Folding egg whites

Beaten egg whites hold a great deal of air, and that air is what lifts soufflés, sponge cakes and mousses. Stirring would knock it out, so the whites are folded in instead.

The heavier mixture, such as the egg yolk mixture, is poured over the whipped whites, not the other way around. With a rubber spatula the cook cuts down through the middle, sweeps across the bottom, and brings the batter up the side and over the top. The bowl is turned often and the spatula lifts through the center every few strokes.

Folding stops as soon as the mixture is one even color with no white streaks left. Moving gently and stopping early protects the volume, which is why cooks say to fold gently to maintain volume.