The most effective means of controlling the rate and extent of flocculation in an emulsion is to regulate the colloidal interactions between the droplets. Flocculation can be prevented by designing an emulsion in which the repulsive interactions between the droplets are significantly greater than the attractive interactions. A wide variety of different types of colloidal interaction can act between the droplets in an emulsion (Chapter 3). Which of these is important in a given system depends on the type of ingredients present, the microstructure of the emulsion, and the prevailing environmental conditions. To control flocculation in a particular system, it is necessary to identify the most important types of colloidal interaction.
Electrostatic Interactions. Many oil-in-water emulsions used in the food industry are stabilized against flocculation by using electrically charged emulsifiers which generate an electrostatic repulsion between the droplets (e.g., ionic surfactants, proteins, or polysaccharides) (Dickinson 1992, Das and Kinsella 1990). The flocculation stability of electrostatically stabilized emulsions depends mainly on the electrical properties of the emulsifier and the pH and ionic strength of the aqueous phase (Hunter 1986). The number, position, sign, and dissociation constants of the ionizable groups on an emulsifier determine its electrical behavior under different environmental conditions. For each type of food product, it is therefore necessary to select an emulsifier with appropriate electrical characteristics.
Hydrogen ions are potential determining ions for many food emulsifiers (e.g., COOH ^ COO- + H+ or NH2 + H+ ^ NH+), and therefore the sign and magnitude of the electrical charge on emulsion droplets are determined principally by the pH of the surrounding solution (Section 3.4.1). The influence of pH on droplet flocculation in a protein-stabilized emulsion is illustrated in Figure 7.10. At pH values sufficiently above or below the isoelectric point (IEP) of the whey proteins (IEP ~ pH 5), the droplet charge is large enough to prevent flocculation because of the strong electrostatic repulsion between the droplets (Figure 7.10). At pH values near the isoelectric point (IEP ± 1), the net charge on the proteins is relatively low and the electrostatic repulsion between the droplets is no longer sufficiently strong to prevent flocculation. Droplet flocculation leads to a pronounced increase in the viscosity of an emulsion, as well as a decrease in creaming stability, and therefore has important implications for food quality (Demetriades et al. 1997a, Agboola and Dalgleish 1996d).
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