This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
To make an emulsion it is necessary to break or separate the dispersed phase into small globules. The work for this separation can be done in different ways. Machines of different types are used for making commercial emulsions. They are all designed to break the liquid into globules, either by a rotary motion, pressure, or by some other means. Agitation is used in all of these. Homogenization is used by most commercial firms for making mayonnaise. In making emulsions in the home, different methods are followed. For mayonnaise, an egg beater is often used, French dressing is often shaken, and gravy and sauces are made by stirring, usually with a spoon.
Optimum degree of agitation for each emulsion. Clayton says it is well known that agitation can both break and make an emulsion. The amount of agitation required for a given emulsion depends upon the particular emulsion being made and the kind of mixing utensil used. Clayton states: "It is quite reasonable to believe that for any given emulsifying apparatus there exists an optimum speed or degree of agitation or mixing, and an optimum time of mixing or running, whereby the most perfect emulsion can be obtained in a given system. Experiments prove this."
Stamm has reported that the method of preparation affects the size of the particles of the dispersed phase. Harkins (1928) states that the "method used in preparing an emulsion is one of the factors determining the distribution, but the most striking feature of the present work is that the shift of the number maximum is so slight with the different methods of stirring employed." By number maximum Harkins refers to the number of particles of the dispersed phase in a definite volume. Thus the apparatus, its speed, and the time used in mixing emulsions like mayonnaise, may or may not determine to a great extent the size of the particles formed and the stability of the emulsion.
Intermittent mixing for emulsions. Clayton states that what may be termed the mechanics of emulsification are far from being understood even now. In some experiments in making different emulsions, investigators have reported that intermittent shaking is more effective than continuous shaking or agitation. Clayton believes that shaking is an inferior method of making emulsions, but that continuous shaking should give equally as good results as the intermittent shaking, provided the emulsified portions are continuously removed from the mass. Intermittent agitation is explained as being more effective than continuous agitation, because of the rest periods, which allow time for adsorption of the emulsifying agent.
In making mayonnaise, the beating is often intermittent, for in stopping to add oil to the mixture, short rest periods occur.
Foams and formation of emulsions. Harkins in commenting on the method of preparation of emulsions, states that in a number of cases the emulsion failed to form after stirring rapidly with a motor-driven egg beater for 5 minutes, which was not in accord with the usual emulsifica-tion in a few seconds. He found that in all these cases no foam was produced, and that as soon as a foam formed, emulsification occurred in a few seconds. This may be a factor in the formation of some emulsions in food preparation.