden, 1988)

and tend to swell on moisture absorption (Larrea et al., 1997). These two ingredients i.e.

starch and fibers are probably the reason for the swelling of the cocoa powder particles.

4.4.5 Structure of Coatings

A large number of SEM images of dry and hydrated coatings were obtained to

determine the structure of generic coating and a coating with 50% cocoa powder. The

structure of a generic coating as seen under the SEM is shown in Figure 4–21 and that of

a coating with 50% cocoa powder is shown in Figure 4–22. The images show that there

is a network of crystal spherulites in both types of the coatings. There seem to be small

cracks in certain parts of the crystals. These cracks could have occurred due to quick

cooling of the coatings with liquid nitrogen.

Loisel et al. (1997) suggested that most of pores in chocolates are less than 0.4

µm and cannot be observed under SEM. But after looking at the swelling of the coatings

in the WVP experiments, it was hypothesized that the change in the pore size will be

Table 4–10: Increase in the diameter of coatings stored at 75% RH Coating Sample Initial Diameter

(mm) Final Diameter (mm)

% Increase in the Diameter

Generic (12.5% cocoa powder)

91.025 91.237 0.47

20% cocoa powder 91.00 91.358 0.79 40% cocoa powder 91.05 91.95 1.99

155

large enough to be seen under the SEM. However, on comparing the images of dry and

hydrated coatings, no visual differences were observed. From the increase in the

diameter of the coatings (Table 4–10), it can be seen that the swelling of the coatings is

very small (<2%) and hence no change was seen in the SEM images. There was also no

observed differences in the coating thickness because the maximum variation in thickness

due to swelling (<2%) (Table 4–10) is less than the variation inherent variation in the

thickness of the coating.

Figure 4–21: Structure of a section of dry generic coating seen using cryo SEM

156

4.4.6 Effect of Solid Fat Content

The effect of solid fat content (SFC) on the WVP is shown in Figure 4–23. The

data shows that the WVP of the coating decreases with increase in the SFC. Martin-Polo

et al. (1992) observed that for a mixture of paraffin wax and oil blends the WVP

decreased with an increase in SFC and the results observed here are similar to their

results. The trend line shows that there is a large decrease in the WVP with small

Figure 4–22: Structure of a dry coating with 50% cocoa powder as seen under cryo SEM

157

increase in SFC when the SFC is less than 50%. Upon increasing the SFC beyond 50%,

the decrease in the WVP with increase in SFC is smaller. It was also very difficult to

make coatings with the fats having high SFC values (>60%). These fats had a tendency

to crack and several coatings had to be made to obtain one intact (without cracks)

coatings.

4.4.7 Effect of Cocoa Powder type, Sugar, and Emulsifier

The effect of natural and alkalized cocoa powder, sugars, and Citrem is shown in

Table 4–11. There is a significant increase in WVP when dextrose was used. Dextrose

adsorbs more moisture than sucrose (Iglesias and Chirife, 1982) at the same aw and this

can cause some structural changes which might have increased the WVP of the coating.

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90

Solid Fat Content (%)

Perm

eabi

lity

(g m

il/[m

2 da

y m

mH

g])

Figure 4–23: Effect of SFC on the WVP of fat coatings

158

There was a significant increase in the WVP with lactose and Citrem. Ctirem is polar

than lecithin and therefore more moisture will adsorb to its surface and therefore there is

an increase in the WVP.

There is no significant change (p>0.05) in the WVP of coatings with change in

any of the other ingredients. The generic coating was made using defatted cocoa powder.

The normal and alkalized cocoa powder contains 10-12% cocoa butter, but there is no

change in the SFC on the coatings. The average SFC of the generic coating was 54.2%,

while the average SFC of the coatings containing either alkalized or natural cocoa

powder was 53.5%. Since the SFC remains essentially constant, there is no change in the

WVP of the coatings with change in the cocoa powder type.

Previous studies had shown that lecithin had a significant effect on the diffusion

behavior when the relative humidity on one side of the coating is 85%. So, an attempt

was made to measure the WVP when the humidity on one side of the coating was 85%

and on the other side it was 13%. Duplicate samples of generic coating and coating made

with Citrem were studied in this setup. All the samples cracked within three days.

Before the samples had cracked sucrose crystals were seen in the surface of the coatings

Table 4–11: Effect of cocoa powder, sugar and emulsifier type on the WVP of coatings at 18.5oC Sample WVP (g mil m-2 day-1 mmHg-1) Generic 10.67 ± 1.06A

Lactose 12.62 ± 1.13B

Dextrose 14.88 ± 1.47B

Natural 10.57 ± 1.63A

Alkalized 10.52 ± 0.21A

Citrem 12.66 ± 0.67B Note: Samples with different letters are significantly different. The significance was determined using a 2-sample t-test and assuming equal variances.

159

(Figure 4–24). This suggested that the moisture diffusing through the coating had

dissolved the sucrose particles and the sucrose migrated to the surface of the coating

along with the moisture and crystallized at the surface.

4.5 Conclusions

The ingredients and the storage environment affect the WVP of the coatings.

Among the ingredients studied sucrose and fat decreased the WVP while cocoa powder

Figure 4–24: Picture of a cup with a generic coating that has a humidity of 85% inside the cup and 13% outside

160

and lecithin increased the WVP of the coatings. Lecithin has a bigger impact on the

WVP when the water activity on one side of the coating is higher than 0.85. The WVP of

fat films decreased with increase in the solid fat content (SFC). The type of cocoa

powder (natural and alkalized) did not have a significant effect on the WVP, while

lactose, dextrose and Citrem, significantly increased the WVP. The WVP of the coatings

also depended on the absolute relative humidity and the temperature of storage.

4.6 References

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Biquet, B. and Labuza, T. P. (1988). Evaluation of the moisture permeability


989 - 998.

Cadden, A. M. (1988). Moisture sorption characteristics of several food fibers. J. Food

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Callegarin, F., Gallo, J. Q., Debeaufort, F. and Voilley, A. (1997). Lipids and

biopackaging. J. Am. Oil Chem. Soc. 74(10): 1183 - 1192.

Cornell, J. A. (2002). Experiments with Mixtures: Designs, Models, and the Analysis of

Mixture Data. New York, NY, Wiley-Interscience.

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Tomorrow's packagings: A review. CRC Critical Reviews in Food Science 38(4):

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161

Fennema, O., Donhowe, I. G. and Kester, J. J. (1994). Lipid type and location of the

relative humidity gradient on the barrier properties of lipids to water vapor. J.

Food Eng. 22: 225-239.

Full, N. (2003). Swelling of starch molecules in different solvents. Personal

Communication.

Garbolino, C. (2002). The influence of surfactants and moisture on the colloidal and

rheological properties of model chocolate dispersions. Ph.D. thesis. Department

of Food Science. Pennsylvania State Univerisity, University Park, PA

Ghosh, V., Ziegler, G. R. and Anantheswaran, R. C. (2002). Fat, moisture, and ethanol

migration through chocolates and confectionery Coatings. Crit. Rev. Food Sci.

Nutr. 42(6): 583 - 626.

Guilbert, S. and Biquet, B. (1995). Edible films and coatings. Food Packaging

Technology. G. Bureau and J.-L. Multon. New York, NY, VCH Publishers, Inc.:

315 - 353.

Iglesias, H. A. and Chirife, J. (1982). Handbook of Food Isotherms: Water Sorption

Parameters for Food and Food Components. New York, NY, Academic Press.

Kamper, S. L. and Fennema, O. (1984). Water vapor permeability of an edible, fatty acid,

bilayer film. J. Food Sci. 49(6): 1482 - 1485.

Kester, J. J. and Fennema, O. R. (1986). Edible films and coatings: A review. Food

Technol.(December): 47 - 59.

Koelsch, C. (1994). Edible water vapor barriers: Properties and promise. Trends Food

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162

Krochta, J. M. and Mulder-Johnston, C. D. (1997). Edible and biodegradable polymer

films: Challenges and opportunities. Food Technol. 51(2): 61 - 74.

Labuza, T. P. and Hyman, C. R. (1998). Moisture migration and control in multi-domain


Landmann, W., Lovegreen, N. V. and Feuge, R. O. (1960). Permeability of some fat

products to moisture. J. Am. Oil Chem. Soc. 37(1): 1 - 4.

Larrea, M. A., Grossmann, M. V. E., Beleia, A. P. and Tavares, D. Q. (1997). Changes in

water absorption and swollen volume in extruded alkaline peroxide pretreated rice

hulls. Cereal Chemistry 74(2): 98-101.

Loisel, C., Lecq, G., Ponchel, G., Keller, G. and Ollivon, M. (1997). Fat bloom and

chocolate structure studied by mercury porosimetry. J. Food Sci. 62(4): 781 - 788.

Martin-Polo, M., Voilley, A., Blond, G., Colas, B., Mesnier, M. and Floquet, N. (1992).

Hydrophobic films and their efficiency against moisture transfer. 2. Influence of

the physical state. J. Agric. Food Chem. 40: 413 - 418.

Matissek, R. 2002. Zwischen zwei Phasen – Citrem. SÜSSWAREN (2002) Heft 5: 6

Montgomery, D. C. (1997). Design And Analysis Of Experiments. New York, NY, Wiley

Interscience, Inc.

Morillon, V., Debeaufort, F., Blond, G., Capelle, M. and Voilley, A. (2002). Factors

affecting the moisture permeability of lipid-based edible films: A review. Crit.

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163

Morillon, V., Debeaufort, F., Capelle, M., Blond, G. and Voilley, A. (2000). Influence of



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the gas and vapor permeability of plastic films and coated papers. Part VI. The

permeation of water vapor. Tappi 44(1): 58 - 64.

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NY, Marcel Dekker.

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in cocoa beans and chocolate products. J. Food Sci. 45(3): 555-557, 563.

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Cacao De Zaan.

Chapter 5

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH


investigated in this study. An approach, developed by Weisz (1967) to understand

diffusion in heterogeneous materials, was used to elucidate the mechanism of moisture

migration through chocolate-flavored coatings. It was found that Weisz (1967) approach

could be used to model the diffusion through heterogeneous food materials. The

diffusion of moisture through the dispersed phase occurred through the cocoa powder

particles and along the surface of the sucrose particles. The approach outlined in this

work can be extended to understand the mechanism of moisture migration in other food

systems.

The time lag experiments used to measure the diffusion coefficients took a long

time to complete because of the accuracy of the balance. Another problem that occurred

was that the salt would diffuse through the coating material, thus altering the structure of

the coating. During the experiment, the Thwing Albert cups were removed from the

environmentally controlled chamber for weighing. The salt solution at the bottom of the

cups were disturbed when the cups were removed for weighing and the salt particle might

have gotten attached to the surface of the coating and then diffused into the coating

material. Therefore, it is recommended to develop a faster and flow-through method for

measuring the diffusivity of the coating materials.

165

The presence of lecithin increased the moisture adsorption capacity of sugar. The

moisture adsorption isotherm for a coating made with 70% coconut oil + 0.5% lecithin

and 30% sugar was determined. The equilibrium moisture content, for this coating, at

each water activity level was higher than the individual constituents, i.e., sugar or

coconut oil + 0.5% lecithin. On of the possible reasons for the observed data is that there

is a layer of moisture that is present in between the surface of the sugar particles and the

polar regions of lecithin.

The effect of sucrose, cocoa powder, emulsifier, fat type, and storage environment

on the water vapor permeability (WVP) of a chocolate-flavored coating was also studied.

It was determined that the presence of lecithin and cocoa powder increases the water

vapor permeability. The effect of lecithin becomes prominent only when the water

activity on one side of the coating is higher than 0.85. The WVP of fat films decreased

with increase in the solid fat content (SFC) and reached a minimum at 80% SFC.

Therefore, a coating that is very resistant to moisture diffusion should have a high SFC

fat in the continuous phase and large quantity of sugar (>60%) , while it should not have

any lecithin or cocoa powder.

One of the advantages of lecithin is that it changes favorably the rheological

properties of the chocolate-flavored coatings during processing. During processing,

moisture at the surface of the sugar increases the viscosity. This is because the moisture

at the surface of the sugar particles increases friction between them. This results in a

greater resistance when the particles move among themselves and produces the effect of

increased viscosity. The viscosity is decreased by addition of lecithin, since the

166

hydrophilic groups of the molecules attach themselves firmly with water molecules.

Further research needs to be done using alternative emulsifiers that produces affects

favorably the rheological properties, but does not affect the moisture migration rates. It

has been found that PGPR adsorb less moisture than lecithin and therefore has a good

potential for being used as an alternative emulsifier. Research also needs to be done on

coatings with any cocoa powder and their rheological properties.

The WVP of fat based films decreased with an increase in the solid fat content

(SFC) to 80%. The WVP of fat films reached a minimum at 80% SFC. Studies need to

be done to find out the reason for a decrease in the WVP of coatings above the 80% SFC.

Information that will help in answering the behavior observed in this study is the

correlation between structure and moisture migration rate. Therefore, a studies need to

be done to understand the relationship between the structure of the coating and diffusion

behavior. Porosity of the fats can be determined using mercury porosimetry.

The data at sub-zero temperatures show that the presence of cocoa powder (>40%

w/w) causes cracks in the coating during storage. Research needs to be done to

determine the moisture sorption behavior of sugar and cocoa powder at sub-zero

temperatures.

Appendix A

DEVELOPMENT OF A METHOD TO MEASURE THE WATER VAPOR TRANSMISSION RATE

In order to study the effect of the ingredients on the WVP through the

coatings studies were conducted using the ASTM E-96 method. A schematic of the test

cell is shown in Figure A–1. During the initial studies the cups were put in desiccators

containing saturated salt solutions (Figure A–2). The weight gain with time was

monitored till the rate of change of weight became constant with time.

Figure A–1: Schematic of a test cell

168

The assumption in using the desiccator for WVP studies was that the humidity

inside the desiccator reached the desired humidity very quickly. A study done using a

handheld hygrometer showed that it took about 90 to 120 minutes before the humidity

inside the desiccator reached the desired value (Figure A–3). On the other hand when a

fan was put inside the setup (Figure A–4), it took only 5 to 10 minutes to reach the

desired value.

Figure A–2: Setup for measuring WVTR using desiccators

169

0102030405060708090

0 50 100 150 200

time (minutes)

Rel

ativ

e H

umid

ity (%

)

desiccatorconvection

Figure A–3: Time to reach equilibrium humidity in a desiccator and the box fitted with a fan (convection)

170

The following studies were performed to check the setup for measuring the WVP:

(i) compare the water vapor transmission rate (WVTR) values with and without

convection and (ii) compare the WVTR of the coatings in the presence of one and two

fans. The result of the first study is shown in Table A–1and the result of the second study

is shown in Table A–2. The data shown that there are considerable errors when using the

desiccator method. The error for the sample having a lower WVTR value is less than that

with higher WVTR value. This behavior is expected because the lower the WVTR value,

the closer it is to the assumption that the time taken by the moisture to get saturated

Figure A–4: Setup for measuring the water vapor transmission rate at 20oC

171

inside the chamber is negligible compared to the time taken to diffuse through the

coating.

The data in Table A–2 shows that the addition of an extra fan does not change the

experimental WVTR. Addition of an extra fan increases the convection and if the

WVTR has not reached its limiting value then the WVTR value should increase with

increasing level of convection as seen in Table A–1. This does not happen in the second

experiment (Table A–2), hence, the WVTR data obtained using one fan is the limiting

WVTR value of the coatings. Hence, a setup with one fan is suitable for studying the ater

vapor transmission rates of the coatings under consideration.

Table A–1: Comparison of WVTR (g m-2 day-1) data obtained with and without convection

Sample # Desiccator (No Convection) Convection % Error

1 3.713 4.739 21.65

2 3.106 3.559 12.73

Table A–2: Comparison of WVTR (g m-2 day-1) data obtained when one or two fans are used for creating convection inside the setup

Sample # One fan Two fans

1 3.542 3.431

2 4.062 4.238

Appendix B

MOISTURE SORPTION AND DIFFUSION COEFFICIENT DATA

The moisture content data obtained from the sorption experiments and weight loss

data obtained from the time-lag experiments are given in this Appendix.

B.1 Moisture Sorption Isotherms

B.1.1 Coconut Oil

The moisture content data obtained for coconut oil is shown in Table B–1.

B.1.2 Coconut Oil + 0.5% lecithin

The moisture content data obtained for coconut oil + 0.5% lecithin is shown in

Table B–2.

Table B–1: Equilibrium moisture content for the coconut oil samples Water Activity Moisture Content (%db)

Set 1 Set 2 Average Moisture Content

0.11 0.0415 0.0447 0.0431 0.75 0.0778 0.0810 0.0794 0.85 0.0975 0.1185 0.1080

173

B.1.3 Sugar

The moisture content data obtained for sugar is shown in Table B–3

B.1.4 Cocoa Powder

The moisture content data obtained for cocoa powder is shown in Table B–4

Table B–2: Equilibrium moisture content data obtained for coconut oil containing 0.5% lecithin Water Activity Moisture Content (%db)


0.11 0.0603 0.0575 0.0589 0.75 0.210 0.190 0.2000 0.85 0.315 0.267 0.2910

Table B–3: Equilibrium moisture content for sugar at different water activities Water Activity

Sorption Moisture Content (%db)

Set 1 Set 2 Average

Desorption Moisture Content (%db)

0.11 0.09 0.10 0.0905 0.100 0.54 0.10 0.11 0.105 0.105 0.70 0.12 0.12 0.120 0.120 0.75 0.14 0.13 0.135 0.135 0.79 0.18 0.20 0.190 0.190 0.85 0.73 0.82 0.775 0.775

174

B.1.5 Coconut oil + 30% Sugar + 0.5% Lecithin

The moisture content data obtained for coconut oil containing 30% sugar in

coconut oil +0.5% lecithin is shown in Table B–5.

B.2 Diffusion Coefficients

B.2.1 Coconut Oil and Coconut Oil + 0.5% Lecithin

The weight loss data and diffusivity through coconut oil and coconut oil + 0.5%

lecithin is shown in Table B–6.

Table B–4: Equilibrium moisture content of cocoa powder at different water activities Water Activity

Sorption Set 1 Set 2 Average

Desorption Set 1 Set 2 Average

0.11 4.40 3.86 4.13 6.30 5.21 5.750.33 6.68 6.76 6.72 7.94 8.65 8.290.54 8.53 8.39 8.46 9.45 10.74 10.100.70 9.72 10.19 9.95 11.21 11.46 11.330.75 11.41 10.89 11.15 12.34 12.20 12.270.79 11.64 12.44 12.04 12.90 13.23 13.060.85 13.43 14.96 14.20 13.43 14.96 14.20

Table B–5: Equilibrium moisture content for a coating containing 30% sugar in coconut oil + 0.5% lecithin Water Activity Moisture Content (%db)


0.11 0.180 0.205 0.193 0.33 0.236 0.170 0.203 0.75 0.272 0.251 0.262 0.85 1.033 1.521 1.277

175

B.2.2 Coatings Containing Sugar

The weight loss data and diffusivity through coatings containing 30% and 40%

sugar in coconut oil is shown in Table B–7.

Table B–6: Weight loss data obtained for coatings made from coconut oil and coconut oil + 0.5% lecithin

Coconut Oil Coconut Oil + 0.5% Lecithin Time (h) Sample 1 Sample 2 Sample 1 Sample 2

0 0 0 0 00.5 0.0012 0.0012 0.001 0.001

17.75 0.0049 0.0045 0.004 0.004844 0.0137 0.0145 0.0125 0.0125

68.25 0.0216 0.0216 0.0193 0.0291.5 0.0314 0.0298 0.0277 0.0278

114.5 0.0415 0.0387 0.035 0.0355138.75 0.0565 0.0503 0.0426 0.042

Time Lag (h) 5.028 4.365 5.674 3.929Thickness (mm) 2.067 1.940 1.798 1.918Diffusivity (m2s-

1) 3.93 x 10-11 3.99 x 10-11 3.00 x 10-11 3.81 x 10-11

Permeability (g mil m-2 day-1 mmHg-1) 13.52 12.07 10.49 10.98

176

B.2.3 Coatings Containing Sugar and Lecithin

The weight loss data and diffusivity through coatings containing 30% and 40%

sugar in coconut oil + 0.5% lecithin is shown in Table B–8 - B–10.

Table B–7: Weight loss data obtained for coatings containing 30% and 40% sugar in coconut oil. These data were obtained when the relative humidity on the higher humidity side was 75%

30% sugar 40% sugar Time (h) Sample 1 Sample 2 Sample 1 Sample 2

0 0 0 0 023.67 0.0037 0.0047 0.0031 0.004444.17 0.008 0.0089 0.0071 0.009472.42 0.0138 0.0158 0.0125 0.015893.67 0.018 0.0219 0.0174 0.0225

114.67 0.0232 0.0273 0.0216 0.0276Time Lag (h) 7.453 10.957 10.645 9.521

Thickness (mm) 1.803 2.047 1.910 1.743Diffusivity (m2s-1) 2.02 x 10-11 1.77x 10-11 1.59x 10-11 1.48 x 10-11

Permeability (g mil m-2 day-1

mmHg-1) 7.99 11.18 10.41 7.51

177

Table B–8: Weight loss versus time data for coatings containing 30% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 85%

Sample 1 Sample 2 Time (h) Weight loss (g) Time (h) Weight loss (g)

0 0 0 0 2 0.0005 0.5 0.0003 4 0.001 3.67 0.0016

6.08 0.0013 12.92 0.0054 9 0.002 19.42 0.0079

12.5 0.0046 26.58 0.013 16 0.0051 36.82 0.0192

23.75 0.009 46.82 0.0242 33 0.0163 61.82 0.0379

39.58 0.0205 75.58 0.0505 51.5 0.0332 86.15 0.067

59.83 0.0436 95.42 0.0816 71.5 0.0602 112.17 0.1246

129.83 0.1754 157.67 0.2668

Time Lag (h) 17.968 11.907 Thickness (mm) 1.790 1.942

Diffusivity (m2s-1) 8.26 x 10-12 1.47 x 10-11 Permeability (g

mil m-2 day-1 mmHg-1) 37.27 29.10

178

Table B–9: Weight loss versus time data for coatings containing 40% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 85%. Time (h) Sample 1 Sample 2

0 0 020 0.0056 0.0052

42.25 0.0137 0.012468 0.0271 0.0238

93.17 0.046 0.0404117.82 0.0678 0.057

Time Lag (h) 26.524 24.326Thickness (mm) 2.045 2.048

Diffusivity (m2s-1) 7.3 x 10-12 7.98 x 10-12


mmHg-1) 26.65 22.16

Table B–10: Weight loss versus time data for coatings containing 30% sugar in coconut oil + 0.5% lecithin. These data were obtained when the relative humidity on the higher humidity side was 75%

30% sugar 40% sugar Time (h) Sample 1 Sample 2 Sample 1 Sample 2

0 0 0 0 022.67 0.0079 0.0077 0.01 0.005944.17 0.0337 0.0233 0.0205 0.019666.66 0.0814 0.0466 0.0403 0.042993.5 0.1495 0.0782 0.069 0.0753115 0.1882 0.1075 0.0962 0.105162 0.3098 0.1739 0.1704 0.1901

Time Lag (h) 32.492 33.770 41.116 42.994Thickness (mm) 2.0925 1.9775 2.0025 1.9325

Diffusivity (m2s-1) 6.24 x 10-12 5.36 x 10-12 4.52 x 10-12 4.02 x 10-12


mmHg-1) 103.13 55.23 57.34 62.56

179

B.2.4 Coatings Containing Cocoa Powder

The weight loss data and diffusivity through coatings containing cocoa powder in

coconut oil is shown in Tables B–11 - B–14.

Table B–11: Weight loss versus time for coatings containing 2.5% cocoa powder in coconut oil

Time (h) Sample 1 0 0

15.17 0.008625.5 0.0135

39 0.022951.25 0.031

Time Lag (h) 2.638Thickness (mm) 0.805

Diffusivity (m2s-1) 1.14 x 10-11

Permeability (g mil m-2 day-1 mmHg-1) 10.91

180

Table B–12: Weight loss versus time for coatings containing 20% cocoa powder in coconut oil


0 0 0 0 2.17 0 7 0.0052 7.58 0.0013 13.5 0.0067 22.5 0.008 27 0.0125

33 0.0149 35 0.015 45.58 0.0246 50 0.0239

57 0.0327 57.5 0.0268 70.42 0.0425 72 0.0361

83 0.0422 97.75 0.0517 105 0.0556 122.83 0.0668 147.5 0.0805



mil m-2 day-1 mmHg-1) 11.038 9.16

181


0 0 0 0 3 0 7 0.0006

7.83 0 13.5 0.006 19 0.0022 27 0.012 28 0.0061 35 0.0158

46.75 0.0163 50 0.0252 54.83 0.0203 57.5 0.0302 67.25 0.0282 72 0.0401 82.75 0.0367 83 0.0469

97.75 0.0584 105 0.0627 122.83 0.0754 147.5 0.0918



mil m-2 day-1 mmHg-1) 9.107 13.71


182

B.2.5 Coatings Containing Cocoa Powder and Lecithin

The weight loss data and diffusivity through coatings containing cocoa powder in

coconut oil is shown in Tables B–15 - B–18



0 0 0 0 2.17 0 15.67 0 7.58 0.0004 29.5 0 22.5 0.0012 47 0.0026

33 0.0062 67 0.0207 45.58 0.0144 88.17 0.0322

57 0.0216 114.75 0.05 70.42 0.0325 137.75 0.0655

158 0.0818 Time Lag (h) 24.716 47.218

Thickness (mm) 0.775 1.022 Diffusivity (m2s-1) 1.13 x 10-12 1.03 x 10-11


mmHg-1) 11.22 14.24

183

Table B–15: Weight loss versus time for coatings containing 20% cocoa powder in coconut oil and 0.5% lecithin

Time (h) Sample 1 Sample 2 0 0 0

15.17 0.0049 0.005625.5 0.0086 0.0088

39 0.0142 0.014651.25 0.0185 0.0194

Time Lag (h) 2.455 1.61Thickness (mm) 0.817 0.742

Diffusivity (m2s-1) 1.26 x 10-11 1.59 x 10-11


mmHg-1) 6.69 6.20

184



0 0 0 0 14.92 0.0062 7 0.0068 25.42 0.0131 13.5 0.0096

38 0.023 27 0.017 49.42 0.0321 35 0.0214 62.84 0.0441 50 0.0322

57.5 0.0375 72 0.0484 83 0.0564 97.75 0.0689 105 0.0742 122.83 0.0895 147.5 0.1103



mil m-2 day-1 mmHg-1) 13.13 15.85

185



0 0 0 0 2.17 0 3 0 7.58 0 7.83 0.0003 22.5 0.0115 19 0.0055

33 0.0219 28 0.013 45.58 0.0356 46.75 0.0271

57 0.0471 54.83 0.0337 70.42 0.0617 67.25 0.0426

82.75 0.0541 Time Lag (h) 12.233 10.031

Thickness (mm) 0.874 0.785 Diffusivity (m2s-1) 2.89 x 10-12 2.84 x 10-11


mmHg-1) 19.22 12.14

186

Table B–18: Weight loss versus time for coatings containing 40% cocoa powder in coconut oil and 0.5% lecithin Time (h) Sample 1 Sample 2 Time (h) Sample 3

0 0 0 0 015.67 0 0.0002 3 029.5 0.0028 0.0002 7.83 0.0002

47 0.0136 0.0124 19 0.000267 0.0667 0.0678 28 0.0046

88.17 0.1011 0.1023 46.75 0.0193114.75 0.1532 0.1582 54.83 0.0269137.75 0.2009 0.2128 67.25 0.0401

158 0.2528 0.2672 82.75 0.0584Time Lag (h) 42.529 46.052 29.716

Thickness (mm) 0.842 1.063 1.005Diffusivity (m2s-1) 7.72 x 10-13 1.14x 10-12 1.57 x 10-12


mmHg-1) 35.59 48.49 22.76

187

Figure B–1: Adsorption of lecithin on sugar at water activities > 0.8. Model proposed by Garbolino (2002)

Appendix C

WATER VAPOR PERMEABILITY DATA

The raw data on weight gain obtained for the WVTR experiments and SEM

pictures are given in this appendix. The WVTR is calculated using the equation C.1

daymg

AslopeWVTR 2= ( C.1 )

where A is the area of the coating (m2) and slope is the slope of the straight line portion

of the plot of weight gain versus time. The water vapor permeability (WVP) was

calculated using equation C.2.

mmHgdaymmilg

pxWVTRWVP 2∆

∆•= ( C.2 )

where ∆x is the thickness of the coating (mil) and ∆p is the vapor pressure difference

(mm Hg). The equilibrium relative humidity above the calcium chloride particles was

found to be 3.5%. Hence, in the calculations the relative humidity at the low humidity

end was taken as 3.5%. The vapor pressure at the high humidity end was obtained using

the average temperature and humidity during the experiment. The temperature and

humidity information was obtained from the data logger. The exposed area of the coating

was 46 cm2.

189

C.1 Effect of Coating Thickness

The WVTR and permeability data for a generic coating at 17oC are shown in

Table C–1.

Table C–1: Weight gain versus time data at 17oC for coatings with different thickness and its effect on water vapor permeability.

Time (h) Sample 1 Sample 2 Sample 3 Sample 4 0 0 0 0 0

26.33 0.0545 0.0592 0.0545 0.059247.3 0.0724 0.0783 0.0724 0.0783

73 0.0873 0.0936 0.0873 0.093697.25 0.098 0.1047 0.098 0.1047

124 0.1079 0.1147 0.1079 0.1147144 0.1186 0.1256 0.1186 0.1256169 0.127 0.1327 0.127 0.1327

192.33 0.1374 0.1422 0.1374 0.1422222.42 0.1427 0.1496 0.1427 0.1496239.58 0.1463 0.156 0.1463 0.156

263 0.1536 0.1627 0.1536 0.1627294.42 0.1587 0.1669 0.1587 0.1669

313 0.1649 0.1731 0.1649 0.1731Thickness (mm) 2.014 2.012 0.880 0.808WVTR (g m-2 day-1) 0.96 1.34 2.24 2.46Permeability (g mil m-2 day-1 mmHg-1) 7.44 8.80 7.60 7.68

190

C.2 Mixture Experiment

The WVTR and permeability data for the mixture experiments are shown in

Tables C–2- C–21. The permeability data for the mixture experiments were obtained at

20oC.

Table C–2: Weight gain versus time obtained for coating made from trial 1 formulation Time (h) Sample 1 Sample 2

0 0 028 0.0469 0.0408

52.33 0.0666 0.06174.75 0.0793 0.077599.33 0.0915 0.0905

120 0.1043 0.0998145.25 0.115 0.112

Thickness (mm) 1.30 1.41WVTR (g m-2 day-1) 2.688 2.536Permeability (g mil m-2 day-

1 mmHg-1) 10.95 11.23


0 0 012 0.0056 0.009824 0.0124 0.011336 0.017 0.018148 0.0282 0.026260 0.0329 0.030472 0.0406 0.038384 0.0474 0.046996 0.0538 0.052


1 mmHg-1) 9.50 10.17

191


0 0 012 0.0173 0.017724 0.031 0.032436 0.0402 0.043148 0.0513 0.056960 0.0606 0.066472 0.0699 0.0762


1 mmHg-1) 12.90 12.63


0 0 018.17 0.0348 0.0409

26 0.0435 0.043742 0.0648 0.0632

49.17 0.0726 0.071866.17 0.0924 0.0851

73 0.0984 0.091988.17 0.1087 0.1053

111.82 0.133 0.1263124.5 0.1463 0.138


1 mmHg-1) 13.97 14.01

192


0 0 012 0.0118 0.012824 0.0217 0.024636 0.0304 0.03148 0.0398 0.0460 0.0424 0.045372 0.0498 0.054584 0.0548 0.060496 0.0624 0.0668

108 0.0657 0.0727Thickness (mm) 1.19 1.06WVTR (g m-2 day-1) 2.404 2.652Permeability (g mil m-2 day-

1 mmHg-1) 8.99 8.83


0 0 012 0.0207 0.020824 0.0316 0.031936 0.0386 0.040948 0.0498 0.05160 0.0581 0.057372 0.0671 0.067684 0.0784 0.077796 0.0824 0.0836

108 0.0943 0.0906120 0.1006 0.1016132 0.1068 0.1093

Thickness (mm) 0.962 0.977WVTR (g m2 day-1) 3.49 3.46WVTR (g m-2 day-1) 10.57 10.64Permeability (g mil m-2 day-

1 mmHg-1)

193


0 0 018.42 0.0209 0.01847.17 0.048 0.039362.83 0.0621 0.051273.5 0.0724 0.0585

87 0.0729 0.0655Thickness (mm) 0.90 0.96WVTR (g m2 day-1) 4.86 3.84WVTR (g m-2 day-1) 13.75 11.62Permeability (g mil m-2 day-

1 mmHg-1)


0 0 012 0.0207 0.020824 0.0316 0.031936 0.0386 0.040948 0.0498 0.05160 0.0581 0.057372 0.0671 0.067684 0.0784 0.077796 0.0824 0.0836

108 0.0943 0.0906120 0.1006 0.1016132 0.1068 0.1093


1 mmHg-1) 8.52 11.70

194


0 0 012 0.0143 0.020224 0.0224 0.0359

47.75 0.0319 0.048860 0.0413 0.059172 0.0476 0.070984 0.0514 0.0811

96.5 0.0581 0.0918Thickness (mm) 1.091 1.296WVTR (g m-2 day-1) 2.33 4.65Permeability (g mil m-2 day-

1 mmHg-1) 8.78 20.82


0 0 012 0.0166 0.020424 0.0303 0.034936 0.0417 0.046448 0.0551 0.061360 0.0667 0.073872 0.0925 0.098184 0.1037 0.109496 0.1135 0.1215


1 mmHg-1) 11.62 11.02

195


0 0 012 0.0072 0.013124 0.0136 0.014236 0.0203 0.021948 0.0291 0.029860 0.0368 0.036672 0.0467 0.04784 0.0523 0.055196 0.0592 0.059


1 mmHg-1) 9.53 8.53

Table C–13: Weight gain versus time obtained for coating made from trial 12 formulation

Time (h) Sample 1 Sample 2 0 0 0

12 0.0106 0.014624 0.0228 0.025636 0.0317 0.031848 0.0387 0.039760 0.0424 0.046972 0.0524 0.05784 0.0595 0.0623


1 mmHg-1) 9.44 9.28

196


0 0 012 0.0147 0.016224 0.0264 0.028236 0.0373 0.038548 0.0491 0.050760 0.0568 0.06172 0.0687 0.071784 0.0789 0.0811


1 mmHg-1) 11.10 11.83


0 0 012 0.0094 0.008524 0.0182 0.015536 0.0243 0.023648 0.0322 0.029860 0.0407 0.035872 0.0465 0.043


1 mmHg-1) 8.82 8.80

197

Table C–16: Weight gain versus time obtained for coating made from trial 15 formulation


0 0 0 028 0.0117 24.33 0.0341

52.33 0.0221 46.75 0.046274.75 0.0315 71.33 0.059599.33 0.0399 92 0.07

120 0.0479 117.25 0.0815145.25 0.0561

Thickness (mm) 1.417 1.244WVTR (g m-2 day-1) 1.84 2.61Permeability (g mil m-2 day-1 mmHg-1) 8.17 10.22

Table C–17: Weight gain versus time obtained for coating made from trial 1 formulation. Second replicate Time (h) Sample 1 Sample 2

0 0 012 0.0153 0.017424 0.026 0.0257

47.75 0.0363 0.032660 0.0432 0.0472 0.0512 0.049184 0.0568 0.0522


1 mmHg-1) 10.25 10.24

198


0 0 012 0.0054 0.006824 0.0134 0.014236 0.0215 0.020648 0.0305 0.029


1 mmHg-1) 11.41 10.37


0 0 012 0.0189 0.015224 0.0325 0.0336 0.0437 0.036748 0.0569 0.045860 0.0676 0.054972 0.0827 0.070584 0.0975 0.0783


1 mmHg-1) 15.88 9.91

199

Table C–20: Weight gain versus time obtained for coating made from trial 4 formulation. Second replicate


0 0 0 018.17 0.0337 12 0.0156

26 0.0417 24 0.027942 0.058 36 0.0356

49.17 0.0665 48 0.047266.17 0.0798 60 0.0575

73 0.085 72 0.069988.17 0.0948 84 0.0803

111.82 0.1128 124.5 0.1244



0 0 012 0.0167 0.018724 0.0298 0.034136 0.0392 0.044548 0.0505 0.05960 0.0598 0.067972 0.0714 0.078684 0.0768 0.086696 0.0866 0.0992


1 mmHg-1) 11.41 12.04

200

C.3 Effect of Ingredients

The effect of ingredients on the WVP was obtained at temperature of 18.5oC. The

data is shown in Tables C–22 - C–30.

Table C–22: Weight gain versus time obtained for a generic coating.(Set 1) Time (h) Sample 1 Sample 2

0 0 023 0.0153 0.0144

45.75 0.0286 0.02679.5 0.0531 0.0486

102.66 0.0667 0.0627123.67 0.0792 0.0777


1 mmHg-1) 10.83 10.73


0 0 023 0.0158 0.0104

45.75 0.0287 0.020279.5 0.0548 0.0374

102.66 0.0696 0.0511123.67 0.085 0.0652


1 mmHg-1) 9.74 9.25

201


0 0 019.83 0.026 0.026543.17 0.045 0.04765.75 0.0651 0.065288.58 0.0823 0.0812

115.43 0.1022 0.1024138.83 0.1216 0.1209162.25 0.1385 0.1353


1 mmHg-1) 12.21 11.76

Table C–25: Weight gain versus time obtained for a generic coating.(Set 4) Time (h) Sample 7

0 026.33 0.053347.33 0.075372.83 0.0979

97 0.116123.83 0.1321143.67 0.1395

169 0.1538193 0.1679

222.83 0.1841246.17 0.1965263.58 0.2037

294 0.2221315.33 0.2313

Thickness (mm) 1.085WVTR (g m-2 day-1) 2.72Permeability (g mil m-2 day-

1 mmHg-1) 10.19

202

Table C–26: Weight gain versus time obtained for a coating made using Citrem Time (h) Sample 1 Sample 2

0 0 012.5 0.0193 0.0197

23.58 0.0293 0.029431 0.0365 0.0366

48.5 0.0492 0.050661 0.0616 0.0627

73.5 0.0681 0.068882.17 0.0718 0.0733


1 mmHg-1) 12.19 13.14

Table C–27: Weight gain versus time obtained for a coating made using commercial natural cocoa powder Time (h) Sample 1 Sample 2

0 0 021.5 0.0493 0.0453

43.45 0.0784 0.066666.58 0.0867 0.083789.33 0.0986 0.0952

114.58 0.116 0.1068138.16 0.1305 0.1182


1 mmHg-1) 11.72 9.42

203

Table C–28: Weight gain versus time obtained for a coating made using commercial alkalized cocoa powder Time (h) Sample 1 Sample 2

0 0 021.5 0.0918 0.0612

43.45 0.1139 0.102566.58 0.1222 0.123689.33 0.1357 0.1327

114.58 0.1584 0.1523138.16 0.1715 0.1743


1 mmHg-1) 10.67 10.37

Table C–29: Weight gain versus time obtained for a coating made using lactose Time (h) Sample 1 Sample 2

0 0 07.5 0.0174 0.0156

21.5 0.0301 0.02931.67 0.0385 0.037245.83 0.049 0.048355.67 0.0563 0.0547


1 mmHg-1) 11.82 13.41

204

C.3.1 Effect of Solid Fat Content on Water Vapor Permeability

The weight gain versus time data for different fats are shown in Tables C–31- C–

36. The SFC of each fat was also determined at the temperature at which the experiment

was conducted.

Table C–30: Weight gain versus time obtained for a coating made using dextrose Time (h) Sample 1 Sample 2

0 0 07.5 0.0193 0.0165

21.5 0.0338 0.030731.67 0.0429 0.041545.83 0.0541 0.05355.67 0.0617 0.0606


1 mmHg-1) 13.83 15.93

Table C–31: Weight gain versus time obtained for the AARHUSTM fat coating Time (h) Sample 1 Sample 2

0 0 015.58 0.013 0.011726.58 0.0231 0.0204

38 0.0329 0.029950.5 0.045 0.0397


1 mmHg-1) 15.06 14.53

Note: Average temperature during the experiment was 19.6 oC. The SFC of the coating at 20oC was 25.2

205

Table C–32: Weight gain versus time obtained for the Victory-76 fat coating Time (h) Sample 1 Sample 2

0 0 015.58 0.0119 0.012926.58 0.0207 0.021

38 0.0274 0.027850.5 0.036 0.0371


1 mmHg-1) 10.54 10.91

Note: Average temperature during the experiment was 19.6 oC. The SFC of the coating at 20oC was 35.1

Table C–33: Weight gain versus time obtained for the Karlshamns fat coating Time (h) Sample 1 Sample 2

0 0 030.35 0.0072 0.010852.75 0.0118 0.017276.33 0.0172 0.0233122.5 0.0263 0.0336

171 0.0373 0.0484236.25 0.0525 0.0653306.42 0.0653 0.0823


1 mmHg-1) 3.36 4.27

Note: Average temperature during the experiment was 18.9 oC. The SFC of the coating at 18.5oC was 46.3

206

Table C–34: Weight gain versus time obtained for the DP1192 fat coating Time (h) Sample 1 Sample 2

0 0 025.5 0.0001 0.0002

55 0.0028 0.001884.33 0.0032 0.0027

126 0.0065 0.0046144.17 0.0074 0.0056171.5 0.0105 0.0086

196.25 0.0125 0.0088288.67 0.0139 0.0226415.17 0.0278 0.0415504.75 0.0384 0.0551633.17 0.0521 0.0741718.25 0.0689 0.0868


1 mmHg-1) 2.69 3.06

Note: The average temperature during the experiment was 16.5 oC. The SFC of the fat at 16.5oC was 68%.

207

Table C–35: Weight gain versus time obtained for the DP1193 fat coating Time (h) Sample 1 Sample 2

0 0 025.5 0.0001 0.0002

55 0.0028 0.001884.33 0.0032 0.0027

126 0.0065 0.0046144.17 0.0074 0.0056171.5 0.0105 0.0086

196.25 0.0125 0.0088288.67 0.0139 0.0226415.17 0.0278 0.0415504.75 0.0384 0.0551633.17 0.0521 0.0741718.25 0.0689 0.0868


1 mmHg-1) 2.69 3.06

Note: The average temperature during the experiment was 16.5 oC. The SFC of the fat at 16.5oC was 79.2%.

Table C–36: Weight gain versus time obtained for the DP1194 fat coating


0 0 0 030.35 0.0037 46.17 0.010752.75 0.0061 94.25 0.023476.33 0.0097 160 0.0387122.5 0.0144 230.25 0.0528

171 0.0209 236.25 0.0291 306.42 0.0356


Note: The average temperature during the experiment was 18.9 oC. The SFC of the fat at 16.5oC was 68%.

208

C.4 Effect of Humidity on Water Vapor Permeability

The weight gain data for a generic coating subjected to different relative humidity

at the bottom of the cell is shown in Table C–37 and Table C–38.

Table C–37: Weight gain versus time for a generic coating when the humidity at the bottom of the cell was 33% Time (h) Sample 1 Sample 2

0 0 012 0.0409 0.037424 0.0572 0.052736 0.0634 0.06148 0.0693 0.068960 0.0772 0.07672 0.0834 0.079384 0.0891 0.0871


1 mmHg-1) 12.88 11.88

209

C.5 Augmented Design Data

The weight gain data for the different experiments done for augmenting the

mixture design are shown in Tables C–39 - C–48.

Table C–38: Weight gain versus time for a generic coating when the humidity at the bottom of the cell was 54% Time (h) Sample 1 Sample 2

0 0 012 0.0363 0.032324 0.0509 0.051736 0.0688 0.059748 0.0833 0.064960 0.1044 0.075472 0.1277 0.087484 0.1437 0.103


1 mmHg-1) 78.98 44.94

210

Table C–39: Weight gain versus time for a coating made with 20% sugar and 80% coconut oil Time (h) Sample 1 Sample 2

0 0 023.33 0.0193 0.020236.17 0.029 0.031848.33 0.039 0.042263.58 0.0517 0.053774.82 0.0602 0.062587.33 0.0705 0.072899.67 0.0816 0.0829


1 mmHg-1) 10.05 10.15

Table C–40: Weight gain versus time for a coating made with 30% sugar and 70% coconut oil Time (h) Sample 1 Sample 2

0 0 023.33 0.0182 0.018536.17 0.0259 0.031548.33 0.0333 0.040463.58 0.0447 0.053274.82 0.0518 0.063187.33 0.0596 0.071899.67 0.0675 0.0808


1 mmHg-1) 9.95 9.50

211

Table C–41: Weight gain versus time for a coating made with 50% sugar, 49.5% coconut oil and 0.5% lecithin Time (h) Sample 1 Sample 2

0 0 014.25 0 0.0037

24 0.0035 0.004148 0.0124 0.013672 0.0206 0.020796 0.0287 0.0278

119.25 0.037 0.035141.58 0.0436 0.0414


1 mmHg-1) 7.06 7.36

Table C–42: Weight gain versus time for a coating made with 60% sugar, 39.5% coconut oil and 0.5% lecithin Time (h) Sample 1 Sample 2

0 0 014.25 0.0061 0.0109

24 0.0125 0.017948 0.0184 0.024972 0.0246 0.031896 0.0308 0.0388

119.25 0.0367 0.0456141.58 0.0425 0.052


1 mmHg-1) 5.95 6.16

212

Table C–43: Weight gain versus time for a coating made with 20% cocoa powder and 80% coconut oil Time (h) Sample 1 Sample 2

0 0 023.42 0.0693 0.068635.67 0.0789 0.0774

48 0.0909 0.08962.92 0.1042 0.103473.25 0.1143 0.110386.75 0.1292 0.124199.5 0.1401 0.1363


1 mmHg-1) 11.79 11.49

Table C–44: Weight gain versus time for a coating made with 30% cocoa powder and 70% coconut oil Time (h) Sample 1 Sample 2

0 0 023.42 0.0727 0.069235.67 0.0872 0.0855

48 0.0985 0.099962.92 0.1144 0.119673.25 0.1263 0.131286.75 0.1401 0.144499.5 0.1532 0.1606


1 mmHg-1) 12.67 12.22

213

Table C–45: Weight gain versus time for a coating made with 30% cocoa powder, 69.5% coconut oil, and 0.5% lecithin


0 0 0 09.5 0.0142 7.42 0.0067

24.47 0.0337 21.83 0.027133.42 0.051 31.33 0.039945.75 0.0713 46.3 0.0605

55.25 0.0827 67.58 0.0976


Table C–46: Weight gain versus time for a coating made with 40% cocoa powder, 59.5% coconut oil, and 0.5% lecithin Time (h) Sample 1 Sample 2

0 0 07.42 0.0383 0.037

21.83 0.0737 0.075431.33 0.0906 0.087546.3 0.1192 0.1119

55.25 0.1377 0.135967.58 0.1577 0.1624


1 mmHg-1) 29.64 27.04

214

C.6 Qualitative Study Pictures

The pictures of the coating after 14 weeks of storage are shown in Figures C–1 to C–5.

Table C–47: Weight gain versus time for a coating made with 50% sugar, 10% cocoa powder, 39.5% coconut oil, and 0.5% lecithin Time (h) Sample 1 Sample 2

0 0 012.25 0.0192 0.000324.5 0.0287 0.0096

32.33 0.0329 0.015246.75 0.0399 0.024361.67 0.0496 0.031269.17 0.053 0.0338


1 mmHg-1) 9.74 9.98

Table C–48: Weight gain versus time for a coating made with 30% sugar, 30% cocoa powder, 39.5% coconut oil, and 0.5% lecithin Time (h) Sample 1 Sample 2

0 0 012.25 0.0287 0.041824.5 0.0491 0.0528

32.33 0.0626 0.063546.75 0.0762 0.081661.67 0.0908 0.099869.17 0.0997 0.1028


1 mmHg-1) 18.35 20.53

215

Figure C–1: Coating with 50% cocoa powder after 14 weeks of storage at -5C

216


217


218


219

Figure C–5: Coating with 20% sucrose after 14 weeks of storage at -5C

VITA

EDUCATION M. S. Food Science (December 1998), Pennsylvania State University B. Tech. (Honors) (July 1995), Agricultural Engineering, Indian Institute of Technology, Kharagpur, India EXPERIENCE Jul. 1999 – May 2003: Research Assistant, Department of Food Science, Pennsylvania

State University Sep. 1998 - May 1999: Stagaire, R&D, Nestle, Beauvais, France Aug. 1995 - Aug 1998: Research Assistant, Department of Food Science, Pennsylvania

State University PUBLICATIONS Ghosh, V. and Anantheswaran, R. C. 2001. Experimental setup for measuring the oxygen transmission rate of micro-perforated films. Journal of Food Process Engineering. 24: 113 – 133 Ghosh, V., Ziegler, G. R. and Anantheswaran, R. C. 2002. A review of fat, moisture and ethanol migration through chocolates. CRC reviews in Food Science and Nutrition. 42(6): 583 - 626 Ghosh, V., Floros, J. and Anantheswaran, R. C. 2003. Refrigerants. In Encyclopedia of Agricultural and Food Engineering Ghosh, V. and Anantheswaran, R. C. 2003. Unsteady State Heat Transfer. In Encyclopedia of Agricultural and Food Engineering PATENT Anantheswaran, R. C. and Ghosh, V. 2002. Rapid method to experimentally measure the gas permeability of micro-perforated films. US Patent No. 6, 422, 063 AWARDS

1. Graduate Fellowship, Food Packaging Division, IFT (2002). 2. Institute Silver Medal, Indian Institute of Technology (1995). 3. Part of the product development team that won the “Most Creative Product”

award in DMI.

moisture migration through chocolate-flavored …

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