Getting baked goods to rise properly could be tricky were it not for the egg. Leavening is only one of the egg's many roles in baking, but in some cases, it's the egg's most essential function. Some recipes such as sponge cake use nothing more than beaten egg whites as their sole leavening ingredient. Whole eggs and egg yolks alone can't serve as leavening because they carry the fat that makes batter smooth, rather than make it rise.
Whip 'Em Up
Egg whites can leaven baked goods because they foam when vigorously beaten. Unwhipped egg whites add protein, but it's the whites' ability to capture and hold air that makes it a leavening agent. Whipped egg whites can increase in volume as much as eight times, and are essential to sponge cakes, angel food cakes and souffles. Two proteins in egg whites, albumin and ovalbumin, make this leavening possible. When air is added through beating, albumin proteins create a mass of tiny air bubbles. At the same time, some albumin molecules bond to form a structure that holds in water, which makes up 85 percent of the egg white. Albumin's partner, ovalbumin, doesn't aid the beating process, but it coagulates during baking, which helps the batter stand up as water evaporates. These qualities of egg whites cause the batter to rise.