Why do sponge cakes often rise without chemical leavening agents?

Sponge cakes rise without chemical leavening agents because they rely on air and steam for expansion. The process begins with vigorous beating, which incorporates air into the batter to create a foam.12

Egg whites are essential to this method because their protein structure is capable of forming strong, voluminous foams that retain these air bubbles. During the baking process, the heat causes the entrapped air to expand. Additionally, as the interior of the cake approaches the boiling point, water vapor exerts further pressure on the walls of these pre-existing bubbles, causing the cake to rise.123

To maintain the cake's structure, the foam is typically supported by the gluten of flour, which is gently folded into the mixture to prevent the foam from collapsing due to the presence of lipids.4