Wait, isn’t fat the foam foiler?

…asks reader Marci. How is it that you can add a fatty mixture to a batch of egg white foam and not have it collapse? That’s an excellent question, for indeed egg white foam and fat don’t mix…at least until the foam has already formed.

Those of you who recall other posts on egg white foam may remember the mechanics involved. Each individual bubble inside the foam stays intact because it’s being reinforced by a mesh of egg proteins. The long protein molecules are attracted to the bubble surface because some parts of them are water-loving (hydrophilic) and some are water-hating (hydrophobic). The surface of a bubble is therefore an ideal spot for them. They can stick their water-loving parts in water, their water-hating parts in the air, and they’re 100% happy. Additionally, they can (and do) bond with one another side-to-side, the result being a film that helps keep the bubble from popping.

Fats are similar to proteins in that they also have water-loving and water-hating parts. Which means they too are attracted to bubble surfaces. However unlike proteins they don’t bond with each other (or to the proteins). They only get in the way as the proteins try to hold hands with one another. The result is an incomplete mesh that doesn’t do a very good job of reinforcing the bubble. This is why you don’t want fat in a mixer bowl when you’re whipping up an egg white foam (however a little bit of fat, contrary to popular myth, won’t hurt anything).

The good news is that once the protein network is established, fats have a very hard time inserting themselves between the proteins on a bubble surface. This is why you can add butter and cheese sauce to a soufflé batter…but only after you’ve made a stable foam.