How do egg whites help to keep fats inside a cake?

Question

Recently I tried to bake a cake (Caprese) from:

ground almonds
butter
dark chocolate
sugar
eggs

I melted chocolate and butter, mixed all of this with whipped sugar yolks, and then... totally screwed the whipped egg whites, and decided (for science!) to skip folding the egg whites as part of the recipe and just add almonds. As a result, when the cake has actually started to bake, most of the incorporated butter rose to the surface as one slimy puddle.
Why and how egg whites help keep fats inside of cake? I'm mostly interested in chemical or physical explanations of the effect.

Mark Wildon · Accepted Answer

Egg whites are mostly protein. Long chain polymers like proteins typically have regions that are both hydrophilic (binding to water) and hydrophobic (preferring to bind to fats), so are great at forming emulsions and stable gels. When cooked (or even beaten long enough) the long chains unwind and the proteins denature, making it easier for these emulsions to form.
In your cake, the role of the egg whites is to form a stable emulsion trapping fats from the butter and chocolate, as well as air. As you found, the egg whites are a structural essential.

How do egg whites help to keep fats inside a cake?

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