This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Emulsions may be stabilized by different substances. They may be classified as follows, the basis being upon the type of emulsifler: (1) those stabilized by an electric charge, (2) those stabilized by colloids, and (3) those stabilized by powders.
Stabilization of emulsion by an electric charge. Mineral oil emulsions in which the oil is present in very small amounts belong in the group stabilized by an electric charge. The oil particles are negatively charged. Hydrophilic sols are stabilized by hydration and an electric charge. Ghosh and Dhar suggest that emulsions are similar to sols and that the stability, the separation or coagulation, and the reversal of the emulsion are markedly influenced by its electric charge.
Emulsions stabilized by colloids. So many of the emulsifiers are colloidal in nature that this group is the most important in food preparation. The following are commonly used: eggs, gelatin, flour, starch, and milk. Gum arabic, gum tragacanth, Irish moss, and other substances are used less frequently. Pectin is being used to some extent. Gum arabic and gum tragacanth are used principally in cookery of diabetic foods, the Irish moss in puddings.
Stabilization by powders. Finely ground particles or powders such as lampblack, mustard, and paprika are a third class of substances used for emulsifying agents. The best examples of this type of emulsifler in prepared foods are the French dressings. At the present time several brands are on the market that are fairly stable. All are deep red in color, so that the emulsion must be partially and probably wholly stabilized with particles of paprika and mustard. This kind of emulsion is formed by the powder film around the drops of oil which keeps them from coalescing. Clayton reports the results of using finely divided solids as emulsifiers by Bechhold, Dede, and Reiner. 'They found that the formation of emulsion depends upon: (1) the grain size of the powder. The smaller the grain the better the emulsion, until an optimum is reached, after which smaller grains have inferior emulsifying properties. (2) The quantity of powder. The more powder there is available the more globules there can be covered, providing the powder is sufficiently fine." They report that zinc dust, iron powder, clay, Kieselguhr, and yeast made very efficient emulsifiers.