By Harold McGee
There are three enemies to the successful mounting of a foam which the cook should be careful to exclude from the bowl: egg yolk, oil or fat, and detergent. All are chemical relatives, and interfere with foaming in the same ways: by competing with the proteins for a place at the air-water interface without offering any structural reinforcement; and by interfering with the bonding of the protein molecules. Traces of these troublemakers won’t absolutely prevent you from making a foam, but they’ll make you work harder and longer, and the foam won’t be as light or stable. Of course yolk and fat can safely be mixed with a finished foam, as happens in many recipes for soufflés and egg-leavened batters.