This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Water may separate at the bottom of a mayonnaise after long standing. Robinson states that this seldom occurs if the amount of water in the mayonnaise does not exceed 15 per cent. With the percentage of water higher than 15 the separation occurs more frequently.
Oil may separate at the top of the mayonnaise. Anything that destroys the film-forming properties of the emulsifying agent and lets the dispersed phase run together destroys the emulsion. Over-heating, when the oil and liquid expand at different rates, the additions of salts, drying of the surface, freezing, and jarring may all cause separation in mayonnaise. The addition of enough salt (sodium chloride) to "salt out" the emulsifier will break emulsions stabilized with proteins, but this requires far more salt than is ordinarily used in mayonnaise. The amount commonly used aids in stabilizing the mayonnaise.
Freezing dehydrates the egg, because the water separates from the egg protein in ice crystals, thus breaking the film. Agitation or shaking in shipping tends to break the emulsion. Evaporation from the surface of the mayonnaise may cause drying of the emulsifying agent and consequently coalescing of the oil on the surface. Mayonnaise keeps better in covered containers which prevent evaporation. Fischer and Hooker state that, if the protein emulsifier is diluted beyond the point at which it will take up all the water, the emulsion tends to break. Heating, resulting in too much evaporation of the liquid, causes separation of the fat in gravies, sauces, and cream puffs.
Re-forming broken mayonnaise. The usual procedure to re-form broken mayonnaise is to take a new egg yolk and add the curdled, broken mayonnaise to this gradually. It is added and beaten in the same manner as when the oil is originally added to the egg yolk. But it is not necessary to use egg yolk to re-form the emulsion. It can be re-formed by adding the broken emulsion gradually to a tablespoon of water or vinegar. The broken emulsion must be added gradually to the water or vinegar and beaten during the addition. It will not re-form if the water or vinegar is added to the mass of broken emulsion. See Experiment 52,D.