A raw egg-white foam will eventually coarsen, settle, and separate. It must therefore be reinforced when it is turned into a final dish. This may be done by adding other thickening ingredients—such things as flour, cornstarch, chocolate, or gelatin. But if the foam is to be used relatively pure, as in a meringue or a flourless soufflé, the egg proteins have to do the job themselves. With the help of heat, they do beautifully.
Ovalbumin, the major protein in egg white, is relatively immune to beating and doesn’t contribute much to the raw foam. But it is sensitive to heat, which causes it to unfold and coagulate. So when the raw foam is cooked, ovalbumin more than doubles the amount of solid protein reinforcement in the bubble walls. At the same time, much of the free water in the foam evaporates. Heat thus allows the cook to transform a transient semiliquid foam into a permanent solid one.