Several readers have written in to ask if it’s true that a tiny bit of fat in a mixer bowl will ruin a batch of whipped egg whites. The answer: absolutely not. A small blot of, say, egg yolk will do virtually nothing to impede a batch of whites from whipping up to a nice, voluminous foam.
Egg white foams work because the bubbles that make them up are reinforced by a mesh of string-like protein molecules, molecules which have been coaxed into untangling by the whipping action. At that point they begin to collect around air bubbles because certain regions along their length are attracted to air (are hydrophobic) and others are attracted to water (hydrophilic). Thus the surface of the bubble is a desirable spot for them, as all their different regions are happy, and they can bond to each other side-by-side while they’re there.
Fats are attracted to bubbles for similar reasons. They therefore pose something of a problem for the proteins because they compete for space at the bubble surfaces. Once there, they interfere with the proteins’ ability to bond to one another. However they don’t actually undermine the protein bonding, so if a few lipids are present here and there, it’s no big deal. They don’t bond to one another like proteins do, and once the proteins mesh is formed they can’t really worm their way in (this is why soufflé batters work, because once the egg white foam is formed, you can add plenty of fat without ruining it).
So while you don’t want to ignore fat in your mixer bowl, you shouldn’t freak out about it either. Egg white foams are more forgiving than you might think.