3 Formulation of Emulsions

3

Formulation of Emulsions Marie Wahlgren, Björn Bergenståhl, Lars Nilsson, and Marilyn Rayner

Contents 3.1 3.2

Introduction .................................................................................................... 52 Functionality that Ingredients should give to Emulsions ............................... 52 3.2.1 Nutrition and Health ........................................................................... 52 3.2.2 Texture and Flavor .............................................................................. 53 3.2.3 Shelf-Life Stability ............................................................................. 54 3.2.3.1 Emulsion Stability................................................................ 54 3.2.3.2 Chemical Stability ............................................................... 56 3.2.3.3 Microbiological Stability ..................................................... 57 3.2.3.4 Freeze-Thaw Stability .......................................................... 58 3.3 Issues to Consider when choosing Ingredients for Emulsions........................ 59 3.4 Key Ingredients in Emulsions......................................................................... 62 3.4.1 Fats and Oils ....................................................................................... 62 3.4.2 Low Molar Mass Emulsifiers.............................................................. 63 3.4.3 Proteins ............................................................................................... 67 3.4.4 Polysaccharides................................................................................... 72 3.4.5 Protein-Polysaccharide Complexes .................................................... 75 3.4.6 Particles .............................................................................................. 76 3.5 Evaluation of Emulsion Formulation and Ingredient Performance ................ 78 3.5.1 Emulsification Capacity ...................................................................... 81 3.5.2 Emulsion Stability Index .................................................................... 83 3.5.3 Assessing Gravitational Separation—Creaming Index......................84 3.5.4 Accelerated and Environmental Stress Tests ...................................... 87 3.5.5 Evaluation of Texture .......................................................................... 89 References ................................................................................................................92 ABSTRACT In this chapter, we describe some of the main concerns when it comes to formulating emulsions. This includes the choice of ingredients, such as emulsifiers, oils, preservatives, and thickener. This is done with a focus on how these ingredients can give the desired properties of the emulsions, such as texture, flavor, nutrition, and stability. Commonly encountered thickeners and emulsifiers are described, and the methods to characterize the key properties of emulsion and ingredient are discussed.

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Engineering Aspects of Food Emulsification and Homogenization

3.1 IntroduCtIon Almost all industrially processed emulsion-based food products are made up of a wide variety of constituents, including fats and oils, emulsifiers, texture modifiers, preservatives, antimicrobial agents, antioxidants, pH adjusters, sweeteners, salts, coloring agents, flavors, and, of course, water. Each of these has been included in the food product due to its intrinsic function or a combination of functions with other compounds in the formulation. They are there to provide the overall quality of food products such as nutritional value, flavor, texture, and shelf life. In this chapter, we will discuss how the ingredients deliver these quality attributes to emulsion, and we will also give a more general description of some of the key ingredients in emulsions, primarily oils, emulsifiers, and texture modifiers. Food ingredients can be described on several levels: 1. 2. 3. 4.

Molecular (e.g., H2O, glucose, kappa casein, etc.) Nutritional (e.g., proteins, lipids, carbohydrates, minerals, etc.) Composite ingredients or recipe (e.g., milk, eggs, flour, salt, etc.) Functional ingredients (e.g., emulsifiers, thickeners, preservatives, etc.)

Food manufacturers, product developers, and formulators are generally concerned with the mass fraction of composite ingredients and functional ingredients because they are normally purchased and used in this form.

3.2 FunCtIonalIty that IngredIents should gIve to emulsIons 3.2.1 NutritioN aNd HealtH A key function of any food emulsion is certainly its nutritional value. As emulsions contain both lipophilic and hydrophilic regions, they have the capability to include both water-soluble and oil-soluble components of high nutritional value. Emulsions can increase the bioavailability of lipophilic nutrients such as vitamin E (Mayer, Weiss, and McClements 2013, Yang and McClements 2013) or other beneficial components, such as curcumin, that have low solubility in water (Ting et al. 2014). One of the main nutritional concerns when it comes to emulsions is the composition of the oil phase. Health benefits can be obtained, for example, by formulating products containAQ 1 ing omega-3 oils (Berasategi et al. 2014, Moore et al. 2012). Another important health aspect of food emulsions is the development of low-calorie products. In this case, one often tries to manufacture products with low oil content, such as low-fat spreads, that still has a texture similar to the original high-fat product (Chronakis 1997, Kasapis 2000). The aim is to formulate a product with low oil content that still has comparable texture, flavor, mouthfeel, and visual aspects as its traditional high-fat product. However, as the volume fraction of oil phase often is important for emulsion structure, this poses specific problems that need to be addressed; for example, the addition AQ 2 of texturizing macromolecules (polysaccharides and proteins) will compensate the lower oil fraction in maintaining the microstructure in low-calorie products.

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Formulation of Emulsions

53

The effectiveness of emulsions as vehicles for the delivery of individual nutritional compounds is affected by emulsion properties such as the surface area of the oil droplets and the availability of the oil interface for digestive enzymes. Furthermore, in the in vivo situation, the properties of the emulsion will also affect the gastric emptying of the stomach, where emulsions prone to phase separation in the stomach show a more rapid empting than emulsions that are stable (Golding and Wooster 2010). The structure of the fat used for the emulsion will also affect digestion; for example, solid fat is digested more slowly than liquid fat (Michalski 2009).

3.2.2 texture aNd Flavor In this section, a short overview of the area is given, and for a more thorough reading, we recommend some recent reviews on the topic (Chung and McClements 2013, Stokes, Boehm, and Baier 2013). The texture of emulsions is strongly dependent on its rheological properties. Rheology of the emulsion is in turn dependent on the volume fraction of the dispersed phase, the degree of flocculation of the dispersed phase, and rheological properties of the continuous phase. In most cases, especially if the drops are small, the rheological properties of the dispersed phase are less important. Another factor that influences the mouthfeel and taste is how the emulsion may aggregate and coalesce in the mouth due to mixing with saliva, interactions with the mucosa, the change in temperature, and the mechanical treatment while eating (Benjamins et al. 2009). In oil-in-water (O/W) emulsions, the rheological properties of the continuous phase are often modified by the addition of polymers. As the degree of flocculation of the oil droplets may also influence rheology, factors that affect flocculation such as the type of emulsifier, pH, and salt should also be considered. The physical properties of dispersed oil phase could affect the rheology by the formation of crystalline bridges between different oil droplets leading to semicoalesced drops (Fredrick, Walstra, and Dewettinck 2010). Thus, the melting temperature of the oil phase may affect the texture. The fraction of oil also influences mouthfeel, where high-fat O/W emulsions are usually perceived to have high creaminess, to be smooth and rich in flavor (Chung and McClements 2013). The critical level of fat content to achieve the mouthfeel related to fattiness seems to be around 15% (Malone, Appelqvist, and Norton 2003). In low-fat products, increasing the viscosity in the continuous phase can to some degree compensate the low-fat content and give products that have similar flavor and mouthfeel as high-fat products. However, the key parameter may not be the rheology as a bulk property but more on the rheology of the film formed in the mouth cavity upon eating the food product (Malone, Appelqvist, and Norton 2003). In the case of water-in-oil (W/O) emulsions, such as spreads, the state of the fats are important not only for mouthfeel but also for spreadability. The ratio between liquid and solid fat will thus affect the rheology. Also, for these types of products, the type of polymorphic form of the lipid crystals will influence the property of the product, as a transition from β′ crystals (preferred in margarine-type products) to β crystals is associated with larger crystals (greater than20 µm), giving a gritty or sandy mouthfeel, low spreadability, and oil–fat separation (Heertje 2014, Sato and Ueno 2011). Mouthfeel of emulsions can also be altered by the presence of particles

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or fat crystals. Large particles will give a sandy mouthfeel usually described as tallowness (Watanabe et al. 1992). When it comes to flavor, the release of flavoring components from the dispersed phase is important. The release will be affected by how these molecules are transported out of the dispersed phase and thus by properties such as diffusion coefficient of the component, droplet size of the dispersed phase, and interaction with other ingredients in the emulsions (such as the emulsifier). The release will also be influenced by partitioning of the flavoring ingredient into two phases and thus be affected by the concentration of the dispersed phase. This is especially important for O/W emulsions, as it is the concentration of aroma in the water phase and the head space (gas phase above the emulsion) that influences its taste. Low-fat products can show a burst of flavor due to the quick release of the oil-soluble components whereas highfat products often display a more continuous release of components that partition to the oil phase (Bayarri, Taylor, and Hort 2006). When designing and producing low-fat products, the release profile of the oil-soluble components may have to be modulated, for example, by encapsulation. It has also been seen that in systems that have the same release of aroma components into the gas phase, changes in the rheology of the emulsion still can affect taste. This could be attributed to the difference in the release pattern between volatile aroma compounds and more water-soluble taste compounds such as sugar (Bayarri et al. 2006), where the latter is more sensitive to the rheology. When it comes to the water-soluble components, they will predominately be in the water phase, and thus O/W emulsions will have a quick influence on the taste. However, if taste masking is desired, the water-soluble components can sometimes be encapsulated in double emulsions.

3.2.3 SHelF-liFe Stability The shelf-life stability of food emulsions is governed by factors that affect both chemical and microbiological stability, in addition to issues that have to do with the stability of the emulsion as such. There are also special issues, for instance, the stability of emulsions in frozen food, which are of technical and industrial importance. 3.2.3.1 emulsion stability In Chapter 2 Bergenståhl describes factors that lead to the destabilization of emulsions in more detail. Destabilization of emulsions is mainly caused by creaming/ AQ 3 sedimentation, coalescence, and Ostwald ripening. For macroscopic emulsions, creaming/sedimentation occurs due to the density difference between the oil and water fraction; it can partially be reduced by decreasing the droplet size of the emulsion, increasing the viscosity of the continuous phase, or by decreasing the difference in densities between the two phases (see Section 3.5.3). Coalescence leads to the formation of larger oil droplets, which may eventually lead to a complete AQ 4 phase separation of the emulsion. This can mainly be controlled by the adsorption of surface-active compounds to the interface that hinders drop–drop contact through either a steric barrier or an electrostatic repulsion, thus resulting in

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the process of coalescence. Increased viscosity of the continuous phase can also decrease coalescence (as well as the rate of creaming/sedimentation) to some extent. Another mechanism that drives the evolution of droplet size is caused by the pressure difference between the inside and outside of a curved surface. This so-called Laplace pressure is higher for a more curved surface, for example, small droplets; this is the driving force Ostwald ripening, which leads to an increase in particle size of the emulsions at the expense of smaller droplets. In this case, the solubility of the dispersed phase in the continuous phase is of major importance; that is, a low solubility slows down or prevents Ostwald ripening. Hence, Ostwald ripening is typically not observed in triglyceride O/W emulsions but, for instance, can occur for more soluble oils such as aromatic and essential oils. Ostwald ripening can also be decreased by increasing the viscosity in the continuous phase (decreases diffusion) and systems with low curvature. Pickering emulsions, for example, have been suggested to decrease Ostwald ripening, as they might have a local zero curvature (Tcholakova, Denkov, and Lips 2008). Flocculation is the aggregation of droplets. Flocculated systems may have desired properties for formulation such as beneficial rheology, but extensive flocculation might lead to increased creaming and thus may lead to coalescence. Changes in the degree of flocculation can also affect the rheology of the emulsions, changing properties such as mouthfeel. The colloidal stability of the emulsion will be governed by the repulsive/attractive forces between individual droplets of dispersed phase, the energy and rate of droplet collisions, the viscoelastic properties of the interface between oil and water, and the solubility of the dispersed phase in the continuous one. The choice of an emulsifier could influence all of these, and a proper choice of viscosity modifier will influence all kinetic factors such as collision of droplets and diffusion of dissolved molecules. The most important repulsive and attractive forces between emulsions droplets are summarized as follows: Hydrophobic effect. This is the main reason for the instability of emulsions. The hydrophobic interaction is based on the exclusion of nonpolar components from water. van der Waals attraction. These forces exist in all systems. Between small molecules, van der Walls forces are of short range and the decay with the distance between the molecules as the power of minus six. However, in a colloidal system, they can be of a much more long range, decaying with the reciprocal of distance. Together with the electrostatic forces, it is the basis for the DLVO theory (Verwey and Overbeek 1948). Electrostatic repulsion. This can be an important stabilizing force for food emulsions. Both proteins and ionic emulsifiers can be charged, depending on the pH, and when adsorbed, at the droplet interface giving rise to electrostatic repulsion. Emulsions stabilized by electrostatic repulsions are sensitive to salt and, in many cases, sensitive to pH. This sensitivity toward salt is due to the decay in the range of the electrostatic repulsion caused by the presence of ions and the effect strongly increases with the

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AQ 6

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AQ 7


valance of the ions. Thus, it is the ionic strength that is the key issue when it comes to stability in emulsions based on ionic emulsifiers. One should be aware that the ionic strength of buffers changes with pH. In some cases, there could also be specific ion interactions; for example, an interaction between calcium ions and casein that leads to aggregation (Dickinson and Davies 1999). Depletion attraction and steric repulsion. These interactions are caused by the presence of macromolecules in the continuous phase. Depletion attraction is due to the fact that macromolecules (proteins, polymers, and colloidal particles) having no affinity toward the interface will be excluded in the space between two approaching emulsion drops; this will lead to an osmotic pressure gradient, which then favors aggregation. Depletion attraction is typically observed in emulsions containing dissolved neutral polysaccharides. Thus, the addition of polysaccharides to alter the rheology or to form complexes with emulsifying agents may lead to depletion aggregation (Magnusson and Nilsson 2011). Steric repulsion is induced by macromolecules adsorbed at the interface; this is mainly due to the excluded volume effect, as adsorbed molecules come close together (Israelachvili 1985). Steric repulsion thus requires not only the affinity of the macromolecule to the interface but also a high solubility of the macromolecule in the continuous phase. The latter allows parts of the adsorbed macromolecule to protrude into the continuous phase, giving rise to steric hindrance. Nonionic emulsifiers both low molecular and polymers might stabilize the emulsion through steric repulsion. These systems are less sensitive to salt and pH than electrostatic stabilized emulsions.

3.2.3.2 Chemical stability One key issue for the chemical stability of emulsions is the oxidation of fats; especially fats with a high degree of unsaturation are susceptible to this problem (Moore et al. 2012, Waraho, McClements, and Decker 2011). The most common cause of fat oxidation in O/W emulsions is the interaction between transition metals and lipid hydroperoxides located at the oil–water interface, which produces highly reactive peroxyl and alkoxyl radicals (Frankel 1998, McClements and Decker 2000). One way to handle fat oxidation could be to add antioxidants such as vitamin E or phenolic substances, but a proper choice of ingredients and ingredient quality can also be of importance. For example, Charoen et al. (2012) showed that different biopolymers used as emulsifiers for rice oils differed in their capability to protect AQ 8 the oil from oxidation. They speculate that this could partly be due to the degree of specific binding of iron to the polymers, but they could not exclude that it was caused by impurities such as heavy metal ions used in the polymers. This illustrates that it is important to be aware of the oxidative impurities in the ingredients that are AQ 9 used; this could be heavy metal ions as well as peroxides. Waraho, McClements, AQ 10 and Decker (2011) review the oxidation of lipids in emulsions and point out that there will be a difference in the oxidation process in pure oil when compared with one in an emulsion. This is because the water phase in the emulsion may include

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oxidative agents such as transition metals and iron and ingredients such as EDTA AQ 11 and iron-binding proteins (e.g., lactoferrin) that may decrease oxidation in emulsions (Waraho, McClements, and Decker 2011). Phenolic compounds have been seen to be pro-oxidatives, especially in the presence of iron (Medina et al. 2012, Sørensen et al. 2008). Antioxidants can, as mentioned, be added to the formulation, and the activity of these antioxidants will depend on their location in the emulsion and solution conditions such as pH. It has been shown that nonpolar antioxidants are more effective in emulsions as compared to nonpolar oxidants, which are more effective in bulk oils (Frankel 1998). This is called the polar paradox and is probably related to the fact that the antioxidant has to be close to the lipids that it should protect. There is a growing interest to use naturally occurring phenolic compounds such as caffeine, coumaric acid, and rutin as antioxidants (Kikuzaki et al. 2002, Medina et al. 2012, Sørensen et al. 2008). These compounds have, however, also been seen to, at some conditions, be pro-oxidative (Sørensen et al. 2008), again highlighting the AQ 12 importance to know the function of the specific additive at the conditions used for each food product. Another problem with several phenolic compounds is their low solubility (Löf, Schillén, and Nilsson 2011) and, in these cases, their existence as dispersed particles in the continuous phase, which, of course, reduces there antioxidative capacity. 3.2.3.3 microbiological stability Microbiological stability is especially important for O/W emulsions as they often have a high water activity. The shelf life from a microbiological viewpoint will be dependent on packaging, processing (e.g., pasteurization), and the choice of ingredients. When it comes to ingredients, both their intrinsic microbiological load and their ability to function as antimicrobiological ingredients are important. Furthermore, from a formulation viewpoint, ingredients other than preservatives can give bactericidal effects. For example, components in essential oils are antimicrobial (Burt 2004) and so are some emulsifiers, for example, lysozyme–xanthan gum conjugates (Hashemi, Aminlari, and Moosavinasab 2014) and monocaprylate (Hyldgaard et al. 2012). Some of the more common food preservatives such as ascorbic acids and its salts (Lück 1990) are also used in emulsions. Other bactericides are the peptide nisin (Castro et al. 2009) that is common in several food emulsions such as dairy products and sausages (Abee, Krockel, and Hill 1995). When choosing preservatives, it is important to understand how the property of the emulsions such as pH and salt content will affect the preservative. The choice of other ingredients and their concentration might also affect the action of the preservative. Nisin, for example, has been seen to be strongly affected by the composition of the emulsion such as oil content and oil/surfactant ratio (Castro et al. 2009). The homogenization as such may also affect the preservative, especially if it is sensitive to surface adsorption, heat, or shear (Zapico et al. 1999). In W/O emulsions, the microbiological growth is reduced due to the limited space in the water droplets and the inability for the microorganisms to transport themselves in between droplets; however, as pointed out by others, W/O emulsions such as margarine and spreads also need to show how microbiological safety is obtained

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during the shelf life (Charteris 1996, Delamarre and Batt 1999). For these products, spoilage is often due to moulds and can be reduced by the addition of preservatives such as sorbates and benzoates (Delamarre and Batt 1999). 3.2.3.4 Freeze-thaw stability A special case of stability is the freeze-thaw stability of emulsions (Degner et al. 2014). It is often seen that frozen emulsions changes when thawed, for example, manifested as a full phase separation or that the emulsion becomes grainy and watery. There are several reasons for these effects; one is that the lipid crystals can form and lead to partial coalescence of the semifrozen emulsion, which upon reheating goes forward to full coalescence and phase separation. Crystallization pattern of the lipids is one of the main factors that will determine if a freeze-stable emulsion can be obtained. Magnusson, Rosén, and Nilsson (2011) have shown that for high dispersed volume fraction O/W emulsions, oils contain high amounts of unsaturated fatty acids, have a high percentage of crystallized triglycerides at −25°C, and thus have a high rate of susceptibility for freeze-thaw instabilities. The volume fraction of oil will also affect the freeze-thaw stability; in an unpublished study, it was shown that the freeze-thaw stability of mayonnaise was increased by decreasing the oil fraction. This is probably because of the reduced contact time between oil droplets. Another mechanism that is seen for emulsions with oils that do not crystallize before ice formation is that coalescence is triggered by increasing the concentration of the dispersed phase when larger and larger volumes of the water phase are removed due to ice formation. In this case, freeze-thaw instabilities can be decreased by not only the right freezing conditions but also the right choice of the product composition. The freeze-thaw stability of emulsions can be increased by the addition of cryprotectants such as polyols (sucrose, glucose, fructose, trehalose, and maltose), antifreeze proteins, gelatin, and some carbohydrates (Degner et al. 2014). These alter the crystallization of water and the morphology of the ice crystals; however, some of them can also function by increasing the viscosity, and thus decreasing the number of oil droplet collisions leading to coalescence. Addition of polysaccharides has also been seen to improve the freeze-thaw stability. This could be due to several factors; however, an increased viscosity of the nonfrozen phase and the capability of some polysaccharides to form protective layers around the dispersed phase hindering coalescence play a major role (Degner et al. 2014). The emulsifier is critical when it comes to destabilization due to increased concentration, but can also be important for lipid crystallizationinduced freeze-thaw instabilities. Emulsifiers are able to stabilize the emulsion also at a high concentration, for example, some Pickering emulsions using quinoa starch granules or egg yolk granules (Marefati et al. 2013, Rayner et al. 2014) proteins such as caseins (Degner et al. 2014) and hydrophobic starch that give a thicker interfacial coatings around the fat droplets are good in this sense. In frozen and cold-stored foods, it is especially important to understand how the emulsifier itself is affected by the decrease in temperature; for example, several lowmolecular emulsifiers lose their solubility below the so-called Kraft point and thus the function of these emulsifiers will decrease.

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3.3

59

Issues to ConsIder when ChoosIng IngredIents For emulsIons

One of the key ingredients to choose for an emulsion formulation is the emulsifier. The emulsifier lowers the surface energy between the two phases and thus affects the size of the emulsion droplets. It should also create a barrier for coalescence and droplet growth during storage. Emulsions can be stabilized by low-molecular emulsifiers, proteins and other polymers, and particles. Table 3.1 gives a comparison between them. As a rule-of-thumb, low-molecular emulsifiers lowers the surface tension more than surface-active macromolecules, but they adsorb reversibly to the interface; they often form complexes with other ingredients in products such as proteins, and might be less good than high-molecular emulsifiers when it comes to reducing coalescence during storage. When choosing an emulsifier, it is important to consider its compatibility with other ingredients of the product; many components, such as some preservatives, are surface active themselves and might interact with the interface. Another issue is salt concentration and pH. Both charged small emulsifiers and proteins are AQ 13 strongly affected by salt concentration and proteins are especially strongly affected by pH. When choosing ingredients for a multicomponent system as an emulsion, it is very important to not only understand the solubility of components in the two phases but also to partition the ingredients into each phase and to the interface between the two phases. For example, whatever thickener is used, it should be partitioned into the continuous phase of the formulation. One also has to be aware that the partitioning of components might change the phase behavior of the ingredient. One simple example of this is that the partitioning of small surface-active substances to the oil–water interface will shift the apparent critical micelle concentration (CMC) for these components to a higher concentration, which is dependent on the surface area of the dispersed phase, and thus affected by the droplet size and the amount of dispersed phase. For shelf life, the purity of the ingredient is critical, especially components that trigger oxidation of the oil, for example, heavy metal ions or peroxides. One should also be aware of the concentration of surface-active substances such as fatty acids in the oil. The latter could interact in different ways with the mechanisms of stabilization of the oil droplets either by competing with the chosen emulsifier at the oil–water interface or by interacting with it thus changing its properties. This is of special importance for particles, but could also likely affect, for example, fatty acidbinding proteins or biopolymers that form inclusion complex with the surface-active components. Also, the stability of the ingredients during homogenization has to be considered. As discussed previously, the homogenization process negatively affects the preservative effect of nisin. Another example is the hydrophobically modified starch, which has been shown to decrease its molecular weight during homogenization (Modig et al. 2006, Nilsson, Leeman et al. 2007). Soy proteins have shown disruption as well as aggregation induced by high-pressure homogenization (Roesch and Corredig 2003). Several issues such as heat, shearing, and the adsorption into surfaces of the

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Fast in dynamic equilibrium Low CFC

Water

Oil

O O

O O

O O N

Water

O O

Lecithin

O/W ~0.05 Poor Poor at I > CFC

Water O/W ~1 to 1.5 Good Good

Water

Oil

Water Particle

O/W ~0.02 to 1 Variable Variable

Water

W/O ~0.02 to 1 Good Good

Oil

θ < 90° oil in water emulsion (or if θ > 90° water in oil emulsion)

θ

Oil

Water

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equipment can affect the ingredients in the emulsion. Another issue could be that the surface-active components might induce the leakage of components from gaskets and other plastic and rubber parts of the equipment. Thus, an incompatibility between the process and the ingredients used has to be considered during the development process. In food products, a further complication is that many of the ingredients used are often very complex mixtures, for example, mixture of proteins, polar lipids, and polysaccharides. One good example here is egg yolk, which is used to stabilize mayonnaise-type emulsions. In such cases, it may be difficult to know which ingredient actually contributes to the emulsification and stabilization actions; to complicate things further, the components at the interface can vary with solution properties such as pH (Magnusson and Nilsson 2013, Nilsson et al. 2006, Nilsson, Osmark et al. 2007). The interaction between ingredient components may enhance the stability of emulsions as well as cause instability, and the effect of adding individual ingredients can be difficult to predict. Finally, one should be aware of the variation and inhomogeneity of the ingredients and at least have some knowledge if this might affect batch-to-batch variation of products. Several commercial emulsifiers are mixtures that show a variation in chain length of the hydrophobic tail (cf. sorbitan esters and ethoxylated sorbitan esters) or variation in molecular weight (cf. all polymers) and even variation in the composition, for example, lecithin and whey proteins. These variations can affect the composition of the molecules at the interface and could influence issues such as shelf-life stability, rheology, droplet size, and so on.

3.4 Key IngredIents In emulsIons 3.4.1

FatS aNd oilS

Oil is not only often the major source of energy in a food emulsion, but it also acts as the phase where key nutritional components such as antioxidants and fat-soluble vitamins will be dissolved. Thus, the choice of fats used in the emulsion process will influence the nutritional value of the final product. Generally speaking, a high degree of saturated lipids, especially omega-3 and omega-6 lipids, is considered to be linked to health benefits, and a factor that is used to improve the nutritional value of some foods (Berasategi et al. 2014, Moore et al. 2012, Sørensen et al. 2008) The most common oils in food emulsions are triglycerides (see Figure 3.1). The properties of the triglycerides are governed by the chain length and the degree of saturation of the lipophilic part of the molecule (Larsson 1986).The longer the chain length and the more saturated fatty acids are in a triglyceride, the higher will be the melting point. Triglycerides are often classified depending on the chain length of the fatty acid part into high- (>12), medium- (6–12), and low-chain ( 2

V: volume of hydrophobic chain CPP = V l: length of hydrophobic chain al a: area of head group

FIgure 3.2 Schematic description of some self-assembled lipid structures and an explana- AQ 18 tion of packing parameter: (a) CPP 2.

phases. The amphiphilic character of these molecules also triggers the formation of different self-assembled structures in solution (see Figure 3.2). The type of selfassembled structures that are obtained depends on the concentration of the emulsifiers, as well as is partly affected by the intrinsic properties of the molecules. The type of structures formed ranges from micellar structures to reversed structures (reversed micelles, L2 phases, or reversed hexagonal phases). Emulsifiers with an even bal- AQ 19 ance between hydrophilic and hydrophobic parts often form lamellar liquid crystals, which forms vesicles when dispersed in water. The critical packing parameter (CPP) is a generalization of the self-assembling properties of surfactants, describing the properties as a geometrical balance between the area needed for the polar group relative and the area needed for the hydrophobic group (Israelachvili, Mitchell, and Ninham 1976) (see Figure 3.2). This can be used to estimate what type of structure an emulsifier will give. When CPP is equal to one, it will favor lamellar structure, whereas if the CPP is lesser than 1/3, it will favor micelles; a CPP of above 2 will favor reversed micelles. For more details on the liquid crystalline phases formed by common food emulsifiers, there is a review by Krog (1997). Friberg and Wilton

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(1970) suggested that the presence of lamellar liquid crystalline phases is a strong indication of a good emulsifier in simple systems. The functionality of low-molecular emulsifiers is, in a wide interpretation, determined by their solution properties. Although the character of low-molecular emulsifiers is such that they contain regions that are water soluble and those that are more lipophillic, their overall character can make them more soluble in one of the two phases. Thus, emulsifiers can be found in a range from highly soluble in the oil to more soluble in the water phase. The effect of solubility on emulsion character was AQ 20 first expressed in the Bancroft (1913) rule, stating that “hydrophilic colloid will tend to make water the dispersing phase while a hydrophobic colloid will tend to make water the disperse phase.” To describe the degree of hydrophilicity contra lipophilicity, it is very popular to use the hydrophilic–lipophilic balance system according AQ 21 to Griffin (1954). The HLB ratio is expressed as a number based on the molecular weight of hydrophobic components compared to the molecular weight of the molecule. The HLB number can also be estimated from the chemical structure according to molecular group contributions as stated by Davies (1957): HLB = ∑ Hydrophilic group numbers − ∑ Hydrophobic group numbers + 7 (3.1) The HLB value of an emulsifier is often used as a rule of thumb (see Table 3.3). However, one should be aware of the fact that solution conditions might change the HLB balance of a system; for example, the addition of salt can screen charge groups, making the system appear less hydrophilic. Another factor of importance is the temperature. The effective HLB value is strongly temperature dependent. For ethoxylated emulsifiers, the emulsifier gets less hydrophilic with increasing temperature and finally becomes insoluble in water at a temperature denoted as the cloud point. In an emulsion system, this can be followed by the phase inversion temperature (PIT), which corresponds to the temperature at which the effective HLB is about 6 (Shinoda and Sato 1969). Emulsions stored at a temperature of 25°C–60°C below the PIT are usually more stable. However, in food applications, this is rarely used, as ethoxylated surfactants are uncommon for food applications. Another important temperature to consider for emulsifiers is the Krafft point (Krafft and Wiglow 1895). The Krafft point is the temperature below

taBle 3.3 hlB values as Predictor for the use of emulsifiers hlB value

applications

example of emulsifiersa

3.5–6 7–9 8–18 13–15 15–18

W/O emulsifier Wetting agent O/W emulsifier Detergent Solubilization

Glycerol monostearate Sorbitan monolaurate Tween 80 Tween 81 Sodium Oleate

Source:

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Davies, J.T., Proceedings of 2nd International Congress Surface Activity, Butterworths, London, 1957.

a

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67

which the surfactant has low solubility and, hence, cannot form micelles. Technical functionality (such as foaming and emulsifying action) is only obtained above the Krafft temperature. High-melting fat bases (fully hardened C18-dominated fats) or long paraffinic chains creates high-melting emulsifiers with Krafft temperatures in the range of 40°C–60°C. Precipitating emulsifiers may contribute to fat crystallization and solid emulsifier may have a textural functionality; however, for most applications, such high melting points are unsuitable. Intermediate melting fat bases (C14–C18 fats with some unsaturation) give emulsifiers with Krafft or transition temperatures between 30°C and 50°C. These emulsifiers could be used to create stable α-gels and usually display well-performing properties in baking applications. Low-melting fat (highly unsaturated fat), branched hydrocarbons and inclusion of aromatic groups, gives low Krafft points, sometimes below 0°C. Table 3.4 summarizes some examples and usages of common low-molecular weight emulsifiers.

3.4.3

ProteiNS

Proteins function both as emulsifiers and as rheological modifiers in the formulation of food emulsions. The character of proteins in emulsions will be based on their tertiary structure in the solution and at the interface, their size, the net charge and charge distribution, their capability to form gels, and the distribution of hydrophilic and hydrophobic groups. There are numerous proteins that are used in emulsion, and the choice of protein emulsifier is often not only based on function but also based on what food group the emulsion is related. There are several traditional or natural-occurring emulsions that, at least, are partially stabilized by proteins, such as mayonnaise, dairy products, and sausages. Table 3.5 presents some of the more common protein emulsifiers. Most of these emulsifiers are mixtures of several different protein species. Thus, depending on the production and formulation conditions, the actual proteins at the interface may differ although the same protein emulsifier is used. Factors affecting the protein adsorption into the interface during competitive adsorption from solution are size, charge and hydrophobicity of the protein, the transport conditions of proteins to the surface during emulsification, if adsorbed proteins can be exchanged by proteins in solution, and the degree of conformational changes of the protein at the interface (Nilsson et al. 2006, Nilsson, Osmark et al. 2007, Wahlgren and Arnebrant 1991). Thus, it is a complex issue to understand what proteins are actually adsorbed at the interface. Magnusson and Nilsson (2013) reviewed this recently for egg yolk in high internal phase emulsions and discussed that the main property governing adsorption was the hydrophobicity of the proteins and that there is a preference for HDL and LDL proteins to adsorb at the interface. In the case of milk proteins, Surel et al. (2014) have seen that in mixtures of casein micelles and whey proteins, casein dominates at the interface when the fraction of casein in the solution is above 25%. Proteins get their amphiphilic character from the mixture of hydrophilic and hydrophobic amino acids. The amino acid sequence (secondary structure) also gives the template for the three-dimensional structure of the protein (tertiary structure). However, one should be aware that the tertiary structure will vary due to solution conditions, and that proteins in solution have a well-defined tertiary structure, which

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AQ 22

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E322 184.1400

E470 172.863

E481 172.846

E472 172.832

E471 184.1505

Fatty acid salts

Sodium stearoyl lactylate

Citric acid esters of MG

Mono and diglycerides

number eu/usa

Lecithin

name

Oil

Dispersible in hot water, insoluble in cold water, and soluble in edible oils and fats.

Dispersible but insoluble in water, where it swells on hydration. Soluble in oils and fats. Sodium and potassium salts are soluble in water. Calcium salts are insoluble in water. Dispersible in warm water and soluble in hot edible oils and fats.

solubility

taBle 3.4 Common low-molecular weight emulsifiers uses

Baked goods (e.g., bread and cakes), confectionery, dairy products, margarines, spreads, shortenings, salad dressings, and sauces Fine bakery wares, emulsified liqueur, fat emulsions, desserts, beverage whiteners, and minced and diced canned meat products Fats for stabilizing, also as synergists for antioxidants, baking fat emulsions, bakery margarines and shortening for stabilizing, margarine, mayonnaise, salad dressings, sauces, and in low-calorie foods Baked goods, confectionery (e.g., chewing gum, toffees, and caramels), dairy products, creams, desserts, edible ices, margarines, shortenings

Margarine, chocolate, breads and cakes, bubble gum, salad dressings, and sauces

Comment

Nonionic

Negatively charged

Negatively charged

Mixture of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids Charged at normal and low pH

68 Engineering Aspects of Food Emulsification and Homogenization

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E475 172.854 E477 172.856 E435 172.836

E433 172.840

Polyglycerol esters of FA Propylene glycol esters of FA Polyoxyethylene (20) sorbitan monooleate

Polyoxyethylene (80) sorbitan monostearate

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Whippable icing Fine bakery wares, fat emulsions for baking purposes, milk and cream analogues, emulsified sauces, soups, dietary food supplements, carriers and solvents for colors, fat-soluble antioxidants, and antifoaming agents Fine bakery wares, fat emulsions for baking purposes, milk and cream analogues, emulsified sauces, soups, dietary food supplements, dietetic foods, carriers and solvents for colors, fat-soluble antioxidants, and antifoaming agents

Oil

Water

Water

Cakes and icings, margarine, and salad oils

Water

Nonionic cloud point around

Nonionic cloud point around

Nonionic

Nonionic


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taBle 3.5 Common Commercial Proteins emulsifiers and example of Proteins that are Part of the emulsifier AQ 23

emulsifier

Key Proteins

Whey protein

a

Caseinsb

Egg whitec

Egg yolkc

Soy proteind

molecular weight

IP

18.6 14.2

5.3 4.8

Bovine serum albumin

66

5.1

α1-Casein α2-Casein

23 25

4.1 5.3

β-Casein

24

5.1

κ-Casein

19

5.6

45 77.7

4.5 6.0

β-lactoglobulin α-lactalbumin

Ovalbumin Ovotransferrin Ovomucoid

28

4.1

Lysozyme

14.3

10.7

Phosvitin

160–190

Low-density lipoproteins

16–135

Cobalamin-binding proteins

39

Riboflavin-binding proteins

37

Biotin-binding proteins

72

α- and β-Lipovitellins

400

α-Conglycinin

18–33

β-Conglycinin

104

σ-Conglycinin

141–171

Glycinin

317–360

aKinsella, J.E. and Whitehead, D.M., Advances in Food and Nutrition Research, Academic Press, San Diego, CA 1989. Swaisgood, H.E., J. Dairy Sc., 76, 10, 3054–3061, 1993. Awade, A.C., Z. Lebensm. Unters. For., 202, 1–14, 1996. Clarke, E.J. and Wiseman, J., J. Agr. Sci., 134, 111–124, 2000.

Sources: b c d

could be considerably changed and even lost when adsorbing at an interface. Proteins are often divided into different categories based on their tertiary structure. The most common structures are random coil (casein), globular proteins (whey proteins and egg proteins), and rod-like structures (fibrinogen, collagen, and gelatin). In many cases, the protein has a defined molecular weight but for some food proteins such as gelatin, this is not the case. The distribution of hydrophobic groups within the polymer is important. For globular proteins, the hydrophobic groups are mainly found inside the core of the protein shielding them from water. Upon adsorption into the oil–water interface, these hydrophobic groups could orient themselves toward the oil, which might lead to conformational changes of the protein. A few proteins especially κ-Casein has very distinctive hydrophilic and hydrophobic domains, which together

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with its semirandom coil structure makes them especially suitable as emulsifiers. The casein proteins α1−, α2−, β−, κ−Caseins form complex called casein micelles. Although these proteins play a large biological and a technical role, the structure of the casein micelles is still debated (Dalgleish 2011, Horne 2002). The main difference between low-molecular weight emulsifiers and proteins is that while the adsorption of the former is completely reversible, when the concentration is lowered, proteins have a tendency to adsorb irreversibly. This makes them less sensitive to changes such as dilution. However, even if the adsorption is irreversible toward the lowering of concentration, the protein could still be exchanged by other species (proteins or low-molecular ones) that have higher driving force for adsorption, for example, a higher reduction of the surface tension at the oil–water interface. The kinetics of these events and the conformational changes of the proteins can be slow, on the time scale of hours to days, and can lead to postproduction changes of the emulsion. Furthermore, proteins are sensitive to heat, enzymes, and solution conditions such as pH and ionic strength, which lead to degradation, aggregation, and other protein changes. These events can also lead to long-term change of emulsions stabilized by proteins. Another difference between low molar mass (or small) emulsifiers and proteins is the rheology of the adsorbed layer. Proteins often form thicker, more viscous layers than small emulsifiers (Bosa and van Vlieta 2001). This AQ 24 is in most cases positive for the long-term stability of the emulsion. Proteins might stabilize emulsions through electrostatic repulsion and, thus, several protein systems show tendencies to aggregate at pH close to the isoelectric point of the protein emulsifier. If such aggregation does not lead to coalescence, it could lead to an increase in the viscoelasticity of the emulsion (Wu, Degner, and McClements 2013). In systems where both small molecular emulsifiers and proteins are present, there might be a competition between the components at the interface or there might be a cooperative adsorption (Maldonado-Valderrama and Patino 2010, Nylander et al. 2008, Rodríguez, García, and Niño 2001, Waninge et al. 2005). The competitive AQ 25 adsorption of proteins and small emulsifiers are strongly concentration dependent, and at concentrations below the CMC of the emulsifiers, proteins often dominate at the interface (Wahlgren and Arnebrant 1992). The order in which the components reach the surface might also be important as small surface-active components cannot always remove already adsorbed proteins (Karlsson, Wahlgren, and Trägårdh 1996, Wahlgren 1995). Cooperative adsorption may occur when the protein complexes with the low-molecular emulsifier, which, for example, is common for many ionic surfactants (Maldonado-Valderrama and Patino 2010). It is often seen that the adsorption of low-molecular emulsifiers to protein-stabilized emulsions have a detrimental effect on the emulsion stability (Wilde et al. 2004). Furthermore, there could be strong interactions between proteins and surfactants in solution, changing the structure and behavior of the proteins (Nylander et al. 2008). Proteins also have the capability to change the rheology of the emulsions, especially if they are triggered to aggregate and to form a gel. Gel formation is often induced by heating and denaturation of the proteins but could also be an effect of pH, for example, the change in the rheology between milk and yoghurt. For example, increased viscosity through the addition of proteins is important in low-fat products such as margarines, sausages, and spreads (Chronakis 1997). Common proteins used

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to form gel structures are milk-based systems such as whey proteins (Chronakis 1997, Youssef and Barbut 2011) and soy proteins (Youssef and Barbut 2011).

3.4.4

PolySaccHarideS

Polysaccharides primarily function as viscosity modifiers in emulsions. However, hydrophobically modified polysaccharides are also used as emulsifiers. A thorough description of polysaccharides is given in Food Polysaccharides and Their Applications (Stephen, Phillips, and Williams 2006). The properties of a polysaccharide is given by the structure of the smallest repeating saccharide units, the degree of branching of the polymer, and its molecular size. Differing from proteins, polysaccharides typically have a high degree of polydispersity when it comes to branching and molecular weight. There can also be a large batch-to-batch variation, which might lead to variation in performance. Table 3.6 presents some of the more common polysaccharide groups and these will also be discussed subsequently. AQ 26 Traditionally, exudate gums, such as gum arabic, have been used as emulsifiers especially in flavored beverages (Dickinson 2003). These are natural polysaccharides that are produced by plants as a protection against bacteria and dehydration. One has to be aware that there is a very high variation in the composition between gums obtained from different species (Stephen, Phillips, and Williams 2006) and that this variation might affect the emulsion produced. These contain a heterogeneous mixture of highly branched polysaccharides and a small amount of proteins (2% for gum arabic) covalent attached to the polysaccharides. Gum arabic is thought to have a water blossom structure built up of an amino acid core of around 400 units onto which bulky polysaccharide units of 250 kDaltons are grafted (Stephen, Phillips, and Williams 2006). It is the protein moieties of the exudated gums that make them surface active. Alftrén showed that for gum arabic and mesquite gum, the amount of protein in the polysaccharide fraction increased with increasing molecular mass and that these high protein content/high-molecular weight fractions were preferentially adsorbed into emulsion droplets (Alftrén et al. 2012; Evans, Ratcliffe, and Williams 2013). The emulsification capacity of gum arabic is lost upon heating (Williams, Phillips, and Randall 1990). Another group of polysaccharide that is dependent on a protein fraction for its emulsifying properties are modified pectins (Akhtar et al. 2002, Dickinson 2003). Although pectin is mainly used as a rheological modifier in emulsions, if they are modified, they might work as emulsifiers (Dickinson 2003). The modification is, in most cases, acetylation; however, depolymerization using acids has also been used (Dickinson 2003). Akhtar et al. (2002) have shown that depolymerized citrus pectin of 70% esterification gives good stable emulsions, although only 25% of the pectin is adsorbed into the interface and that upon storage, there are some flocculation that increase particle size. For example, hydrophobic modification of polysaccharides starch can also produce molecules that are surface active and can be used as emulsifiers. Nilsson and Bergenståhl (2006, 2007) have done extensive studies of hydrophobically modified starch and shown that they are good emulsifiers and that the surface load of OSAstarch can be as high as 16 mg/m2. They have also shown that it is the high molar mass components of the polymer that are selectively adsorbed to the emulsion droplets (Nilsson, Leeman et al. 2007).

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Carrageenans

Chitosan

Carboxymethylcellulose

Modified starches

Sulfated D-galactans, units of (1 → 3)-β-D-Gal and (1 → 4)-3,6-anhydro-a-D-Gal alternating; pyruvate and Me groups

Clusters of short (1 → 4)-α-D-Glc chains attached by a-linkages of 0–6 of other chains Native starch both amylose and amylopectin-modified by chemicals for example acetate, phosphate, and sodium octenyl succinate HO2CCH2-groups at 0–6 of linear (1 → 4)-β-D-glucan (1 → 4)-2-acetamido-2-deoxy-β -D-glucose and 2-amino-2-deoxy-b-D-glucose

Starch amylopectin

molecular structure

Essentially linear (1 → 4)-α-D-glucan

Starch amylase

name

taBle 3.6 some Key Polysaccharides Function

Stabilizer, thickener, and gelation

Stabilizer, thickener, and water retention Emulsifier and rheological modifier

Stabilizers, thickener, and hydrophobically modified starches, which function as emulsifiers

viscosity modification

(Continued)

Semisoluble polymer with a wide range of viscosities, viscosity decreases with temperature. Positively charged, partly hydrophobized polymer that can gel depending on pH and the presence of multivalent negative ions Forms salt- or cold-setting reversible gels in an aqueous environment. Gelling ability is seen for Carrageenans that form helical structures.

Starch gelatinization; the ordered crystalline regions undergo melting, permitting granule swelling. This can be followed by the recrystallization and formation of helix structure. Modification can make the starch cold, swelling or nonswelling.


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Linear and branched (1 → 4)-α-Dgalacturonan (partly methyl esterified and acetylated); chains include (1 → 2)-L-Rhap, and branches D-Galp, L-Araf, D-Xylp, D-GlcA Cellulosic structure, D-Manp (two) and GlcAcontaining side chains, acetylated and pyruvylated on Man

Pectins

Stabilizer and thickener

Emulsifier, encapsulating agent, stabilizer, and thickener Gelation, thickener, and stabilizer

Function

viscosity modification

High-molecular weight pectins form gels at low pH (2.5–3.5) and in the presence of high sucrose concentration (>55%), or other cosolutes (e.g., sorbitol, and ethylene glycol) Solutions have a thixotropic behavior; gels are formed at high concentration or in the presence of plant galactomannans such as locust bean gum.

Low viscosity at high concentrations, less than 40% rheology strongly affected by pH and electrolyte.

Source: Data from Stephen, A.M. et al., Food Polysaccharides and Their Applications, CRC Press, Boca Raton, FL.

Xanthan gum

Acidic L-arabino-, (1 → 3)- and (1 → 6)-β- D-galactan, highly branched with peripheral L-Rhap attached to D-GlcA. Minor components of glycoproteins

molecular structure

Gum arabic

name

taBle 3.6 (Continued ) some Key Polysaccharides


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75

According to Dickinson (2013), xanthan gum, which has high viscosity at low shear, is established as the first choice when it comes to using them as rheological modifiers for stabilizing emulsions, but there are a large range of polysaccharides that are used for improving texture in food emulsions. Polysaccharides can increase the viscosity of an emulsion either by some gelation mechanism, such as those triggered by the addition of calcium ions (alginate, pectins, and carrageenan) and the formation of double helices and crystallization (starch), or by nonspecific chain–chain interactions determining the viscosity. Nongelling polysaccharides, especially if they are linear and are good solvents, often behave as random coil polymers. At low concentrations, such polymers behave more or less as Newtonian liquids but as the concentration increases, the polymer chains start to overlap and the rheological behavior of the polymer changes. Above the so-called overlap concentration, the viscosity becomes shear AQ 27 thinning and the viscosity increases more steeply with polymer concentration. Gelling of polysaccharide can also form continuous water-swollen networks at low concentrations. To obtain such systems, the polysaccharides contain both regions that form the physicochemical bonds between the polymers and the nonbinding regions that primarily hold the water. The regions that form the bonds are usually well ordered, allowing for helices, egg-box, ribbon–ribbon, and double helix–ribbon structures. In complex food products, for example, the interaction with the polysaccharides with other components of the product might lead to segregated networks when proteins and polysaccharides phase segregate. The polysaccharides, especially starch, can also form inclusion complex with small emulsifiers such as monoglycerides and fatty acids (Eliasson 1986, Tufvesson, Wahlgren, and Eliasson 2003). These interactions are often stronger than the tendency for the emulsifier to adsorb into interfaces and thus it will lower the amount of the emulsifier available (Lundqvist, Eliasson, and Olofsson 2002). The complexes so formed will also have additional properties AQ 28 such as melting point than the double helices normally formed in starch.

3.4.5

ProteiN-PolySaccHaride coMPlexeS

Compatibility between proteins and polysaccharides becomes important during the modification of textures in food products. Mixtures of proteins and polysaccharides can lead to three major scenarios (Schuh et al. 2013): 1. A single homogenous phase is formed. 2. The polysaccharide and proteins phase segregates into different phases as the mixtures are not thermodynamically compatible. 3. Protein and polysaccharides aggregate and form insoluble complex. In complex food systems, these types of interactions might either lead to the stabilization of the emulsion or, especially in the latter case, destabilization. Lately, there has been an interest in using the protein–polysaccharide complex as emulsifiers in food systems (Evans, Ratcliffe, and Williams 2013). As discussed previously, some traditional polysaccharide emulsifiers are probably protein– polysaccharide complexes; other methods to obtain such complexes could be the formation of Maillard conjugates (Akhtar and Dickinson 2007, Zhang, Chi, and

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Li 2013) or electrostatic complexes (Harnsilawat, Pongsawatmanit, and McClements 2006, Koupantsis, Pavlidou, and Paraskevopoulou 2014, Xu et al. 2014). Protein– polysaccharide complexes are used in the encapsulation of emulsion droplets, leading to the protection of sensitive substances (Xu et al. 2014), or the encapsulation of AQ 29 flavors (Koupantsis, Pavlidou, and Paraskevopoulou 2014). Polysaccharide–proteinstabilized emulsions have been shown to be more stable to stress, for example, heat than emulsions only stabilized by the protein (Harnsilawat, Pongsawatmanit, and McClements 2006). They have also been seen to be insensitive to pH and salt concentration (Zhang, Chi, and Li 2013).

3.4.6

ParticleS

In addition to small molecular-weight surfactants and macromolecules, colloidal particles can be utilized to stabilize emulsions. Particle-stabilized emulsions (commonly referred to as Pickering-type emulsions) are possible as a result of the properties of the particles, where a combination of size, form, and partial dual wettability of both the oil and water phases confers Pickering particles several useful properties and the ability to create emulsion droplets that are highly stable against coalescence (Rayner et al. 2014). Particle-stabilized emulsions in general, and in food-based particles in particular, have received an increasing interest in recent years and for this reason, a separate chapter has been devoted to particle-stabilized emulsions in this book (see Chapter 4). However, particle-stabilized emulsions are by no means a recent discovery, being reported in the scientific literature during the early twentieth century. One of the first particle-stabilized food products was mayonnaise, a popular condiment based on an O/W emulsion, which was first formulated in 1756, where the finely ground (and somewhat hydrophobic) mustard particles adsorb at the oil–water interface and cover a fraction of the oil droplets preventing coalescence, in addition to other surface-active components found in egg and other ingredients (Binks 2007). The commonly used types of particles in fundamental studies (where there is an abundance of literature to be found in fields of soft matter and physical chemistry) are often inorganic/synthetic such as clays, silica, alumina, titanium oxides, and latexbased particles (Aveyard, Binks, and Clint 2003). However, there has been a recent and an increasing trend toward developing suitable food-based particles, which are not only edible but also maintain the consumer perception of being wholesome or natural. Three general approaches can be taken to obtain food-based particles. They can be built up or synthesized from molecules extracted from food-based materials (e.g., aggregation, crystallization, cross-linking, precipitation, etc.); they can be obtained by reducing the size of existing structures (e.g., milling, crushing, hydrolyzing, etc.); and they can be isolated as with their innate biological structures intact. Also, in many cases, a breaking-down step (i.e., to dissolve the working material) is a precursor for synthesis. Examples of a synthesis approach for generating edible particles include starch nanocrystals (Li, Sun, and Yang 2012), chemically modified starch nanospheres (Li, Sun, and Yang 2012, Tan et al. 2012), flavonoid particles (Luo et al. 2011, 2012), chitin nanocrystals (Tzoumaki et al. 2011, 2013), soy protein particles (Paunov et al. 2007), whey protein microgels (Destribats et al. 2014), insoluble corn protein (zein) particles (De Folter, Van Ruijven, and Velikov 2012), solid lipid particles, and fat crystals

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(Gupta and Rousseau 2012). Examples of particles formed from the breakdown of larger structures include cellulose nanocrystals (Kalashnikova et al. 2011, 2013), cocoa particles (Gould, Vieira, and Wolf 2013), ethyl cellulose particles (Jin et al. 2012), and cryomilled fractured modified starch particles (Yusoff and Murray 2011). Examples of particles directly isolated include lactoferrin nanoparticles (Shimoni et al. 2013), bacteria chitosan networks (Wongkongkatep et al. 2012), natural spore particles (Binks et al. 2005, 2011) hydrophobic bacteria (Dorobantu et al. 2004), egg yolk granules (Ercelebi and Ibanoglu 2010, Eriksson 2013, Laca et al. 2010, Rayner et al. 2014), and starch granules isolated from a variety of botanical sources (Li et al. 2013, Rayner, Timgren et al. 2012, Timgren et al. 2011), with or without hydrophobic modification (Rayner, Sjöö et al. 2012, Song et al. 2014, Timgren et al. 2013). Generating food-grade particles for stabilizing emulsions has been of a recent interest because Pickering-type emulsions are generally more stable against coalescence and Ostwald ripening, and have the potential to enhance oxidative stability (Aveyard, Binks, and Clint 2003, Binks 2002, Kargar et al. 2012). The reason for their high stability is due to the particles preventing interfacial interaction by volume exclusion; that is, particles adsorbed at the oil–water interface create a physical barrier preventing drop–drop contact. Because Pickering particles are often tens to thousands of nanometres in diameter, this physical layer is quite large in comparison to surfactant molecules (~1 nm) and protein molecules (~5 nm). As in most types of emulsion formulations, the size of emulsions droplets in Pickering emulsions is governed either by the amount of emulsifier relative to the dispersed phase or by the intensity of the emulsification device (Chevalier and Bolzinger 2013, Tcholakova, Denkov, and Lips 2008). Generally droplet size decreases with increasing particle concentration (at fixed dispersed phase content) to a certain extent after which it levels out, and excess particles begin to accumulate in one of the phases. Beyond this level of particle-to-dispersed phase ratio, a further reduction in droplet size can only be achieved by improving the emulsification conditions (Frelichowska, Bolzinger, and Chevalier 2010). However, in Pickering emulsions, the size of the stabilizing particles ultimately limits the size of the emulsions drops that can be formed. Generating small particles is a common objective of many studies, as it reduces the amount required (milligram of particles per milliliter of dispersed phase) to stabilize a given emulsion droplet interface, that is, emulsification capacity. Requiring a high concentration of emulsifier for creating a stable emulsion is generally undesirable from a formulation viewpoint, as emulsifiers can be expensive, have a negative impact on the overall taste, or have their concentration limited by regulatory aspects. On the other hand, if the particles themselves are food components (i.e., starch granules, egg yolk granules, or fat crystals), this may not necessarily be the case, as the particles themselves contribute to the nutritional profile, the perceived product quality, and/or the sensory properties of the formulation in a positive way. Furthermore, for Pickering emulsions, achieving a small droplet size may not be as crucial as in conventional emulsion formulations, where small droplet size is often correlated with improved emulsion stability, as creaming is often a precursor to coalescence. In contrast, Pickering emulsions droplets of large size (even on the millimeter scale) if successfully formed, seem to be stable to coalescence over extended periods of time (Aveyard, Binks, and Clint 2003, Binks 2007, Laredj-Bourezg et al. 2012, Marku

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et al. 2012, Timgren et al. 2013). However, large droplets are also susceptible to creaming, which is a major drawback of this type of emulsion. Some examples of how gravimetric separation is reduced in Pickering formulations include the careful AQ 30 choice of particle size and the amount to generate droplets that are neutral buoyancy (Rayner, Sjöö et al. 2012), or in cases where particle properties create weak gel (Dickinson 2010, 2012), which is further improved in cases having high level of dispersed phase (drops + particles), that is, space-filling conditions. Particles as emulsion stabilizers have enabled formulators in the reduction or the removal of low-molecular weight surfactants, which, in some cases, have a limit on the amount that can be used in food emulsion recipes. Due to the relatively large size of the stabilizing particles (in comparison to molecular surfactants and polymers emulsifiers), they make up a significant volume fraction of the emulsion formulation per se, and once adsorbed at the oil–water interface, they are essentially thermodynamically trapped there (Aveyard, Binks, and Clint 2003). Furthermore, if they possess a sufficient level hydrophobicity, it can also lead to particle aggregation. Because if it is energetically favorable for a particle to adsorb at the oil–water interface, it is also favorable for the particles to adsorb to each other and tend to exist in a state of weak aggregation in the continuous phase (Dickinson 2013), and the particles may form a network or a gel-like structure with increased viscoelastic moduli (Dickinson 2013).This can also result in the emulsion having a yield stress, which, even if relatively small, will assist in preventing creaming, drop–drop contact, and coalescence under quiescent storage conditions (Dickinson 2012). This may also prove to be the reason why particle-stabilized emulsions, even when the surface coverage of particles at the oil–water interface is much less than a closely packed monolayer, can remain stable over several years of storage (Timgren et al. 2013). Rheological properties resulting from particle–particle interactions may also have the added benefit of reducing the need for additional thickeners and viscosity modifiers in particle-stabilized formulations (Dickinson 2013).

3.5 evaluatIon oF emulsIon FormulatIon and IngredIent PerFormanCe The evaluation and characterization of emulsions and ingredient performance in food formulations can be performed on several different time and length scales. The complexity of these expensive type measurements or characterization methods can vary from using simple visual observations of creaming in a glass container to elaborate neutron scattering experiments. In any case, the method and time window of evaluation should reflect the formulations’ fitness of intended use. For example, diary emulsions as refrigerated products need to be stable over the span of its best before date (due to microbial spoilage), but not necessarily too much beyond that. Furthermore, the emulsions should remain stable when exposed to likely environmental stresses encountered during processing, packaging, storage, and consumption. There are numerous methods to characterize emulsions and the ingredients making up emulsion formulations, all of which cannot be described in detail here. For a more thorough description of characterization method, we recommend McClements’s (2005a) work and references therein. However, in Table 3.7,

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AQ 32

AQ 31

DSC, ultrasonics, and heat-controlled microscopy

Melting point and amount of solid materialf

Crystallinity and polymorphismd

Interfacial rheology can be measured as dilatational deformation (oscillating increase and decrease of surface, for example, a pendant drop) Shearing deformation where the area is constant but the shape is changed (deep-channel surface viscometer) X-ray diffraction is the preferred method for identifying crystalline structure but DSC, and FTIR can be used as complementary techniques

Wilhelm plate, drop volume, Pendent drop

examples of methods

Interfacial rheology that is especially important for biopolymers and proteinsb. Presence of low-molecular emulsifiers often decrease interfacial elasticityc

Surface tension of biopolymers, surfactants and proteinsa

Key Properties (reviews)

taBle 3.7 Characterization of emulsion Ingredient Properties

(Continued)

The use of synchrotron radiation enables analyses on a short time scale and thus kinetic phenomena can be studied. X-ray can also be used to study self-associated structures of lipids and synchrotron radiation can be used to study structures at interfacese One should be aware that the scanning rate used in DSC sometimes are too fast to have the sample in equilibrium and this can affect the measured melting temperature

Pendent drop works well for soluble substances and for oil–water interface. It is also good for measuring dynamic change in surface tension. Wilhelm plate are easier to use for nonsoluble materials The methods listed here are suitable for measurements close to equilibrium Langmuir trough gives both dilatation and shearing deformation

Comments


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AQ 33

There are numerous methods for molecular weight determination; for high-molecular weight molecules, FFFg, size exclusionh, gel electrophoreses, rheology of diluted solutions, and analytical ultracentrifugation can be used. Mass spectroscopy such as MALDI TOFi

examples of methods

Comments Size exclusion and FFF can be linked to light-scattering detectors to get additional information such as shape. FFF is especially well suited for larger polymers and proteins such as starch molecules. MALDI TOF is primarily used to get composition and structural information

Sources: a Drelich, J. et al., Encyclopedia of Surface and Colloid Science, Marcel Dekker, 2002. Pelipenko, J. et al., Acta Pharm., 62, 2,123–140, 2012. c Maldonado-Valderrama, J. and Patino, J.M.R., Curr. Opin. Colloid Interface Sci., 15, 4, 271–282, 2010. d Sato, K., Chem. Eng. Sci., 56, 2255–2265, 2001. e Cristofolini, L., Curr. Opin. Colloid Interface Sci., 19, 3, 228–241, 2014. f McClements, D.J., Food Emulsions Principles, Practices, and Techniques, CRC Press, Boca Raton, FL, 2005a. g Nilsson, L., Food Hydrocolloids, 30, 1, 1–11, 2013. h Hagel, L., Protein Purification, John Wiley & Sons, Inc., 2011. i Harvey, D.J., Mass Spectrom. Rev., 30, 1:1–100, 2011. DSC, differential scanning calorimetry; FFF, field flow fractionation; FTIR, Fourier transform infrared spectroscopy.

b

Molecular weight/mass distribution is critical properties especially for proteins and polysaccharides. Information on branching and shape can also be important to understand biopolymers

Key Properties (reviews)

taBle 3.7 (Continued ) Characterization of emulsion Ingredient Properties


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we summarize some of the more important ingredient properties investigated and their possible characterization. On a microstructural level, the emulsion droplet size distribution is perhaps the most central quantifying measure in emulsion science, as emulsion characteristics and performance are highly dependent on the droplet size distribution. There are numerous methods to assess particle size distributions in emulsions, including direct droplet measurement by microscopy (light, confocal, electron, etc.), automated particle counters (i.e., Coulter counter), light scattering (i.e., Malvern Mastersizer), dynamic light scattering, diffusional wave spectroscopy, nuclear magnetic resonance, and sedimentation or centrifugation (Walstra 2005). These techniques vary with respect to the range of sizes covered, measurement principles, degree of sample preparation and dilution, as well as physical limitations of the methods. The interested reader is directed to comprehensive and critical reviews on emulsion characterization techniques for more details (Dalgleish 2003, Dickinson 2013, McClements 2005b, 2007, Sherman 1995, Walstra 2005). From a consumers’ perspective, aside from taste, the two most important emulsion properties are physical appearance (creaming, sedimentation, phase separation, graininess, etc.) and texture (mouthfeel, viscosity, etc.). These macroscopic properties are a result of microstructure and molecular interactions within the emulsion formulation (Corredig and Alexander 2008) and are closely related to the ability of the formulation to stabilize and maintain the stability of the emulsion droplets, as well as the rheology of the resulting dispersion. For this reason, in the development of food-based emulsions, the effectiveness of emulsifiers and the evaluation of texture are often studied. From the material presented in the earlier sections of this chapter, it is apparent that there is a large variety of emulsifiers that can be used in formulating food-based emulsions. The fundamental performance of an emulsifier can be described by the emulsifying capacity (EC) as the minimum amount required to produce a stable emulsion and its ability to produce small drops during homogenization. The emulsion stability index (ESI) is a measure of the ability of an emulsifier to prevent droplets from aggregating, flocculating, and coalescing over time.

3.5.1 eMulSiFicatioN caPacity When formulating a food emulsion, it is useful to know the minimum amount of emulsifier required to create a stable emulsion. The EC of a water-soluble emulsifier is defined as the maximum amount of oil that can be dispersed in an aqueous solution containing a specific amount of emulsifier without the emulsion breaking down or inverting into a W/O emulsion (Sherman 1995). The EC of an oil-soluble emulsifier is determined in a similar way, except that water is added to the oil phase and it would invert into an O/W emulsion. This test is practically carried out by placing the continuous phase containing the emulsifier into a vessel with a highspeed mixer (e.g., ultra-turrax) and carefully titrating the dispersed phase into the vessel. Phase inversion can be monitored via electrical conductivity (Allouche et al. 2004, Gu et al. 2000) or optically using a colorimeter or spectrophotometer in reflectance mode (McClements 2002) or by adding a dye to one of the phases (Timgren et al. 2013). The larger the volume of oil that can be added before phase inversion,

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the higher will be the EC of the emulsifier. This test is widely used due to its relative simplicity, but has several drawbacks that prevent its application as a standardized procedure (Dalgleish 2003, McClements 2005b, 2007, Sherman 1995). The main problem identified with the procedure is that the amount of emulsifier required to stabilize an emulsion depends on the oil–water interfacial area rather than the oilvolume fraction, thus EC depends on the size of the droplets produced during agitation. This in turn is highly sensitive to the type of mixing/homogenization apparatus used, its energy intensity (see Chapter 1), the volume of emulsion being processed, the viscosity of the oil phase, and the temperature of processing. As such, EC should be regarded as a qualitative index that depends on the specific setup and conditions used to carry out the test that can be used to compare different emulsifiers tested under the same conditions. An alternative way of estimating the amount of emulsifier required to form an emulsion that takes into account the amount of interfacial area generated is via the surface load, Γs, which corresponds to the mass of emulsifier required to stabilize a unit area of droplet surface (Dickinson 1992). This is determined by first generating a stable emulsion by homogenizing a known amount of oil, water, and emulsifier. Then the amount of emulsifier adsorbed at the oil–water interface is determined via a mass balance of the emulsifier; that is, the amount adsorbed at the oil–water interface, which is found by considering the initial concentration of emulsifier in the continuous phase, Cini , minus the amount remaining in the continuous phase after emulsification, Cend . This is carried out experimentally by carefully separating the droplets from the continuous phase by centrifugation or filtration and determining the remaining concentration of the emulsifier (Tcholakova et al. 2002). The interfacial area to which the emulsifiers are adsorbed is found by measuring the specific surface area of the emulsion by either microscopy or an automated particle size analyzer. The specific surface area, S, is determined from the surface mean diameter, d32 d32

∑Nd = ∑Nd i

i

3 i i

2 i i

(3.2)

where: Ni is the number of drops with diameter di Because the specific surface area, S, is the sum of all the surface areas of all drops divided by the sum of all their volumes (m2/m3), we can calculate S from d32: S=

4π(d32 /2)2 6 = 3 (4/3)π(d32 /2) d32

(3.3)

Now, including this into the mass balance or the emulsifier over the continuous phase and interface, we get ΓS =

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Vcts (Cini − Cend ) (1 − φ) d32 = (Cini − Cend ) φ Vdisp S 6

(3.4)

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Here, Vcts and Vdisp are the volumes of the continuous and dispersed phases, respectively, and ϕ is the volume fraction of dispersed phase, that is, φ=

Vdisp Vcts + Vdisp

(3.5)

Typically, the values of ΓS for molecular food emulsifiers is around a few milligram per square meter, but is much larger for particles, hundreds to thousands milligram per square meter, as ΓS is also directly related to the thickness of the interfacial layer. These estimates of surface load values provide some knowledge with respect to the minimum amount of emulsifier that is required to make an emulsion having droplets of a given size and the dispersed phase fraction. However, in practice, an excess of emulsifier is often used, as not all emulsifiers are ideally adsorbed into the oil–water interface during homogenization due to kinetic limitations, as well as due to the partitioning equilibrium conditions between the interface and the continuous phase. Furthermore, the surface load of some types of emulsifiers is also sensitive to formulation conditions such as ionic strength, pH, the concentration of macromolecules, temperature, and so on.

3.5.2

eMulSioN Stability iNdex

The emulsification capacity, presented in Section 3.5.1, gives information on the ability to create an emulsion with a given formulation, but does not necessarily take into account the evolution of emulsion stability over time. One expression of the emulsion stability over time is the ESI, which is based on particle size measurements performed at given time intervals and is defined as ESI =

d( 0 )t d( t ) − d( 0 )

(3.6)

where: d( 0 ) is the initial mean droplet diameter of the emulsion d( t ) is the mean droplet diameter measured after a storage time, t (McClements 2005b) Some of the main strengths of this method include that the mean droplet diameter can be readily determined in analytical instruments, the evolution of particle size microstructure is often a precursor to quality deterioration on the macrostructure (creaming and phase separation, etc.) and can re-repeated over relevant time scale for the shelf life of the product. A similar index is also sometimes used that compares the specific surface area of the emulsions rather than just mean droplet size as a measure of how much coalescence has taken place. This surface coalescence index (SCI) is more sensitive to the fate of smaller drops (as they have a relatively larger surface are to volume ratio) and can be calculated by SCI =

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S( 0 ) − S( t ) S( 0 )

(3.7)

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where: S( 0 ) is the initial specific surface area of the emulsion S( t ) is new specific surface area of the emulsion measured after a storage time, t (Anton, Beaumal, and Gandemer 2000) S is calculated directly from 6/d32 It should be noted that there is no compelling evidence that a single index such as EC, ESI, or SCI can be used to ultimately compare the effectiveness or the stability of emulsifiers if they have been produced under different homogenization conditions. Still, these indices are very useful when comparing a series of emulsifiers of emulsion formulations produced under standardized conditions or in situations when the influence of specific changes are being made to the formulation, processing conditions, or functionality of a specific ingredient that is being studied (McClements 2005b).

3.5.3 aSSeSSiNg gravitatioNal SeParatioN—creaMiNg iNdex Gravitational separation of emulsions is one of the most common instability mechanisms encountered in food and personal care products, thus formulators need to know at what degree creaming or sedimentation is likely to occur over the shelf life of a relevant product. Due to the fact that emulsion droplets in the context of food emulsions typically never have the same density as the continuous phase and are large enough for the buoyant forces to overcome viscous resistance and Brownian motion, they allow gravitational separation to be observed on a relevant time scale. As most edible oils at room temperature have a lower density than aqueous solutions, oil droplets in O/W emulsions will tend to rise to the top of the container in a process referred to as creaming, leaving the depleted layer by an emulsion drop at the bottom of the container, often referred to as serum. These terms likely originate from the prevalence of diary emulsions. For W/O emulsions, the sedimentation of water droplets is observed, although generally at a much slower rate due to the higher viscosity of the oil. However, the opposite can be observed, where the sedimentation of oil droplet can occur if fat crystals or other weighing agents are added to the oil phase, or in some cases, in particle-stabilized emulsions, where a higher density of the stabilizing particle layer increases the overall density of the droplet causing them to settle (Rayner, Timgren et al. 2012). The rate at which a single spherical droplet or particle will cream (or settle) in a Newtonian fluid can be predicted by the Stokes velocity: υStokes = −

AQ 34

2gr 2 (ρ2 − ρ1 ) 9η1

(3.8)

where: g is acceleration due to gravity r is the particle radius ρ1 and ρ2 are the densities of the continuous and dispersed phases, respectively η1 is the continuous phase viscosity

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The settling or creaming rate of drops and particles indicated by Stokes equation is somewhat idealized, as in reality, emulsions drops are not all the same size and will be interacting during creaming or settling. Furthermore, Stokes law is mainly applicable at low concentrations of the dispersed phase. However, Stokes equation does provide an illustration of the factors that have the most impact on the gravitational tendency, specifically the viscosity of the fluid surrounding the droplets, their relative density, and, to a large degree, the droplet size due to the exponent. For example, an oil droplet creams at a rate of 0.1 mm/day if its diameter is 0.1 µm, and will cream at a rate of 10 mm/day if the diameter is 1 µm, if all other conditions remain constant. In many practical situations, these conditions are not constant and are more complex; for example, there is often an increase in the effective particle size during creaming due to coalescence, flocculation, or Ostwald ripening (see Chapter 2), which results in Stokes under predicting the rate AQ 35 of gravitational separation (McClements 2007). Therefore, it is often more practical to directly quantify gravitational separation of the emulsions during storage. The extent of creaming or sedimentation in an emulsion can be monitored by visual observation. This method is cheap and straightforward, only requiring the emulsions to be stored in an appropriate environment in clear glass vials or test tubes. The layer formed by creamed emulsion droplets can be readily seen, and often the serum layer is transparent or optically distinct to such a degree that its height can be determined, or its volume estimated. The emulsion index (EI) is a measure of the volume of an emulsion layer formed relative to the total volume given by the following equation, with the volumes defined in Figure 3.3: EI =

Vemuls Vtotal

(3.9) AQ 36

The relative heights of the creamy layer is also used to define the creaming index (CI): CI =

Vemuls Vtotal Vserum

EI =

Vemuls Vtotal

Hserum × 100% Hemuls

(3.10)

Hrel oil

Released oil layer

Hemuls

Creamed emulsion Serum layer

Hserum

CI =

Hserum Hemuls

× 100%

FIgure 3.3 (a) Test tube showing creamed emulsions as defined in EI; (b) schematic test AQ 37 tube showing layers as defined in CI.

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Assuming that there is no significant coalescence or creation of an oil later (called oiling-off, schematically illustrated in Figure 3.3b), the CI will start at zero and will increase until all the emulsion drops are packed into the creamy layer, after which the CI index reached a final value. The height of the final creamy layer depends on the volume fraction of the oil, ϕ, and the maximum packing parameter droplets, which is approximately P ~ 0.6, for random closed packing (McClements 2007). φ  CI final =  1 −  × 100% P  

(3.11)

Knowing the expected CI final can be relevant if emulsions with different dispersed phase volumes are to be compared, as the more dispersed phases, the lesser will be the serum. This concept had also been extended to adjust for the fact that particles, and especially the larger food-based ones used in stabilizing Pickering emulsions, also contribute to the amount of dispersed phase observed. The total amount of nonseparated emulsion can be expressed as the relative occluding volume (ROV). ROV =

Vemuls Vdisp + Vparticles

(3.12)

where: Vemuls is the volume of the observed emulsion (i.e., the nonclear fraction) after emulsification Vdisp is the known volume of the added dispersed phase Vparticles is the known volume occupied by the added particle stabilizers

1.1

6

1.0

5

0.9

4 ROV

Emulsion index

In a completely phase-separated system, ROV equals to 1; that is, there is no increase in the emulsion layer beyond that of its constituent phases. Figure 3.4 illustrates the differences in EI and ROV for starch granule-stabilized emulsions

0.8

3

0.7

2

0.6

1

0.5

0.1

1 Storage time (weeks) 12.5% oil

10 16.6% oil

0

0.1 25% oil

1 Storage time (weeks)

10

33.2% oil

FIgure 3.4 EI and ROV of quinoa starch granule-stabilized oil-in-water emulsions. (Data from Timgren, A. et al., Procedia Food Sci., 1, 95–103, 2011.)

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with varying dispersed phase fraction (oil content 12.5%–33.2%), but at constant starch-to-oil ratio of 214 mg/mL oil. Here, we can see that although the EI increases as expected with oil content, the ROV is higher for the less tightly packed systems. It should be noted that in this system, there was no significant change in droplet size over time or between formulations with different dispersed phase fractions. Although the use of digital camera has greatly simplified the analysis of visual observations of gravitational separations, there are several limitations to the visual inspection of gravitational separation. It is often difficult to distinguish between the layers in creaming/settling emulsions by the visual observation of a glass vial or a test tube. One means to overcome this limitation is using an apparatus that scans the height of the glass tube with a monochromatic beam of light near infrared part of the spectrum while monitoring the amount of scattered and transmitted light. This can give an improved accuracy with respect to the boundaries between the creamed and serum layers, as well as the possibility to quantify the concentration of emulsion drops as a function of height. Some modern commercial instruments have also implemented multiple light-scattering techniques (e.g., Turbiscan Lab) that enable the measurement of concentrated emulsions and the measurement of both the particle size and phase thickness continuously over time (Mengual et al. 1999), detecting the creaming long before it is visible to the naked eye. The other major limitation of the gravitational separation analysis is that it takes a significantly long time to monitor instability that may occur after weeks of storage and that the storage conditions in reality are not necessarily those found in a controlled laboratory environment. This can be overcome in two ways: (1) by AQ 38 increasing the gravitational field where the droplets cream/settle to accelerate storage and (2) by exposing the emulsions to environmental stresses that may trigger instability.

3.5.4

accelerated aNd eNviroNMeNtal StreSS teStS

The rate of gravitational separation and droplet coalescence can be accelerated by the centrifugation of emulsions at a fixed speed for a certain length of time (Sherman 1995). After which, the separation is monitored using the same means as in a normal storage trial (visual observation, digital imaging, light scattering etc.). Alternatively, there are commercial analytical instruments that both scan and centrifuge samples to gain further information (i.e., Lumisizer). However, precautionary measures must be taken while using this approach, especially if there is a difference in the rate of droplet size evolution between the normally stored and accelerated samples. For example, if the droplet size is changing due to coalescence or Ostwald ripening, which in turn affects the rate of gravitational separation, then these may or may not be reflected by merely increasing the gravitational field. Furthermore, if the emulsion has a complex rheology (i.e., not just a Newtonian continuous phase), increasing the gravitational field may over exaggerate the separation. For example, if the emulsion has a weak gel structure (a finite yield stress) that can be overcome in the centrifuge (but not under normal storage), the emulsion will be forced to separate in conditions when it normally would not, if it is left to settle over the normal time frame. For these reasons, it is imperative to compare the results of accelerated creaming tests with those made using long-term normal

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storage to validate the methods before routinely using an accelerated test for a given type of formulation (McClements 2007). In addition to gravitational separation, coalescence can also be accelerated using centrifugal methods, as these methods essentially force droplets together. In this case, the coalescence stability is determined by measuring the change in droplet size distribution and/or the extent of oiling-off after the emulsion has been centrifuged for a specific speed and time. Here, the coalescence stability is quantified in terms of the maximum centrifugation force that the emulsion can tolerate before a change in the microstructure is observed (droplet size and or oiling-off). The particle size distribution of the emulsion droplets can be measured before and after centrifugation and the data can be represented as either the entire distribution or ESI. Alternatively, Tcholakova et al. (2002, 2006) have developed a centrifugal method that can provide quantitative data about O/W emulsions stability to coalescence. An emulsion is added to a transparent centrifuge tube and is loaded into a centrifuge. This emulsion is then subjected to a centrifugal acceleration at an appropriate intensity and time. The oils droplets will tend to move toward the axis of rotation (z direction in Figure 3.5) due to their relatively lower density. This is the case for almost all food emulsions. Initially, the emulsion droplets form a AQ 39 creamy layer where they are forced into close proximity but keep their initial form. As the centrifugal force is increased, they are pressed tighter and tighter together and eventually the interfacial layer surrounding and stabilizing the droplets will AQ 40 rupture, releasing a layer of cream on the top of the emulsion column in the tube (see Figure 3.5). The critical pressure that the emulsion can withstand before the oil is released CR when the film ruptures is described as a critical osmotic pressure, POSM . For a full derivation, refer to Tcholakova et al. (2002, 2006) and references therein. Hc CR OSM

P

∫

= ∆ρgk φ( z)dz = ∆ρgk 0

ζ0

0

ζ1

ω

(Voil tot − Voil rel ) ATT

ζ2

(3.13) ζ

Cream Oil

Hrel

Z

Serum

HC

0

FIgure 3.5 Schematic image of the thickness of the oil layer released during a forced coalescence test.

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where: Δρ is the density difference between the oil and aqueous phases gk is the centrifugal acceleration φ(z) is the local volume fraction of oil along the z direction along the centrifugal field Voil tot is the total volume of oil in the emulsion Voil rel is the volume of oil released ATT is the interior cross-sectional area of the test tube containing the emulsion After centrifugation, the height of the creamy layer, HC and oil released Hoil rel can CR be easily measured, where Hoil rel = Voil rel / ATT . POSM may be readily calculated from experimental data, if one assumes that the centrifugal field is homogenous through the column of creamed emulsion, HC, and can be represented by the square of the angular frequency, ω times the mean distance of the emulsion layer from the axis of rotation, ζ: gk ≈

ω2 (ζ1 + ζ 2 ) = constant 2

(3.14)

Tcholakova et al. (2002, 2006) have proven that this is a reasonable assumption, as more precise calculations that take into the account the spatial variation of the field compared to using a mean distance gives a relatively small difference in the result, and the imposed error is within the experimental accuracy of the measurements. This method has been demonstrated to be particularly useful for monitoring the coalescence stability of different types of protein-stabilized emulsion with various compositions (Denkov, Tcholakova, and Ivanov 2006, Tcholakova et al. 2002, 2003, 2006). In addition to centrifugation, other types of accelerated coalescence tests include subjecting the emulsions to other types stress such as mechanical forces (extended homogenization, pumping, vibration, shearing, extruding, whipping, shaking, and mixing) environmental stresses (freeze-thaw cycling, thermal processing, and heat abuse), as well as compositional stresses (drying causing a change in solute composition, changes in pH and ionic strength, etc.). All of which with the purpose of emulating some sort of typical event, environmental stress, or process that the emulsion under consideration should withstand during processing, transport, shelf life, and use. The formulation in general—and the performance of its emulsifier in particular—is evaluated in a variety of conditions depending on its application to establish a design space, in which a particular emulsifier is expected to successfully function. Examples of test methods and experimental conditions/protocols can be found in McClements (2007) and references therein.

3.5.5 evaluatioN oF texture As pointed out previously, the rheology of an emulsion is important not only to the taste, texture, and mouthfeel (Le Révérend et al. 2010) but also for the creaming stability and coalescence (Tadros 2004). In this chapter, we only give a short overview of the rheology of an emulsion; for a more extensive review, we recommend

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Derkach (2009), Tabilo-Munizaga and Barbosa-Cánovas (2005), and Tadros (2004)). Texture can be evaluated using techniques such as rheological and texture analyzers. Although texture analyzers can give a quick information and comparison of systems, the information gained especially from oscillating rheology gives more knowledge. Emulsions droplets can, from a rheological viewpoint, in most cases be considered as hard spheres. The rheology of such systems are strongly dependent on the concentration at low concentrations; where there are no droplet–droplet interactions, the system will follow Einstein’s (1906) law for hard spheres. However, at moderate concentrations, the interactions between the droplets will affect the rheology and in these regimes, the rheology can be described by semiempirical equations such as the Krieger–Dougherty equation (Krieger 1972).  φ  η = η0  1 − e   φc 

−[( 5 / 2 ) φc ]

(3.15)

where: η0 is the viscosity of the continuous phase φc is the is the volume fraction at random close packing of spheres φe is the volume fraction of oil in the emulsion. Addition of rheological modifiers will further complicate the rheological properties of emulsions; for emulsions in the concentrated regime or emulsions containing viscosity modifiers, the system will often become viscoelastic. For such systems, measuring the rheology using oscillating measurements can further elucidate the character of the emulsion. This allows for the use of small strains and stresses on the material leading to measurements that does not destroy the structure of the samples. The sample is subjected to a sinusoidal shear deformation and the resultant stress response is measured. The frequency and the strain/stress on the sample can normally be varied and the response is divided into a viscous component G″ (loss module) and an elastic component G′ (storage module). Such measurements can give information of the viscoelastic character of the emulsion and, for example, describe if the emulsion has a gel-like character or not. A characteristic for gels is that G′ is higher than G″. Oscillating measurements can, for example, be used to follow the buildup of a gel during heating or cooling. In oscillating measurements, one can either change the frequency of the oscillation or the strain/stress. Figure 3.6 shows typical frequency measurements of Pickering emulsions for weak and strong gels and Figure 3.7 shows a strain curves for the same samples. The frequency curve gives information on how the emulsions react to stress during different time frames (time is proportional to 1/frequency). Stress or strain test are often used for gel-like emulsions and preferably measured at a frequency where the rheological properties of the gel are in a linear region. Typically, in strain tests, the strain is increased until the structure of the emulsions is broken and the gel starts to flow, which is shown as a rapid decease of G′. As can be seen in Figures 3.6 and 3.7, even if the gel is stiffer (high G′) and has a smaller linear gel region (frequency), it might still flow at lower strains.

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G′ and G′′ (Pa)

10000 1000 100 10 1 0

0.1

1 Frequency, f (Hz)

10

FIgure 3.6 Elastic modulus (G′, in Pa) and viscous modulus (G″, in Pa) versus frequency (f, in Hz) for Pickering emulsions with 19% Miglyol oil-in-water (O/W) emulsion stabilized by chitosan G″(closed squares), G′ (open squares) with 55% Miglyol O/W emulsion stabilized AQ 41 by OSA-modified quinoa starch G″ (closed circles), G′ (open circles).

10000

G′ and G′′ (Pa)

1000 100 10 1 0.1 0.01 1.00E − 01

Complex shear strain

1.00E + 00

FIgure 3.7 Elastic modulus (G′, in Pa) and viscous modulus (G″, in Pa, versus complex shear strain) for Pickering emulsions with 19% Miglyol O/W emulsion stabilized by chitosan G″ (closed squares), G′ (open squares) with 55% Miglyol O/W emulsion stabilized by OSAmodified quinoa starch G″ (closed circles), G′ (open circles).

The rheology of emulsions is especially important to avoid creaming in nonspace filled systems. Thus, it is of interest to estimate the viscosity needed to arrest creaming. The bouncy force on a droplet will be F = VΔρg stress is F/A thus the stress AQ 42 exerted by the droplet will be RΔρg/3. The stress asserted by normal emulsions droplets will thus be normally below 0.1 Pa. Therefore, to arrest creaming, gelling

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polymers only have to withstand the stress asserted by the droplet. Furthermore, this also means that to predict the resistance of the system to creaming, the rheology has to be measured at a low stress, which can be obtained by constant stress or creep measurements. However, normally we would like the system to flow when handled; therefore, a good rheological modifier should yield at higher stresses. In this respect, stress curves are more informative.

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Author Query Sheet Chapter No: 3 Query No. Queries Response AQ 1 “omega three” and “omega six” have been set as “omega-3” and “omega-6,” respectively. Please check and confirm whether this is okay. AQ 2 Can “texturizing macromolecules” be changed to “textured macromolecules” here? Please confirm. AQ 3 “Creaming/sedimentation occurs for macroscopic emulsions . . . difference of the oil” has been changed as “For macroscopic emulsions, creaming/sedimentation occurs due to the density difference between the oil and water fraction . . . ” Please confirm whether OK. AQ 4 In the sentence “This can mainly be . . . ” “that either through a steric barrier or electrostatic repulsion hinders drop-drop contact and thus the process of coalescence” has been changed as “that hinders drop–drop contact through either a steric barrier or an electrostatic repulsion and thus resulting in the process of coalescence.” Please confirm whether OK. AQ 5 Please clarify “and decay with the distance between the . . . ” AQ 6 Please provide an expanded form of DLVO, if available. AQ 7 Please advise whether the sentence “Nonionic emulsifiers both low molecular and polymers . . . ” should read as “Both low-molecular nonionic emulsifiers and polymers might . . . ” AQ 8 Please check whether “degree of specific binding” could be changed to “degree of specific binding capacity” AQ 9 Reference citation “Waraho et al.” has been changed to “Waraho, McClements, and Decker (2011)” with respect to reference list. Please check if this is okay. AQ 10 The sentence “Waraho et al. reviews the oxidation . . . ” has been changed as “Waraho, McClements, and Decker (2011) review the oxidation of lipids in emulsions and point out that there will be a difference in the oxidation process in pure oil when compared with one in an emulsion.” Please confirm whether such changes preserve the intended meaning of the sentence.

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AQ 11 AQ 12

AQ 13

AQ 14 AQ 15

AQ 16 AQ 17

AQ 18 AQ 19 AQ 20 AQ 21 AQ 22 AQ 23 AQ 24

AQ 25

AQ 26

AQ 27 AQ 28

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Perhaps explain EDTA? Please clarify “again highlighting the importance to know the function of the specific additive at the conditions used for each food product.” Please confirm whether “charged small emulsifiers” could be changed to “charged small-molecule emulsifiers.” Perhaps explain CFC? Please clarify the sense in “these can affect the stability of the oil when it comes to oxidation but also when it comes to emulsion stability.” Please provide publisher location for table source Agriculture (2014.) Please provide the organization name, year of publication, article title and access date for URL sources “http://ndb.nal.usda.gov/ndb/foods” and http://www.engineeringtoolbox.com/oil-meltingpoints-d_1088.html. Please check the changes made to the caption of the figure and confirm whether they are fine. Please clarify L2 phases. Please provide a page number for the quote. Perhaps explain HLB ratio? Please advise whether the numbers in this column could be set as E470/172.863 instead. Explain IP in this table. Reference citation “Bosa and Vlieta 2001” has been changed to “Bosa and van Vlieta 2001” with respect to reference list. Please check if this is okay. Reference citation “Rodr ́ıguez Patino, Navarro Garc ́ıa, and Rodr ́ıguez Niño 2001” has been changed to “Rodríguez, García, and Niño 2001” with respect to reference list. Please check if this is okay. “gum arabica” has been changed to “gum arabic” throughout the chapter. Please check and confirm if it is okay as set. Please clarify “the viscosity becomes shear thinning”. I have changed “the formed complexes” to “complexes so formed” and “other properties” to “additional properties”. Please confirm whether OK

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AQ 29 AQ 30 AQ 31

AQ 32 AQ 33 AQ 34 AQ 35 AQ 36 AQ 37 AQ 38

AQ 39 AQ 40 AQ 41 AQ 42 AQ 43 AQ 44 AQ 45 AQ 46 AQ 47

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Kindly clarify the sense of the sentence “Polysaccharide–protein-stabilized . . . ” Please confirm whether the change to “neutral buoyancy” from “buoyance neutral.” Reference citations in table entries have been changed as table footnote sources. Please check whether this is okay. Please check the expansion provided for FTIR in the table footnote. Please provide the publisher location for source references Drelich et al. (2002) and Hagel (2011). Please confirm whether “υ” in υStokes could be changed to “v.” Please clarify “results in Stokes under . . . separation” Should the formula include “× 100”? Please confirm. Should the formula for “EI” be multiplied by 100, too, in the figure? Please clarify. I have changed “The two ways to overcome this are” to “This can be overcome in two ways” and have altered “accelerating storage by increasing . . . ” to “by increasing … the droplets cream/settle to accelerate storage.” Please confirm whether such changes preserve the intended meaning of the sentence. Please clarify “where they are forced into close proximity” Please check whether the inclusion of “cream” after “a layer of” is okay. Perhaps explain OSA here. Please clarify the sense of the sentence “The bouncy force on a droplet . . . ” Please provide the publisher location for reference Agriculture, U.S.D.O. (2014). Please provide the editor name, page number, publisher location for reference Dalgleish (2003). Please provide the proceedings held date for reference Davies (1957). Please provide the congress held date for reference Denkov et al. (2006). Please provide the editor name and page numbers for reference Drelich et al. (2002).

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AQ 48 AQ 49 AQ 50

AQ 51 AQ 52

AQ 53 AQ 54 AQ 55

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Please clarify whether it should be “MSc Thesis” or “MSc Dissertation” here. Please provide publisher location for reference Hagel (2011). Please provide the organization name, year of publication, article title and access date for URLs “http://ndb.nal.usda.gov/ndb/foods” and http://www .engineeringtoolbox.com/oil-melting-points-d_1088 .html. Please provide the publisher location and page numbers for reference Kasapis (2000). Please provide the editor names, page number, publisher and its location for reference Magnusson and Nilsson (2013). Please provide the editor names, page numbers for reference McClements (2005a). Please provide the chapter title and page numbers for reference McClements (2005b). Please provide publisher location for reference Walstra (2005).

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