Beat an egg white for two minutes and it will expand eightfold into a semisolid foam. Beat an egg yolk for ten minutes and you’ll be lucky to double its volume. Yolks are richer in protein than whites, and have the added advantage of emulsifying phospholipids that do a fine job of coating fat droplets: so why can’t they stabilize air bubbles and make a decent foam?
One clue is what happens when you wash out your yolky bowl: the moment you pour in some water, it foams! It turns out that the protein-rich, emulsifier-rich yolk is deficient in water. Not only does it contain about half the water that the white does, but nearly all of it is tightly bound to all the other materials. In one tablespoon/15 ml of yolk, the volume typical of a large egg, there’s about a third of a teaspoon/2 ml of free, foamable water. Add two teaspoons to give it the same free water as a white, and it foams enthusiastically.