Eli5 Why does adding whipped egg whites to a batter make a cake rise?
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Whipped egg whites do two things.
First, the whipping of the egg whites fill them with many many many tiny air bubbles, which helps increase the volume of the cake. Plus those air pockets will want to expand when heated.
Second, the proteins in the egg whites will quickly start to solidify when cooking, which helps give the cake extra structure, holding those air pockets open.
Whipped egg whites are foam – the whites are a protein structure that traps a ton of air.
When the batter is cooked, the air expands making the cake rise. The protein structure stretches to accommodate this, so the air doesn’t escape.
As the cake cools, the protein structure stiffens back up, keeping the risen shape even as the air inside shrinks.
When you whip egg whites, thousands and thousands of tiny air bubbles are formed, and the proteins of the egg white hold them in, without letting them out. This is why you gently fold egg whites into a batter – mix them too hard, and you’ll pop most of those bubbles. Fold them in, and *some* will pop, but many will stay, protected by that egg protein shell they’ve formed.
Then, in the heat of the oven, the air bubbles will expand – this pushes on the egg whites and batter batter, making it rise as it cooks. By the time it’s heated up enough to solidify, it’s been lifted by that hot air, and it’ll solidify in that shape, light and fluffy.
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