Why do sponge cakes fall after cooling?

Q: What happened to my sponge cake midway through baking?

A: Because sponge cakes contain a large amount of air and are based on a high proportion of eggs, they deflate more than other types of cakes when they cool after baking – if you watch carefully, you will notice them pulling away from the sides of the tin and losing some of their height almost immediately after they are removed from the oven. Yet, the major cause for a sponge cake to deflate during baking, causing it to sink in the center, is when a significant amount of heat has been lost from the oven, often owing to the door being left open. The structure of the cake hasn’t set enough to keep its form at this point in baking, and the cake will deflate when the air in the cake cools and compresses temporarily due to heat loss. It will also not rise again even if the oven temperature is restored.

Even though the overall baking time is short, it is preferable to leave sponge cakes in a preheated oven for at least 15 minutes before opening the oven door to ensure equal baking (sponges often only need about 20 minutes). Because of this early period, the structure of the cake has a better chance of setting before the oven door is opened and is less likely to collapse when heat is removed. Also when you do open the oven door make sure it is only for a very short period and the cakes are moved quickly – have your oven mit or tea towel ready and know exactly how / which way you are going to move them to make the process more efficient.