Emulsions And Emulsifiers

An emulsion is a heterogeneous system of two immiscible liquids, one of which is intimately dispersed in the other. The droplets of the dispersed phase generally have a diameter of over 0.1 fim. The emulsions are stabilized by the presence of a third component, the emulsifier, a surface-active agent which is partly soluble in both phases (30). Food emulsions usually contain water and oil as the immiscible phases, giving rise to emulsions of the oil-in-water (O/W) type or water-in-oil (W/O) type. The action of emul-sifiers can be enhanced by the presence of stabilizers. Emulsifiers are surface-active compounds that have the ability to reduce the interfacial tension between liquidliquid and air-liquid interfaces. This ability is the result of an emulsifier's molecular structure: the molecules contain two distinct sections, one having a polar or hydrophilic character, the other having nonpolar or hydrophobic properties. Most surface-active agents reduce the surface tension from about 50 dynes/cm to less than 10 dynes/cm when used in concentrations below 0.2%.

The relative sizes of the hydrophilic and hydrophobic sections of an emulsifier molecule mostly determine its behavior in emulsification. To select the proper emulsifier for a given application, the so-called HLB system was developed (HLB = hydrophile-lipophile balance). It is a numerical expression for the relative simultaneous attraction of an emulsifier for water and for oil. The HLB of an emulsifier is an indication of how it will behave but not how efficient it is. Emulsifiers with low HLB tend to form W/O emulsions, those with intermediate HLB form O/W emulsions, and those with high HLB are solubilizing agents. The HLB value can be either calculated or determined experimentally (31). The scale goes from 0 to 20, at least in theory, since at each end of the scale the compounds would have little emulsifying activity. The HLB value of some commercial nonionic emulsifiers is given in Table 8 (32).

Foods contain many natural emulsifiers, of which phospholipids are the most common.

Emulsions are stabilized by a variety of compounds, mostly macromolecules such as proteins, starches, and gums.

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