Sponge Cake: How Whipped Egg Foam Replaces Chemical Leavening

A sponge cake, also called a foam cake, depends on aeration of eggs plus heat to rise rather than on baking powder or soda. Whipping eggs (or egg whites) incorporates air by agitating the protein albumen into a partially coagulated membrane, which stiffens the mixture and dramatically increases its volume. This egg foam is the cake's structure: properly whipped, eggs with sugar roughly triple in volume and the batter holds a ribbon. Because the foam is the leavening, technique governs the outcome. After whipping, flour is folded in gently — vigorous mixing knocks out the trapped air bubbles the foam depends on. The bare whisked sponge contains no fat; a genoise adds flour and a little melted butter for a moister, slightly denser crumb. Some recipes deliberately bake in ungreased pans so the batter can grip and climb the sides, improving rise when the foam is delicate. The same delicacy explains collapse: a foam that deflates cannot recover, so opening the oven early (causing a temperature drop) or letting heavy, wet fruit weigh the batter down can cause a sunken result. This is the structural basis for maceration draining and flour-coating fruit before adding it to a sponge such as sharlotka.