The stage before a sponge cake goes in the oven can be tricky for a beginner. The mixture shouldn’t be like a dough and it shouldn’t run like cream, so how can you tell if it’s ready? A recipe might call for your sponge to be ‘folded until incorporated’ or ‘mixed until smooth’ without explaining what you are aiming for visually inside the bowl.
The surface of your sponge cake batter can tell you a lot. When the sponge cake is ready, it will usually look even, smooth and lightly glossy without any lumpy bits of dry flour at the top or stuck to the inside of the bowl. When you drag your spatula through your sponge cake mixture it should move together as a soft mass. There may be a slight track left behind that quickly blends back together. If the sponge cake mixture separates and falls back in heavy lumps, your cake batter might be too stiff. If it runs away like milk there may have been an error in measuring or the mixing process.
When you are mixing cake batter by folding you are often at high risk of deflating the sponge mixture. The flour needs to be mixed into the mixture of eggs and sugar but the spatula shouldn’t beat the sponge cake batter as though it were icing. Gently stir the cake batter from the side of the bowl and down through the middle in slow sweeping arcs, with gentle movements to rotate the bowl after each sweep. You are trying to incorporate the flour into the mixture, but not beat the air out of it. Slowly reduce the pace when your sponge mixture is starting to come together. Check carefully for any remaining bits of dry flour.
When it gets to this stage it may help you to do a test before you put the batter into the tin. Before you transfer your cake batter you can lift the spatula from the surface of the cake batter and allow a ribbon of cake batter to slowly fall back into the bowl. You should see your cake batter rest a moment on the surface and then blend in, if the batter immediately vanishes it may be too loose. Alternatively if your batter doesn’t merge with the other cake batter, the mixture might have been overworked and become too dense, or may have become too dry from too much flour. The cake batter should not need to be perfect, you just need to get an idea of what the texture should be like.
If you do not have a spatula that is long enough to reach the bottom and sides of a large bowl it may be a good idea to scrape the bottom of the bowl before you are sure the sponge cake batter is ready. This can often be the hiding place for remaining bits of dry flour if you have been folding too carefully at the sides, which could result in pockets of flour in the finished sponge cake crumb, which can lead to uneven texture and dryness in the sponge cake. Be careful not to fold your sponge cake batter too long either as this can lead to a dense sponge cake that does not rise in the oven.
By this stage your tin should already be lined and ready. It is no time to try and find a suitable tin for your mixture as this will weaken it while your search for the right size cake tin or you may have to try and cut a piece of baking paper that might not fit the tin exactly. If you are relying on the egg whites alone to give the sponge a lift in the oven the air bubbles will quickly disappear if you make the sponge cake batter wait. Pour the sponge cake batter into your tin without delay and just level off the cake mixture to make it flat, don’t over level and avoid dropping the tin.
After the cake is finished you can then assess it against what the sponge cake mixture looked like and the techniques used before. Light crumb, good rise and good texture in the sponge cake usually means the sponge cake batter was handled with a reasonable hand. A dense center, a flat cake, or a texture marred by streaks can all point to a problem with the mixing, the ingredients, the baking time or even bits of unmixed flour in the batter. When you make a sponge cake in the future you can judge whether your sponge cake was good not only by the result in your mouth, but also by what the texture of your sponge cake batter looked like at the point it went into the tin.