When an egg is whipped with an egg beater, or similar utensil, its volume increases, owing to incorporation of air. The egg white because of its low surface tension and the stability of its surface films readily forms a foam.

The essentials for a stable foam have been discussed in Chapter I (The Relation Of Cookery To Colloid Chemistry). They are a low surface tension, a low vapor pressure, and a tendency for the substance in the surface to solidify, hence giving rigidity and permanence.

Stages of stiffness in beating egg whites. With slight whipping the incorporated air bubbles are large, the egg white appears foamy yet transparent, is very runny, and will flow readily. With longer beating the air cells in the egg white become smaller, the appearance of the egg white is less transparent and more white. It still flows if the bowl is partially inverted. The egg white becomes stiffer with continued beating. The stiffness is due, in part at least, to the finer division of the air bubbles, and thus the amount of egg white utilized in forming films is greater. Many small air bubbles with their fine cell walls may be stronger and more rigid than a few large cells. As beating is continued the egg becomes very white, begins to lose a little of the moist, shiny appearance, and is stiff and rigid. If the bowl is inverted, the egg white does not flow but remains in the bowl and the end of peaks stand up straight. If the egg is left to stand, the watery fluid collects at the bottom of the bowl more slowly. With longer beating the egg white appears dry, loses its shiny appearance, and small white patches that look like small curds may appear. This is the stage called dry in cook books. At this point the white is very rigid and rather brittle so that with a whisk beater it is easily thrown out of the bowl in which it is whipped.

Methods of testing stiffness. There are several ways in which the stiffness may be tested. For household use the tests are (1) appearance, (2) the height of peaks and the extent to which the point bends over when the egg beater or some utensil is lifted out of the beaten white, and (3) the rate of flow when the bowl or plate in which the white is beaten is partially inverted. A skillful operator working constantly with egg white soon learns to judge the degree of stiffness by these common household methods, and the degree of proficiency that can be attained is surprising. But for experimental purposes the same degree of stiffness probably could not be duplicated in another laboratory from the description. In the home the height of the peaks, and particularly the extent the point falls over, is the best criterion for judging stiffness. The rate of flow is not quite so good unless the same quantity of egg white is always used, for a large quantity must be beaten stiffer to flow at the same rate as a smaller quantity.

For laboratory purposes, (1) specific gravity, (2) foaming power, and (3) the amount of drainage during a definite time are used to test the stiffness and stability of the foam. Specific gravity is determined by dividing the weight of a given volume of egg white by the weight of the same volume of water at the same temperature. Barmore reports that foams with specific gravity of 0.15 to 0.16 yield good angel cakes. Bailey calculated foaming power by means of the formula:

Where F = foaming power of egg white, V = volume of dish in cubic centimeters, and W = weight of foam in grams. The specific gravity of the original egg white was taken as 1.04.

The stability of a foam may be tested by putting a given weight of foam in a funnel of known capacity and bore. The funnel is covered to prevent evaporation. If the funnel is placed in a graduated cylinder, the drainage may be read in cubic centimeters or the weight taken at the end of a definite time, usually 30, 40, or 60 minutes. When the egg white is not beaten sufficiently both the unbeaten egg white and the foam drain from the funnel. After a certain stage of beating is reached little drainage occurs. With still longer beating Barmore states the drainage increases. Work of students in the author's laboratory, Keltner, Hoskey, and Loaft, though not extensive enough to be conclusive, indicates that better angel cakes are obtained if a certain percentage of drainage occurs, the exact amount varying with different types of egg beaters.

Salt and the whipping of egg whites. It is traditional that a small amount of salt added to egg white is an aid in increasing foaming and stiffness. This could easily be tested but so far as the writer knows its effect on stability has not been reported. It has been found that electrolytes are necessary for heat coagulation of proteins. They may also aid coagulation by mechanical means or surface denaturation. The more likely explanation is the one previously given for protein solutions and electrolytes. The protein lowers the surface tension but the addition of salt lowers it still more, thus causing a greater concentration of the protein at the air/liquid interface, hence a slight stiffening. The addition of salt may result in a slight salting-out effect and a stiffening of the membrane around the air bubbles. This would have the same result as in emulsions. For, by the last mechanism a certain amount of salt would bring about maximum stabilization of the emulsion or foam, but too large a quantity would have the tendency to break the emulsion or the foam.

Acids and stability of egg white foams. Barmore in his last publication states that when acid substances were added in sufficient quantity to adjust the egg white to pH 8 the stability of the foam was practically the same for acetic and citric acids and for cream of tartar. But at pH 6 the cream of tartar produced the most stable foam. The acid sub-stances increased the stability of the foams.

The addition of the acid delays the formation of the foam, i.e., if beaten for a definite time the foams containing acid are not as stiff as those containing no acid. This is particularly true for both acetic and citric acids.