This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Definition by Food and Drug Administration. "Mayonnaise, mayonnaise dressing, mayonnaise salad dressing, is the semi-solid emulsion of edible vegetable oil, egg yolk, or whole egg, a vinegar, and/or lemon juice, with one or more of the following: salt, other seasoning commonly used in its preparation, sugar and/or dextrose. The finished product contains not less than 50 per cent of edible vegetable oil."
Does peptization play a role in emulsification? In no theory of emulsification that the writer has read is peptization mentioned. Yet the following comments by Bancroft on peptization so vividly describe what happens in making mayonnaise that they are given. Bancroft, in his chapter, "Preparation of Colloidal Solutions," states that the methods of making colloidal solutions may be grouped under two heads, the dispersion and the condensation methods.
The making of mayonnaise, Hollandaise sauce, gravies, and other sauces in which an emulsion is formed comes under dispersion methods, for although the dispersed phase is not sufficiently small to class the product as a sol, the dispersed phase must be broken into many particles. Bancroft states that one means of dispersion is by the addition of a peptizing agent. He says that peptization is always due to adsorption. "If an adsorbed film has a low surface tension on the water side, it will tend to scrunch up and to peptize the solid as internal phase. If the reverse is the case, the solid will tend to form the external phase." If the word oil is substituted for solid in the foregoing quotation, an excellent picture of the formation of mayonnaise emulsion is obtained. For example, if one-fourth cup of oil is added to a cup or more of well-emulsified mayonnaise, the oil may be stirred slowly a few times with a spoon, 15 to 20, and presto, it is all emulsified. The stirring could not possibly break the oil into thousands of small particles. Hence, the function of the stirring is to increase the area for adsorption to occur and the squeezing effect of the adsorbed egg yolk breaks the oil into many spheres. For the first addition of oil in making mayonnaise great care must be taken, but large quantities of oil may be added rapidly in the last part of the process.