Air bubbles trapped in liquid albumen when egg white is beaten. When egg white is beaten, it becomes foamy, increases 6 to 8 times in volume and stands in peaks. When the foam is heated, the tiny air cells expand and the egg protein coagulates around them, giving permanence to the foam. Egg white foam is responsible for the structure of soufflés, angel food cake, puffy omelets and meringue.
Egg whites reach their greatest volume if allowed to stand at room temperature for about 30 minutes before beating.
Fat inhibits the foaming of egg whites, so be sure beaters and bowls are clean and that there is no trace of yolk in the whites. Use only metal or glass bowls because plastic bowls tend to absorb fat.
If egg whites are underbeaten, the volume of the finished product will be less than desired. Overbeaten whites form clumps which are difficult to blend with other ingredients. And, because overbeaten egg whites lack elasticity, they cannot expand properly when heated. The finished product may be dry, of poor volume or may even collapse.
The addition of an acid ingredient helps to stabilize the foam. The most commonly used acid ingredient is cream of tartar (1/8 teaspoon for each 1 to 2 whites) although some recipes call for lemon juice or vinegar.
Since salt decreases foam stability, it is best to add it to other recipe ingredients.
Egg white foams should be combined with other ingredients immediately, before they have time to drain or shrink.
Egg yolk and whole egg will also form foams, but the volume is much less than the foam of beaten white.