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Properties and structure of coconut milk emulsions

Dissertation
Author: Nattapol Tangsuphoom
Abstract:
Coconut milk is the natural oil-in-water emulsion extracted from the endosperm of mature coconut ( Cocos nucifera L.) endosperm. The emulsion is stabilized by coconut proteins, but it poorly stable due to the insufficient quantity and quality of the proteins present. In order to improve the stability and quality of coconut milk products, various processes and food additives are normally used. However, the underlying emulsion science is still unclear. The first objective was to investigate the effects of homogenization and heat treatment. Fresh milk had large but non-flocculated via a bridging mechanism. Homogenization reduced the droplets size, but increased the degree of flocculation, via a bridging mechanism. Extensive flocculation and slight coalescence was observed in coconut milk after treatment at temperatures above the denaturation temperature of coconut proteins. Flocculation was responsible for increased viscosity and retarded creaming. The second objective was to determine the influence of pH, and ionic strength. Coconut milk flocculated when the pH was to close to the isoelectric point of coconut proteins as the electrostatic repulsion between droplets is reduced due to the loss in surface charge. The addition of NaCl induced flocculation only when the surface charge of the emulsion droplets was insufficient to prevent aggregation due to the screening effect. The third objective was to understand the changes in bulk quality and surface properties of coconut milk emulsions due to the addition of model surface-active stabilizers [sodium caseinate, whey protein isolate (WPI), sodium dodecyl sulfate (SDS), or polyoxyethylene sorbitan monolaurate (Tween 20)]. When added after the homogenization, small-molecule surfactants broke up the flocs while protein stabilizers did not. The addition of any surface-active stabilizer before homogenization increased the efficacy of homogenization step and produced stable submicron sized emulsion droplets. The improved stability in all cases resulted from the displacement of interfacial coconut proteins by the added stabilizers. The final objective of this work was to determine the effect of various thermal treatments on the stability of the stable emulsions prepared with surface-active stabilizers. Coconut milk homogenized with proteins were stable to the freeze-thaw cycles while those prepared with small-molecule surfactants were not. The caseinate and SDS emulsions were able to withstand the heat treatments whereas WPI and Tween 20 samples extensively coalesced following autoclave treatment.

vi TABLE OF CONTENTS

LIST OF FIGURES.....................................................................................................viii LIST OF TABLES.......................................................................................................xii ACKNOWLEDGEMENTS.........................................................................................xiii Chapter 1 Statement of the Problem..........................................................................1 Chapter 2 Literature Review.....................................................................................7 2.1 Basic emulsion science...................................................................................7 2.1.1 Definition..............................................................................................7 2.1.2 Emulsion ingredients............................................................................7 2.1.3 Emulsion formation..............................................................................10 2.1.4 Emulsion characterization....................................................................12 2.1.5 Emulsion stability.................................................................................22 2.2 Coconut milk..................................................................................................30 2.2.1 Coconuts...............................................................................................30 2.2.2 General characteristics of coconut milk...............................................31 2.2.3 Coconut milk extraction.......................................................................32 2.2.4 Composition and properties..................................................................32 2.2.4 Coconut milk emulsion.........................................................................41 References.............................................................................................................44 Chapter 3 Effect of Homogenization Process and Heat Treatment on the Stability of Coconut Milk Emulsions...................................................................58 3.1 Introduction.....................................................................................................59 3.2 Materials and methods....................................................................................60 3.3 Results and discussion....................................................................................64 3.4 Conclusions.....................................................................................................77 References.............................................................................................................77 Chapter 4 Effect of pH and Ionic Strength on the Physicochemical Properties of Coconut Milk Emulsions......................................................................................80 4.1 Introduction.....................................................................................................81 4.2 Materials and methods....................................................................................83 4.3 Results and discussion....................................................................................88 4.3.1 Effect of pH..........................................................................................89 4.3.2 Effect of NaCl.......................................................................................99

vii 4.4 Conclusions.....................................................................................................106 References.............................................................................................................106 Chapter 5 Effect of Surface-Active Stabilizers on the Microstructure and Stability of Coconut Milk Emulsions...................................................................111 5.1 Introduction.....................................................................................................112 5.2 Materials and methods....................................................................................114 5.3 Results and discussion....................................................................................119 5.3.1 Effect of processing on coconut milk...................................................119 5.3.2 Addition of stabilizers to non-homogenized coconut milk..................123 5.3.3 Addition of stabilizers to homogenized coconut milk..........................129 5.3.4 Addition of stabilizers to coconut milk prior to homogenization.........130 5.4 Conclusions.....................................................................................................136 References.............................................................................................................138 Chapter 6 Effect of Surface-Active Stabilizers on the Surface Properties of Coconut Milk Emulsions......................................................................................142 6.1 Introduction.....................................................................................................143 6.2 Materials and methods....................................................................................144 6.3 Results and discussion....................................................................................149 6.3.1 Homogenized coconut milk without addition of stabilizers.................149 6.3.2 Coconut milk emulsions stabilized with surfactants............................154 6.3.3 Coconut milk emulsions stabilized with added proteins......................160 6.4 Conclusions.....................................................................................................167 References.............................................................................................................167 Chapter 7 Effect of Thermal Treatments on the Properties of Coconut Milk Emulsions Prepared with Surface-Active Stabilizers...........................................172 7.1 Introduction.....................................................................................................173 7.2 Materials and methods....................................................................................175 7.3 Results and discussion....................................................................................180 7.3.1 Changes in emulsion structure..............................................................180 7.3.2 Thermal properties and discussion.......................................................189 7.4 Conclusions.....................................................................................................203 References.............................................................................................................204 Chapter 8 Conclusions...............................................................................................210

viii LIST OF FIGURES

Figure 2.1: Schematic presentation of the structure of the electric double layer........18 Figure 2.2: Schematic presentations of physical changes in oil-in-water emulsions..............................................................................................................24 Figure 2.3: Model of 11S coconut globulin................................................................40 Figure 3.1: Representative particle size distribution of non-homogenized (a) and homogenized (b) coconut milks dispersed in distilled water (solid line) or SDS solution (dashed line)...................................................................................65 Figure 3.2: Micrographs taken of non-homogenized (a, c, e) and homogenized (at 40/4 MPa) (b, d, f) coconut milks either unheated (a, b) or heated to heated to 50 o C (c, d) or 90 o C (e, f).......................................................................................67 Figure 3.3: Mean particle size of coconut milks homogenized at (●, ○) 20/2, (■, □) 40/4, and (▲, ∆) 60/6 MPa..............................................................................68 Figure 3.4: Mean particle size of (●, ○) non-homogenized and (■, □) homogenized (at 40/4 MPa) coconut milks heated at different temperatures......70 Figure 3.5: Flow behavior index (a), and consistency coefficient (b) of (●) non- homogenized and (○) homogenized (at 40/4 MPa) coconut milks heated at different temperatures...........................................................................................72 Figure 3.6: Creaming index after 24 h storage of (●) non-homogenized and (○) homogenized (at 40/4 MPa) coconut milks heated at different temperatures......74 Figure 3.7: Free oil solvent-extracted from (●) non-homogenized and (○) homogenized (at 40/4 MPa) coconut milks heated at different temperatures......76 Figure 4.1: Mean particle size of coconut milk emulsions adjusted to different pH values....................................................................................................................90 Figure 4.2: Optical micrographs taken of coconut milk emulsions adjusted to pH 3 (a), 3.5 (b), 4 (c), and 6 (d)................................................................................91 Figure 4.3: ζ-potential of coconut milk emulsions adjusted to different pH values...92 Figure 4.4: LSCM images overlaid with corresponding images from DIC transmitted light of coconut milk emulsions adjusted to pH 4 (a), and 6 (b).......95

ix Figure 4.5: Steady shear viscosity of coconut milk emulsions adjusted to pH (●) 3, (♦) 3.5, (■) 4, and (▲) 6...................................................................................96 Figure 4.6: Creaming index after 24 h storage of coconut milk emulsions adjusted to different pH values...........................................................................................98 Figure 4.7: Mean particle size of coconut milk emulsions adjusted to pH (●) 3, (■) 4, and (▲) 6 as affected by the addition of NaCl...........................................100 Figure 4.8: Optical micrographs of coconut milk emulsions adjusted to pH 3 (a to c), 4 (d to f), and 6 (g to i) as affected by the addition of 50 (a, d, g) 100 (b, e, h), and 200 (c, f, i) mM NaCl...............................................................................101 Figure 4.9: ζ-potential of coconut milk emulsions adjusted to pH (●) 3, (■) 4, and (▲) 6 as affected by the addition of NaCl............................................................102 Figure 4.10: Steady shear viscosity of coconut milk emulsions adjusted to pH 3 (a), 4 (b), and 6 (c) as affected by the addition of (●) 0, (♦) 50, (■) 100, and (▲) 200 mM NaCl................................................................................................104 Figure 5.1: Representative particle size distribution of (●, ○) non-homogenized and (■, □) homogenized coconut milks................................................................120 Figure 5.2: Representative images from optical microscopy (a, b), LSCM overlaid with corresponding images from DIC transmitted light (c, d), and cryo-SEM (e, f) taken of non-homogenized (a, c, e), and homogenized (b, d, f) coconut milks....................................................................................................121 Figure 5.3: Creaming index after 24 h storage of (●) non-homogenized and (■) homogenized coconut milks.................................................................................122 Figure 5.4 : Mean particle size of coconut milk emulsions with (●, ○) sodium caseinate, (■, □) WPI, (▲, Δ) SDS, or (♦, ◊) Tween 20 added without (a), after (b), or prior to (c) homogenization...............................................................124 Figure 5.5: Representative micrographs taken of coconut milk emulsions with 1 wt% sodium caseinate (a to c), WPI (d to f), SDS (g to i), or Tween 20 (j to l) added without (a, d, g, j), after (b, e, h, k), or prior to homogenization (c, f, i, l)............................................................................................................................126 Figure 5.6: Creaming index after 24 h storage of coconut milk emulsions stabilized with sodium caseinate (a), WPI (b), SDS (c) or Tween 20 (d) added (●) without, (▲) after, or (■) prior to homogenization........................................127 Figure 5.7: Representative particle size distribution of coconut milk emulsions with sodium caseinate (a), WPI (b), SDS (c) or Tween 20 (d) added at

x concentrations of (●) 0, (∇) 0.1, (■) 0.25, (◊) 0.5, or (▲) 1 wt% prior to homogenization.....................................................................................................131 Figure 5.8: Creaming index of coconut milk emulsions with sodium caseinate (a), WPI (b), SDS (c) or Tween 20 (d) added at concentrations of (●) 0, (∇) 0.1, (■) 0.25, (◊) 0.5, or (▲) 1 wt% prior to homogenization.....................................134 Figure 6.1: Surface protein load of coconut milk emulsions stabilized with SDS (a), Tween 20 (b), WPI (c), or sodium caseinate (d) added (■) after, or (●) prior to homogenization........................................................................................150 Figure 6.2: SDS-PAGE gel and the corresponding densitometric profiles of total (lane A) and interfacial proteins (lane B) in homogenized coconut milk.............153 Figure 6.3: ζ-potential of coconut milk emulsions stabilized with SDS (a), Tween 20 (b), WPI (c), or sodium caseinate (d) added (■) after, or (●) prior to homogenization.....................................................................................................155 Figure 6.4: SDS-PAGE patterns of coconut milk emulsions with WPI (a, c), and sodium caseinate (b, d) at 0.1, 0.25, 0.5, and 1 wt% added after (a, b) or prior to (c, d) homogenization.......................................................................................162 Figure 6.5: SDS-PAGE patterns and the corresponding densitometric profiles of WPI (a), and sodium caseinate (b)........................................................................164 Figure 6.6: Peak area of selected protein fractions in coconut milk emulsion stabilized with WPI (a), and sodium caseinate (b) added (■) after, or (●) prior to homogenization................................................................................................165 Figure 7.1: Visual appearance after thermal treatments of homogenized coconut milk (a), coconut milk emulsions homogenized with 1 wt% sodium caseinate (b), WPI (c), SDS (d), and Tween 20 (e)..............................................................182 Figure 7.2: Typical thermograms of coconut oil.........................................................190 Figure 7.3: Successive cooling curves and a heating curve of homogenized coconut milk repeatedly cycled from 30 o C to -15 o C (a), and -40 o C (b) at 1.5 o C min -1 ............................................................................................................191 Figure 7.4: Differential scanning microcalorimetric thermograms of coconut milk homogenized with 1 wt% no additive (a), sodium caseinate (b), and WPI (c) after heated different temperatures for 1 h...........................................................194 Figure 7.5: Successive cooling curves and a heating curve of coconut milk emulsions homogenized with 1 wt% sodium caseinate repeatedly cycled from 30 o C to -15 o C (a), and -40 o C (b) at 1.5 o C min -1 ...................................................197

xi Figure 7.6: Successive cooling curves and a heating curve of coconut milk emulsions homogenized with 1 wt% WPI repeatedly cycled from 30 o C to - 15 o C (a), and -40 o C (b) at 1.5 o C min -1 ..................................................................198 Figure 7.7: Successive cooling curves and a heating curve of coconut milk emulsions homogenized with 1 wt% SDS repeatedly cycled from 30 min -1o C to -15 o C (a), and -40 o C (b) at 1.5 o C min -1 ............................................................201 Figure 7.8: Successive cooling curves and a heating curve of coconut milk emulsions homogenized with 1 wt% Tween 20 repeatedly cycled from 30 o C to -15 o C (a), and -40 o C (b) at 1.5 o C min -1 ............................................................202

xii LIST OF TABLES

Table 1.1: Schematic outline of the dissertation.........................................................6 Table 2.1: World coconut production between 1997 and 2001..................................31 Table 2.2: Proximate composition of fresh coconut meat...........................................33 Table 2.3: Physical properties and chemical composition of coconut milk................34 Table 2.4: Composition standard of coconut milk products.......................................35 Table 2.5: Fatty acid composition of coconut oil........................................................36 Table 2.6: Amino acid composition of coconut proteins............................................38 Table 5.1: Mean effective and primary particle sizes of coconut milk emulsions with 1 wt% surface-active stabilizers added without, after, or prior to homogenization.....................................................................................................137 Table 5.2: Creaming index after 24 h storage of coconut milk emulsions with 1 wt% surface-active stabilizers added without, after, or prior to homogenization.....................................................................................................138 Table 6.1: Mean effective and primary (in parentheses) particle sizes of coconut milk emulsions with surface-active stabilizers added after or prior to homogenization.....................................................................................................157 Table 6.2: Molar ratio of surface-active stabilizer and coconut proteins in coconut milk emulsions stabilized with different surface-active stabilizers......................158 Table 7.1: Temperature protocols...............................................................................177 Table 7.2: Mean effective and primary particle sizes after thermal treatments of coconut milk emulsions homogenized with 1 wt% stabilizer..............................181 Table 7.3: Summary of stability after thermal treatments of coconut milk emulsions homogenized with 1 wt% stabilizer....................................................188

xiii ACKNOWLEDGEMENTS

I would like to express my deepest gratitude and very special appreciation to my advisor, Dr. John N. Coupland, for his enthusiastic supervision of my dissertation. His helpful guidance, constructive criticism, valuable suggestion, consistent supervision, and encouragement helped me getting through the long journey of my Ph.D. study.

I also would like to express my gratitude to Dr. Robert F. Roberts, Dr. Robert B. Beelman, and Dr. Virendra M. Puri for their useful advices and serving on my committee.

I am grateful to my father Surin, my mother Achara, my brother Yossapol, and my sister Taksachan, for their love, support, perfect understanding, and encouragement that inspired me for the success of this study. I could never have completed this dissertation and would never have got this far without them.

I would like to thank members of the Thai Student Association, particularly Dr. Hathairat Maneetes, Dr. Nateekool Kriangchaiporn, Dr. Yotsanan Meemark, Mookarin Nookong, Ponusa Songtipya, Chumpol Yuangyai, and Ponkamon Aumpansub for their friendship and good company during my memorable years in State College. Special thanks to Kiattikhun Manokruang for sharing laughter and tears, and always being there whenever I needed.

xiv I also would like to thank my past and present labmates, all the faculty members, staffs, and graduate students in the Department of Food Science, especially Dr. Supratim Ghosh, Dr. İbrahim Gülseren, Dr. Ying Wang, Dr. Emily J. Furumoto, and Bonnie C. Ford for their kind help and useful discussions. Dr. Raafat I. Malek of the Materials Research Institute, Dr. Lian-Chao Li, Nicole M. Bem, and Missy L. Hazen of the Huck Institutes of the Life Sciences are also acknowledged for their assistance in the techniques that I am not familiar with.

My heartiest thanks is also extended to all other friends, who are half the world away, for their support and encouragement. In particular, I very much appreciate the kind help and valuable comments of Dr. Visith Chavasit of the Institute of Nutrition, Mahidol University, Thailand.

Finally, I would like to acknowledge the Royal Thai Government for providing scholarship to pursue Ph.D. in the United States under the Committee Staff Development Project of the Commission on Higher Education.

Chapter 1

Statement of the Problem

Coconut milk is a natural oil-in-water emulsion in which coconut oil droplets are dispersed throughout an aqueous continuous phase. It is produced in either household or industrial scale by extracting the endosperm of mature coconut (Cocos nucifera L.) with water. The white, opaque emulsion has been used widely as an important food ingredient, especially in Asia and Pacific regions due to its unique sensory characteristics. Coconut milk is naturally stabilized by coconut proteins (i.e., globulins and albumins), and phospholipids. However, like all emulsions, coconut milk is thermodynamically unstable and readily separates into cream and serum layers, known as coconut cream and coconut skim milk, respectively. The physical instability of coconut milk is suspected to be the consequence of the inadequate quantity and quality of coconut proteins. Fresh coconut milk is also microbiologically unstable as it is a good source of nutrients, especially fat, has a neutral pH and typically carries a large microbial load.

Various attempts have been made to preserve the coconut milk in order to prolong the shelf life for commercial purposes, mostly by using temperature treatments (e.g., heat-treated, dried, and frozen coconut milk). The problem of eliminating microorganisms in coconut milk by thermal processing is aggravated by the instability of the coconut proteins to heat. In order to obtain coconut milk product with good quality

2 and stability, various kinds and amounts of emulsifiers and/or stabilizers have been normally used. It is common practical knowledge that the choice of process and additives affects the stability of the emulsion and the quality and sensory characteristics of the processed coconut milk products. However, the physical basis for the changing functional properties and the underlying emulsion science are still unclear.

My dissertation, therefore, concentrates on determining the properties and structure of coconut milk emulsions prepared with various additives and processes. I have taken a model system approach, and used simple combinations of fundamental process operations (homogenization, temperature, pH, and salt) and surface active stabilizers to understand the mechanisms of change with the expectation that this understanding will be helpful in developing more complex and industrially practical processes. Goal, Objectives, and Dissertation Layout The overall goal of this dissertation is to characterize the effects of processing and ingredient interactions on the microstructure, properties, and colloidal stability of the coconut milk emulsion. In order to accomplish this goal the specific objectives of my research were to: 1) Investigate the effect of heating and homogenization on the stability of coconut milk emulsions. 2) Determine the influence of the addition of acid/base and salt on the stability and structure of coconut milk emulsions

3 3) Determine the influence of the addition of widely-used surface-active stabilizers on the bulk quality, structure, and surface properties of coconut milk emulsions. 4) Examine the changes in stability of coconut milk emulsions homogenized with different surface-active stabilizers due to various cooling and heating treatments.

The stability of coconut milk emulsion, like most protein-stabilized emulsions, is expected to be affected by denaturation of the proteins either by heat, or the loss of its electrical charge. Therefore, I started my dissertation by determining effects of those factors on the stability and properties of coconut milk emulsions. Chapter 3 is concerned with the changes in stability and bulk quality of coconut milk after homogenization and heating (i.e., Objective 1). Such processes were selected since they are the fundamental operations in coconut milk manufacture.

[It should be noted that the coconut milk used only in Chapter 3 was prepared from whole fresh coconut, and the fat content of the samples was 15% to 17%. The method of coconut milk preparation was modified and used in later chapters. The in- house grated coconut meat was replaced with the ready-to-use frozen grated meat, of which the composition is less varied and more controllable. The extracted milk was diluted in buffer to a final fat content of 10% in order to normalize the fat content of the emulsions and make the sample more experimentally convenient.]

4 In Chapter 4, the physicochemical properties of coconut milk emulsions with different pH values and ionic strengths were investigated (i.e., Objective 2). The pH of many coconut milk based products is about 6.2 and any decrease in pH normally is a consequence of microbial contamination or of the addition of acidulants. The level of acidity is known to affect the quality of coconut milk and often used by manufacturers as a measure of product quality.

In Chapter 5 and 6, various types and amounts of model surface-active stabilizers were added to coconut milk in order to improve the stability and quality of the emulsion (i.e., Objective 3). Four commonly used proteins and small-molecule surfactants, namely sodium caseinate (i.e., model disordered protein), whey protein isolate, (WPI, i.e., model globular protein), sodium dodecyl sulfate (SDS, i.e., model anionic surfactant), and polyoxyethylene sorbitan monolaurate (Tween 20, i.e., model uncharged surfactant), were selected. The bulk quality of samples was determined in Chapter 5, and the surface properties of samples were examined in Chapter 6 to provide evidence of the surface chemistry underlying changes in bulk properties.

The susceptibility of coconut milk emulsions prepared with surface-active stabilizers to various thermal treatments normally involved in the processing of coconut milk products, e.g., freezing, chilling, heating, and autoclaving, was investigated in Chapter 7 (i.e., Objective 4). The last chapter (Chapter 8) is the conclusions of my dissertation.

5 Chapters 3 to 7 of this dissertation are presented in form of manuscripts co- authored with my major advisor each with a separate introduction, materials and methods, results and discussion, conclusions, and references. The structure of the dissertation is outlined in Table 1.1.

6

Table 1.1: Schematic outline of the dissertation.

Chapter Paper Objective covered

1

Statement of the problem

2 Literature review

3 Tangsuphoom, N., & Coupland, J. N. (2005). Effect of heating and homogenization on the stability of coconut milk emulsions. Journal of Food Science, 70(8), E466- E470.

1 4 Tangsuphoom, N., & Coupland, J. N. (2008). Effect of pH and ionic strength on the physicochemical properties of coconut milk emulsions. Journal of Food Science, In press.

2 5 Tangsuphoom, N., & Coupland, J. N. (2008). Effect of surface-active stabilizers on the microstructure and stability of coconut milk emulsions. Food Hydrocolloids, 22(7), 1233-1242.

3 6 Tangsuphoom, N., & Coupland, J. N. (2008). Effect of surface-active stabilizers on the surface properties of coconut milk emulsions. Submitted to Food Hydrocolloids.

3 7 Tangsuphoom, N., & Coupland, J. N. (2008). Effect of thermal treatments on the properties of coconut milk emulsions prepared with surface-active stabilizers. Submitted to Food Hydrocolloids.

4 8 Conclusions

Chapter 2

Literature Review

2.1 Basic emulsion science 2.1.1 Definition An emulsion is a mixture of two immiscible liquids - one is the dispersed or internal phase as small spherical droplets, and the other is the continuous, external phase. In food systems, the two liquid phases are usually oil and water. Food emulsions can be categorized as oil-in-water or water-in-oil depending upon which phase is continuous (Dalgleish, 1996; McClements, 2004b). Some examples of oil-in-water emulsions include mayonnaise, milk, cream, soups, salad dressings, and sauces. This work is solely concerned with oil-in-water emulsions which are hereafter simply referred to as emulsions. 2.1.2 Emulsion ingredients In addition to oil and water, a typical food emulsion may contain emulsifiers, thickening agents, buffering systems, preservatives, sweeteners, salt, antioxidants, chelating agents, colorants, and flavors (Dickinson, 1992). The physiochemical and organoleptic properties of an emulsion depends on the type and amount of these

8 ingredients and their physical distribution, i.e., in the dispersed phase, the continuous phase, or at the interface. 2.1.2.1 Fats and oils For oil-in-water emulsions, the dispersed phase consists of food oils (i.e., largely mixed triglycerides). Fats and oils influence the physicochemical, organoleptic, and nutritional properties of food emulsions in a variety of ways. The turbidity, cloudiness, or opaque appearance of emulsion is largely the result of light scattering by the dispersed oil droplets (McClements, 2004b). The lipid content and droplet concentration of emulsions contribute to the rheological properties as well as the perceived creaminess of the emulsions. 2.1.2.2 Water and aqueous solutions The aqueous phase of an oil-in-water emulsion contains water and a variety of water-soluble functional ingredients. The unique molecular and structural properties of water largely determine the solubility, conformation, and interactions of such components, which in turns influence the bulk physicochemical and sensory properties of emulsions (McClements, 2004b).

9 2.1.2.3 Emulsifiers Emulsifiers are important ingredients in emulsion stabilization. Oil and water are almost completely insoluble in each other. Emulsifiers are amphiphilic, surface-active substances that are capable of retarding the phase separation in emulsions by adsorbing to the oil-water interface and hence lowering the interfacial tension. The commonly used emulsifiers for food emulsions are small-molecule surfactants, amphiphilic biopolymers, and solid particles.

Small-molecule surfactants Surfactant molecules consist of a hydrophilic head group attached to a hydrophobic tail group. In addition to their principal role in stabilizing the emulsions, small-molecule surfactants may affect emulsion properties by forming micelles, interacting with biopolymers, or altering the fat crystallization in oil droplets (Krog, 1997; Krog & Sparso, 2004). Small-molecule surfactants can be classified by the characteristics of their head groups into nonionic (e.g., monoglycerides, polyoxyethylene sorbitan fatty acid monoesters, sorbitan fatty acid monoesters, and sucrose fatty acid esters), anionic (e.g., alkyl sulfate salts alkyl benzene sulfonate, fatty acid salts, and stearyl lactylate salts), cationic (e.g., alkyl trimethylammonium salts), and zwitterionic (e.g., lecithin and other phospholipids). Each type of surfactant has functional properties that are determined by its molecular structure and the environment (McClements, 2004b).

10 Amphiphilic biopolymers Biopolymers that have significant amount of both polar and nonpolar residues tend to exhibit surface activity. Proteins (e.g., dairy proteins, gelatin, egg albumin, and plant proteins) and polysaccharides (e.g., gum arabic and modified starches) are the two most important types of amphiphilic biopolymers used in food emulsions. Under appropriate environmental conditions, biopolymers can hydrophobically adsorb to oil- water interfaces and reduces the contact area between the oil and water molecules at the oil-water interface, thus lowering the interfacial tension. The thick interfacial layer formed by biopolymers also provides steric stability to the emulsion. Biopolymers often rearrange their structures after they have adsorbed to an interface to maximize the number of contacts between oil and their nonpolar groups. Such rearrangements depend on the molecular structure and interactions of the biopolymers, which in turn influence the characteristics of the interfacial membrane (Das & Kinsella, 1990; Dickinson, 1992). 2.1.3 Emulsion formation The process of converting bulk oil and bulk water into an emulsion or of reducing the size of the droplets in an existing emulsion is known as homogenization and is normally achieved by applying intense mechanical agitation to a liquid, in the presence of emulsifier, using a homogenizer (Breen, Wason, Kim, Nicolov & Shetty, 1996). Homogenization increases the interfacial area and hence the total interfacial free energy of the emulsion. The formation of emulsions by homogenization is a highly dynamic process that involves the violent disruption of droplets and the rapid movement of

11 surface-active molecules from the bulk liquids to the interfacial region of the newly divided surface.

During the homogenization, the size of the droplets formed depends on two opposing physical processes: droplet disruption and droplet re-coalescence (McClements, 2004b). The disruption of a large droplets into smaller ones depends on the balance between the force that hold the droplets together, i.e., interfacial force; and that tries to pull them apart, i.e., disruptive force generated by the homogenizer (Walstra, 2003). The disruptive force acting on the droplets depends on the nature of flow conditions during homogenization and also the type of homogenizer used. In the presence of emulsifiers, the decrease in interfacial tension also facilitates droplet disruption during homogenization (McClements, 2004b). During homogenization, the emulsifiers present in the solution rapidly adsorb onto the freshly generated droplet surface and thus prevent them against recoalescing. Smaller emulsion droplets can be achieved when the time taken by the emulsifiers to adsorb at the interface is much faster than the time between droplet-droplet collisions (McClements, 2004b).

The mechanical agitation required for homogenization can be applied using a number of different types of devices including high speed mixers, colloid mills, high- pressure valve homogenizers, and ultrasonic homogenizers. High-pressure valve homogenizers are the most common methods of producing fine emulsions in food industry. The homogenizer disrupts the droplets by forcing the coarse emulsion through a narrow gap of the valve where it experiences a combination of intense disruptive forces.

12 Thus, the large droplets are broken down to smaller ones. Two-stage homogenization can be done by forcing the emulsion through two consecutive valves. The first provides the high pressure to break up the droplets and the second valve is at lower pressure to disrupt any flocs formed (McClements, 2004b). 2.1.4 Emulsion characterization 2.1.4.1 Disperse phase volume fraction The concentration of droplets in emulsion plays an important role in determining its appearance, texture and stability. Droplet concentration of an emulsion is usually described in terms of the disperse phase volume (or mass) fraction which is equal to the volume (or mass) of emulsion droplets divided by the total volume (or mass) of the emulsion (McClements, 1998). The dispersed phase volume fraction of an emulsion can be determined using many methods, including the proximate analysis methods for measuring fat content, measurements of emulsion density, and electrical conductivity (McClements, 2004b). 2.1.4.2 Particle size distribution Many of the properties of an emulsion, i.e., stability, appearance, and texture, depend on the droplet size (McClements, 2004b). Most emulsions are polydisperse with droplet sizes; typically between 0.1 and 100 μm (Dickinson, 1992; Dickinson & Stainsby,

13 1982). Therefore, an emulsion should be characterized by a distribution of particle size by expressing the fraction of droplets in different size ranges, rather than a single number for droplet size. The fraction of droplets in different size ranges can be expressed in terms of number, mass, volume, or surface area. However, the size of the droplets in a polydispersed emulsion is often represented by a mean particle size rather than the full distribution (Walstra, 2003). For example, the area-weighed average diameter (d 32 ) is related to the average surface area of droplets exposed to the continuous phase per unit volume of emulsion. Another commonly used method of expressing the mean particle size is the volume-weighed average diameter (d 43 ) which is the sum of the volume ratio of droplets in each size class multiplied by the mid-point diameter of the size class (McClements, 2004b). The d 43 is more sensitive to the presence of large particles in an emulsion than d 32 , hence it its more sensitive to phenomena such as flocculation (Walstra, 2003). The values of each mean can be calculated from the full distribution as follows: ∑ ∑ = 2 3 32 ii ii dn dn d (2.1) ∑ ∑ = 3 4 43 ii ii dn dn d (2.2) where n i is the number of particles in each size class per unit volume of emulsion and d i

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Abstract: Coconut milk is the natural oil-in-water emulsion extracted from the endosperm of mature coconut ( Cocos nucifera L.) endosperm. The emulsion is stabilized by coconut proteins, but it poorly stable due to the insufficient quantity and quality of the proteins present. In order to improve the stability and quality of coconut milk products, various processes and food additives are normally used. However, the underlying emulsion science is still unclear. The first objective was to investigate the effects of homogenization and heat treatment. Fresh milk had large but non-flocculated via a bridging mechanism. Homogenization reduced the droplets size, but increased the degree of flocculation, via a bridging mechanism. Extensive flocculation and slight coalescence was observed in coconut milk after treatment at temperatures above the denaturation temperature of coconut proteins. Flocculation was responsible for increased viscosity and retarded creaming. The second objective was to determine the influence of pH, and ionic strength. Coconut milk flocculated when the pH was to close to the isoelectric point of coconut proteins as the electrostatic repulsion between droplets is reduced due to the loss in surface charge. The addition of NaCl induced flocculation only when the surface charge of the emulsion droplets was insufficient to prevent aggregation due to the screening effect. The third objective was to understand the changes in bulk quality and surface properties of coconut milk emulsions due to the addition of model surface-active stabilizers [sodium caseinate, whey protein isolate (WPI), sodium dodecyl sulfate (SDS), or polyoxyethylene sorbitan monolaurate (Tween 20)]. When added after the homogenization, small-molecule surfactants broke up the flocs while protein stabilizers did not. The addition of any surface-active stabilizer before homogenization increased the efficacy of homogenization step and produced stable submicron sized emulsion droplets. The improved stability in all cases resulted from the displacement of interfacial coconut proteins by the added stabilizers. The final objective of this work was to determine the effect of various thermal treatments on the stability of the stable emulsions prepared with surface-active stabilizers. Coconut milk homogenized with proteins were stable to the freeze-thaw cycles while those prepared with small-molecule surfactants were not. The caseinate and SDS emulsions were able to withstand the heat treatments whereas WPI and Tween 20 samples extensively coalesced following autoclave treatment.