Experimental Measurement of Gravitational Separation

To theoretically predict the rate at which gravitational separation occurs in an emulsion, it is necessary to have information about the densities of the dispersed and continuous phases, the droplet size distribution, and the rheological properties of the continuous phase. The density of the liquids can be measured using a variety of techniques, including density bottles, hydrometers, and oscillating U-tube density meters (Pomeranz and Meloan 1994). The droplet size distribution can be measured...

Measurement Of Surface And Interfacial Tensions

By definition, surface tension is measured at a gas-fluid interface (e.g., air-water or oil-water), while interfacial tension is measured at a fluid-fluid interface (e.g., oil-water). These quantities are measured using instruments called surface or interfacial tensiometers, respectively (Couper 1993). Tensiometers can be used to provide valuable information about the characteristics of surfaces and interfaces and about the properties of emulsifiers in solution, such as the surface excess...

Wilhelmy Plate Method

The Wilhelmy plate method is normally used to determine the static surface or interfacial tensions of liquids (Hiemenz 1986, Couper 1993). Nevertheless, it can also be used to monitor adsorption kinetics provided that the accumulation of the emulsifier at the surface is slow compared to the measurement time (Dickinson 1992). The apparatus consists of a vessel that contains the liquid being analyzed and a plate which is attached to a sensitive force-measuring device (Figure 5.14). The vessel is...

Introduction

Fresh milk is an example of a naturally occurring emulsion that can be consumed directly by human beings (Swaisgood 1996). In practice, however, most milk is subjected to a number of processing operations prior to consumption in order to ensure its safety, to extend its shelf life, and to create new products. Processing operations, such as homogenization, pasteurization, whipping, and churning, are responsible for the wide range of properties exhibited by dairy products (e.g., homogenized milk,...

Hydration Interactions

Hydration interactions arise from the structuring of water molecules around dipolar and ionic groups (in contrast to hydrophobic interactions, which arise from the structuring of water around nonpolar groups). Most food emulsifiers naturally have dipolar or ionic groups that are hydrated (e.g., -OH, -COO-, and -NH+), and some are also capable of binding hydrated ions (e.g., -COO- + Na+ -COO- Na+). As two droplets approach each other, the bonds between the polar groups and the water molecules in...

General Features of Depletion Interactions

The strength of the interaction increases as the concentration of colloidal particles in the continuous phase increases (at constant rc). 2. The strength of the interaction increases as the size of the droplets in an emulsion increases. 3. The strength of the interaction increases as the size of the colloidal particles decreases (at constant c), because osmotic effects are influenced more by the number of particles involved (which increases with decreasing rc) than by the volume of the...

Properties Of Curved Interfaces

The majority of surfaces or interfaces found in food emulsions are curved rather than planar. The curvature of an interface alters its characteristics in a number of ways. The interfacial tension tends to cause an emulsion droplet to shrink in size so as to reduce the unfavorable contact area between the oil and water phases (Hunter 1986, Everett 1988). As the droplet shrinks, there is an increase in its internal pressure because of the compression of the water molecules. Eventually, an...

Higher Order Interactions

When one consults the literature dealing with molecular interactions in foods and other biological systems, one often comes across the terms hydrogen bonding and hydrophobic interactions (Bergethon and Simons 1990, Baianu 1992, Fennema 1996a). In reality, these terms are a shorthand way of describing certain combinations of interactions which occur between specific chemical groups commonly found in food molecules. Both of these higher order interactions consist of contributions from various...

The Structural Organization Of Molecules In Liquids

In food emulsions, we are usually concerned with the interactions of large numbers of molecules in a liquid, rather than between a pair of isolated molecules in a vacuum. We must therefore consider the interaction of a molecule with its neighbors and how these interactions determine the overall organization of the molecules within a liquid (Murrell and Boucher 1982, Murrell and Jenkins 1994, Evans and Wennerstrom 1994). The behavior of large numbers of molecules at equilibrium can be described...

Info

FIGURE 2.1 The molecules in food emulsions may adopt a variety of different structural arrangements depending on the nature of their interactions with their neighbors. FIGURE 2.1 The molecules in food emulsions may adopt a variety of different structural arrangements depending on the nature of their interactions with their neighbors. are insignificant at the molecular level because molecular masses are extremely small. Nevertheless, they do affect the behavior of food emulsions at the...

Molecular Interactions

Although food scientists have some control over the final properties of a product, they must work within the physical constraints set by nature (i.e., the characteristics of the individual molecules and the type of interactions that occur between them). There is an increasing awareness within the food industry that the efficient production of foods with improved quality depends on a better understanding of the molecular basis of their bulk physicochemi-cal and organoleptic properties (Baianu...

Hierarchy Of Emulsion Properties

The bulk physicochemical and organoleptic properties of emulsion-based food products are ultimately determined by the concentration, dimensions, interactions, and dynamics of the various types of structural entities present within them (e.g., atoms, molecules, molecular aggregates, crystals, micelles, droplets, air bubbles, and individual phases) (Figure 1.8). The * It should be noted that the continuous phase of an emulsion is also capable of melting or crystallizing, which can have a profound...

Measurement Of Rheological Properties

Food emulsions can exhibit a wide range of different types of rheological behavior, including liquid, solid, plastic, and viscoelastic (Dickinson and Stainsby 1982, Dickinson 1992). Consequently, a variety of instrumental methods have been developed to characterize their rheological properties. Instruments vary according to the type of deformation they apply to the sample (shear, compression, elongation, or some combination), the property they measure, their cost, their sophistication, and...

Molecular Basis Of Interfacial Properties 521 Interfaces Between Two Pure Liquids

The interface that separates the oil and water phases is often assumed to be a planar surface of infinitesimal thickness (Figure 5.1a). This assumption is convenient for many purposes, but it ignores the highly dynamic nature of the interfacial region, as well as the structure and organization of the various types of molecules involved (Figure 5.1b). On the molecular level, the oil and water molecules intermingle with each other over distances of the order of a few molecular diameters (Everett...

Ultrasonic Homogenizers

The possibility of using ultrasound to form emulsions was realized in the early part of this century, and ultrasonic homogenizers have been widely used for this purpose in both industry and research since then (McCarthy 1964, Gopal 1968). This type of homogenizer utilizes high-intensity ultrasonic waves which generate intense shear and pressure gradients within a material that disrupt the droplets mainly due to cavitational effects (Section 6.3.1.2). A number of methods are available for...

Major Factors That Determine Emulsion Rheology

Dispersed-Phase Volume Fraction The viscosity of an emulsion increases with dispersed-phase volume fraction. At low droplet concentrations, this increase is linearly dependent on volume fraction (Equation 8.19), but it becomes steeper at higher concentrations (Equation 8.30). Above a critical dispersed-phase volume fraction ( c), the droplets are packed so closely together that they cannot easily flow past each other, and so the emulsion has gel-like properties. The precise nature of the...

Modification of Biopolymers

A wide variety of natural biopolymers could be used as functional ingredients in foods however, many of these are too difficult to extract economically or only naturally occur in small quantities. For this reason, only those biopolymers which occur in natural abundance, are easy to extract, and are relatively inexpensive tend to be used as food ingredients. Even so, these ingredients often do not have the functional properties required for a particular product, and so there has been a...

Dispersedphase Volume Fraction

The concentration of droplets in an emulsion can be determined using many of the standard analytical methods developed to determine the composition of foods (Nielsen 1994, Pomeranz and Meloan 1994). A variety of solvent-extraction techniques are available for measuring fat content (Min 1994, Pal 1994). The sample to be analyzed is mixed with a nonpolar organic solvent which extracts the oil. The solvent is then physically separated from the aqueous phase, and the oil content is determined by...

In Homogenization What Should Be The Viscosity Ratio

FIGURE 6.4 Dependence of the critical Weber number on the viscosity of the dispersed and continuous phases under simple shear flow conditions. (Adapted from Schubert and Armbruster 1992.) Here, G is the shear rate and nc is the viscosity of the continuous phase. For a given system, it is possible to define a critical Weber number (Wecritical), which is the value of We where the droplets are just stable to disruption. If an emulsion has a Weber number above this critical value (i.e., high shear...

Fats And Oils

Fats and oils are part of a group of compounds known as lipids (Gunstone and Norris 1983, Weiss 1983, Nawar 1996). By definition, a lipid is a compound which is soluble in organic solvents but insoluble or only sparingly soluble in water (Nawar 1996). This group of compounds contains a large number of different types of molecules, including acylglycerols, fatty acids, and phospholipids (Nawar 1996). Triacylglycerols are by far the most common lipid in foods, and it is this type of molecule that...

Emulsion Appearance

The first impression that a consumer usually has of a food emulsion is a result of its appearance (Francis and Clydesdale 1975, Hutchings 1994). Appearance therefore plays an important role in determining whether or not a consumer will purchase a particular product, as well as his or her perception of the quality once the product is consumed. A number of different characteristics contribute to the overall appearance of a food emulsion, including its opacity, color, and homogeneity. These...

And Flavor

The modern consumer is faced with a huge variety of different types of food products from which to choose, and within each category there are a number of different brand names. The initial choice of a particular product is governed by many factors, including its cost, quality, packaging, ease of preparation, and nutritional value. Once a consumer has made a decision to purchase a certain brand name, the manufacturer wants to ensure that he or she is satisfied with the product and will purchase...

Interfaces with Adsorbed Emulsifiers

So far, we have only considered the molecular characteristics of an interface that separates two pure liquids. In practice, food emulsions contain various types of surface-active mol ecules which can accumulate at the interface and therefore alter its properties (e.g., proteins, polysaccharides, alcohols, and surfactants) (Dickinson and Stainsby 1982, Dickinson 1992). 5.2.2.1. Surface Activity and the Reduction of Interfacial Tension The surface activity of a molecule is a measure of its...

Computer Modeling Of Liquid Properties

Our understanding of the way that molecules organize themselves in a liquid can be greatly enhanced by the use of computer modeling techniques (Murrell and Jenkins 1994, Gelin 1994). Computer simulations of molecular properties have provided a number of valuable insights that are relevant to a better understanding of the behavior of food emulsions, including the miscibility immiscibility of liquids, the formation of surfactant micelles, the adsorption of emulsifiers at an interface, the...

Rheological Properties Of Emulsions

Food emulsions exhibit a wide range of different rheological properties, ranging from low-viscosity liquids to fairly rigid solids. The rheological behavior of a particular food depends on the type and concentration of ingredients it contains, as well as the processing and storage conditions it has experienced. In this section, the relationship between the rheological properties of emulsions and their composition and microstructure is discussed. We begin by considering the rheology of dilute...

Droplet Disruption

Droplet Disruption

The precise nature of the physical processes which occur during emulsion formation depends on the type of homogenizer used. Nevertheless, there are some common aspects of droplet disruption which apply to most types of homogenizer. The initial stages of primary homog-enization involve the breakup and intermingling of the bulk oil and aqueous phases so that fairly large droplets of one of the liquids become dispersed throughout the other liquid Walstra 1983, 1993b . The later stages of primary...

Selection of Most Appropriate Demulsification Technique

In addition to depending on the type of emulsifier present, the choice of an appropriate demulsification technique also depends on the sensitivity of the other components in the system to the separation process. For example, if one is monitoring lipid oxidation or trying to determine the concentration of an oil-soluble volatile component, it is inadvisable to use a demulsification technique that requires excessive heating. On the other hand, if the sample contains a lipid phase that is...

Coalescence as the Result of Hole Formation

In many emulsions in which coalescence is observed, one would not expect it to occur from an examination of the interdroplet pair potential, because of the extremely high steric repulsion that arises when two droplets closely approach each another Section 3.5 . The interdroplet pair potential, which describes the dependence of the colloidal interactions between the emulsion droplets on separation, assumes that the system is at thermodynamic equilibrium. It therefore predicts that droplets will...

Functional Properties

Protein Hydration and Water Solubility Many of the functional properties of biopolymers in food emulsions are governed by their interactions with water e.g., solubility, dispersibility, swelling, thickening, emulsification, foaming, and gelling Suggett 1975a,b Damodaran 1996 Fennema 1996b . Biopolymer ingredients are usually added to the aqueous phase of food emulsions in a powdered form. The functional properties of many of these biopolymers are only exhibited when they are fully...

Overview Of Homogenization

Homogenization

The formation of an emulsion may involve a single step or a number of consecutive steps, depending on the nature of the starting material and the method used to create it. Prior to converting separate oil and aqueous phases into an emulsion, it is usually necessary to disperse the various ingredients into the phase in which they are most soluble. Oil-soluble ingredients, such as vitamins, colors, antioxidants, and surfactants, are usually mixed with the oil, whereas water-soluble ingredients,...

Du Nouy Ring Interfacial Tension

Nouy Ring Method Schematic

Summary of the Instruments Used for Measuring Surface and Interfacial Tensions Du Nouy ring Wilhelmy plate Pendant drop Sessile drop Spinning drop Capillary rise Dynamic Maximum bubble pressure Oscillating jet Drop volume Surface waves Du Nouy ring Wilhelmy plate Pendant drop Sessile drop Spinning drop Drop volume Pendant drop Spinning drop FIGURE 5.13 Du Nouy ring method of determining the interfacial and surface tension of liquids. FIGURE 5.13 Du Nouy ring method of determining the...

High Pressure Valve Homogenizers

High Pressure Valve Homogenizer

High-pressure valve homogenizers are the most commonly used method of producing fine emulsions in the food industry. Like colloid mills, they are more effective at reducing the size of the droplets in a preexisting emulsion than at creating an emulsion directly from two separate liquids Pandolfe 1991, 1995 . A coarse emulsion is usually produced using a highspeed blender and is then fed directly into the input of the high-pressure valve homogenizer. The homogenizer has a pump which pulls the...

Colloidal Interactions

Food emulsions are microheterogeneous materials that contain a variety of different structural entities which range in size, shape, and physicochemical properties, including atoms, molecules, molecular aggregates, micelles, emulsion droplets, crystals, and air cells Dickinson and Stainsby 1982, Dickinson 1992 . Many of these structural entities have at least one dimension that falls within the colloidal size range i.e., between a few nanometers and a few micrometers see Figure 1.8 . The...

Interdroplet Pair Potential

When the separation between two droplets is small compared to their size h lt lt rd , the interdroplet pair potential due to exclusion of the colloidal particles from the depletion zone is given by the following expression Sperry 1982 where POSM is the osmotic pressure arising from the exclusion of the colloidal particles and rc is the radius of the colloidal particles. The osmotic pressure difference is given by the following equation Hiemenz 1986 W I1 2M 3.21 Here, C, M, and v are the...

Capillary Rise And Meniscus Formation

Capillary Rise

The surface tension of a liquid governs the rise of liquids in capillary tubes and the formation of menisci curved surfaces at the top of liquids Hiemenz 1986, Hunter 1986 . When a glass capillary tube is dipped into a beaker of water, the liquid climbs up the tube and forms a curved surface Figure 5.10 . The origin of this phenomenon is the imbalance of intermolecular forces at the various surfaces and interfaces in the system Evans and Wennerstrom 1994 . When water climbs up the capillary...

Polymeric Steric Interactions 351 Polymeric Emulsifiers

In Section 3.3, we saw that van der Waals interactions always operate between emulsion droplets and that these interactions are strong enough to cause droplets to aggregate, unless there is a sufficiently strong repulsive interaction to prevent them from coming close together. When emulsion droplets are surrounded by a layer of electrically charged emulsifier molecules, they may be stabilized against aggregation by electrostatic repulsion Section 3.4 . Nevertheless, many food emulsions are...

Hydrophobic Interactions

Compared to the other major forms of colloidal interaction, the contribution of hydrophobic interactions to emulsion stability has largely been ignored by emulsion scientists. Nevertheless, this type of interaction is of great importance in many types of foods and has recently been shown to promote droplet flocculation in protein-stabilized emulsions Monahan et al. 1996, Demetriades et al. 1997b . Hydrophobic interactions are important when the surfaces of the droplets have some nonpolar...

Energy Input

The size of the droplets in an emulsion can be reduced by increasing the amount of energy supplied during homogenization as long as there is sufficient emulsifier to cover the surfaces of the droplets formed . The energy input can be increased in a number of different ways depending on the nature of the homogenizer. In a high-speed blender, the energy input can be enhanced by increasing the rotation speed or the length of time that the sample is blended. In a high-pressure valve homogenizer, it...

Membrane Homogenizers

An emulsion is formed by forcing one immiscible liquid into another through a glass membrane which contains a uniform pore size Figure 6.10 . The size of the droplets formed FIGURE 6.10 Batch version of a membrane homogenizer. depends on the diameter of the pores in the membrane and the interfacial tensions between the oil and water phases Kandori 1995 . Membranes can be manufactured with different pore diameters so that emulsions with different droplet sizes can be produced Kandori 1995 . The...

Depletion Interactions 361 Origin of Depletion Interactions

Depletion Interaction

Many food emulsions contain small colloidal particles that are dispersed in the continuous phase which surrounds the droplets Figure 3.17 . These colloidal particles may be surfactant FIGURE 3.17 An attractive depletion interaction arises between emulsion droplets when they are surrounded by small nonadsorbing colloidal particles. FIGURE 3.17 An attractive depletion interaction arises between emulsion droplets when they are surrounded by small nonadsorbing colloidal particles. micelles formed...

Sessile and Pendant Drop Methods

Sessile And Pendant Drop

The sessile- and pendant-drop methods can be used to determine the static surface and interfacial tensions of liquids Hunter 1986, Couper 1993 . The shape of a liquid droplet depends on a balance between the gravitational and surface forces. Surface forces favor a spherical droplet because this shape minimizes the contact area between the liquid and its surroundings. On the other hand, gravitational forces tend to cause droplets to become flattened if they are resting on a solid surface or...

Molecular Characteristics

The principal role of surfactants in food emulsions is to enhance their formation and stability Charalambous and Doxastakis 1989, Dickinson 1992, Hasenhuettl and Hartel 1997 however, they may also alter emulsion properties in a variety of other ways e.g., by interacting with proteins or polysaccharides, by forming surfactant micelles, or by modifying the structure of fat crystals Dickinson and McClements 1995, Bergenstahl 1997, Bos et al. 1997, Deffenbaugh 1997 . By definition, a surfactant is...

Surfactant Classification

A food manufacturer must consider a variety of factors when selecting a surfactant for a particular product, including its legal status as a food ingredient its cost the reliability of the supplier the consistency of its quality from batch to batch its ease of handling and dispersion its shelf life its compatibility with other ingredients the processing, storage, and handling conditions it will experience and the expected shelf life and physicochemical properties of the final product. How does...

Emulsion Properties

Dispersed-Phase Volume Fraction The concentration of droplets in an emulsion is usually described in terms of the dispersed-phase volume fraction , which is equal to the volume of emulsion droplets VD divided by the total volume of the emulsion VE VDIVE. Knowledge of the dispersed-phase volume fraction is important because the droplet concentration influences the appearance, texture, flavor, stability, and cost of emulsion-based food products. In some situations, it is more convenient to...

Emulsion Science In The Food Industry

Water Droplet Microscope Margarine

Many natural and processed foods consist either partly or wholly as emulsions or have been in an emulsified state at some time during their production such foods include milk, cream, butter, margarine, fruit beverages, soups, cake batters, mayonnaise, cream liqueurs, sauces, desserts, salad cream, ice cream, and coffee whitener Friberg and Larsson 1997, Krog et al. 1983, Jaynes 1983, Dickinson and Stainsby 1982, Dickinson 1992, Swaisgood 1996 . Emulsion-based food products exhibit a wide...

Table of Contents

Emulsion Science in the Food Industry 1.2. General Characteristics of Food Emulsions 1.2.1. Definitions 1.2.2 Mechanisms of Emulsion Instability 1.2.3. Ingredient Partitioning in Emulsions 1.2.4. Dynamic Nature of Emulsions 1.2.5. Complexity of Food Emulsions 1.3.1. Dispersed-Phase Volume Fraction 1.3.2. Particle Size Distribution 1.4. Hierarchy of Emulsion Properties 1.5. Investigation of Emulsion Properties 2.3. Origin and Nature of Molecular Interactions 2.3.2. Electrostatic...