Overcooked COVID 19 Edition: What makes angel food cake so fluffy?

If you’ve ever baked an angel food cake, meringue, or soufflé, you’ll know that egg whites can be whipped up into a stiff foam that gives the dessert a nice fluffy and delicate texture. So how does beating egg whites transform them from a runny, clear liquid to having white peaks that can stand up on their own? The answer of course is polymer science.

Runny egg whites, fresh out of the egg
Beaten into stiff peaks — aka a good arm workout!

Egg whites contain water and a bunch of proteins, which are chains of amino acids linked together. Some of these amino acids are hydrophilic–meaning they like water–while others are hydrophobic–they avoid water. In water, these proteins collapse to form little globs so that the hydrophilic parts can face the water on the outside of the globule, and the hydrophobic parts can avoid it on the inside.

protein structure.png
Proteins have both hydrophilic and hydrophobic amino acids
protein in water.png
These proteins are normally curled up into globules in water

When you beat egg whites, you’re causing these proteins to unfold and also adding in a ton of air. (You can see this from all the air bubbles soon after you start beating)

Lots of air bubbles!

Now, the protein can also rearrange and spread out, since the hydrophobic parts can orient themselves towards these larger air bubbles. As the chains spread out, they reach out and bond with other protein chains. The more bonds that are formed, the more stable this air trapping foam becomes.

protein in air and water.png
The polymer chains spread out and form a network to hold the air in place, making a foam

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