Deflating sponge cake
Q: Why did my sponge cake deflate halfway through baking?
A: Because sponge cakes are based on a high proportion of eggs (which gives them a relatively flexible structure) and they contain a large amount of air, when they cool after baking they do deflate more than other types of cakes – 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. However, the main reason for a sponge cake to deflate during baking so that it dips in the centre, is that a considerable amount of heat has been lost from the oven, often due to the door being open. At this stage during baking, the structure of the cake hasn’t set enough to hold its shape and, as the air in the cake cools and contracts momentarily due to the loss of heat, the cake will deflate. It also won’t rise again even if the oven regains the correct temperature.
If sponge cakes need to be moved around to ensure even baking they are best left in a preheated oven for at least 15 minutes before opening the oven door even if the total baking time is short (sponges often only need about 20 minutes). This initial time will mean that the structure of the cake has more chance of setting before the oven door is opened and are less likely to collapse when heat is lost. 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.