Why is it that every time I mention egg yolks I hear from VERY regular commentator Mexico Bob? He writes in to (re)remind me:
Don’t forget that egg yokes contain lecithin which is a natural amphiphile which is one of the building blocks of life. In fact the word “lecithin” is Greek for “egg yolk”. The lecithin acting as an amphiphile surrounds the fat molecules and prevents them from mixing with the rest of the egg.
No, I shall not forget that. And along the way I’ll make mention of what it is about yolk that’s bad for egg foams. The most stable egg foams, as mentioned earlier, are those made up solely of whites. This is because whites contain proteins which, when they’re unwound (something that happens when you whip them), reveal water-loving (hydrophilic), and water-hating (hydrophobic) sections along their length. Being somewhat schizophrenic in this way, the surface of an air bubble in a foam is the perfect place for them. They can insert their water-hating portions into the interior of the bubble, and their water-loving portions into the surrounding liquid — and use their side-to-side bonding sites to hold on to one another. Ahhh. What results is a film that surrounds the air bubble, lowering its surface tension and reinforcing it.
The trouble is that there are molecules in egg yolks that also like that same air/water boundary area. Introduce them into the mix and they compete for space at the bubble’s surface. What’s the problem with that? you may ask. The problem is that yolk molecules don’t have the same side-to-side bonding sites as the white proteins. This leaves gaps in the film, which makes the bubbles more likely to break, and that creates the instability I mentioned.
The interesting thing about this process, though, is that once you form an egg white-only foam — and all the spots on the surfaces of the bubbles are occupied — molecules from the yolk can’t worm their way in. The foam will be stable no matter how much fat is added later. Which is is what makes rich egg foam preparations like soufflés possible.
Oh, and if you were wondering what an “amphiphile” is, it’s a molecule with both water-loving (hydrophilic) and fat-loving (lipophilic) structures on it. “Likes both” is what the word roughly translates to in Greek.