Génoise is one of the archetypal examples of so-called mechanical leavening, in which hot, expanding pockets of steam do all the heavy lifting. Those steam pockets, however, don’t create themselves. They have to get their start somewhere, and in a sponge cake like génoise, that start is an egg foam. Génoise is somewhat unusual in that it’s based on a foam of whole eggs versus of foam of just egg whites. Egg white foams, as anyone who’s every made meringue or real buttercream knows, whip up fairly easily and maintain their volume for a good long time. That is, provided there isn’t any fat in the mix.
Interesting you should say that, Joe. Isn’t there fat in egg yolks? Why, yes there is…yes there is. One of the great myths about egg foam making is that the tiniest bit of fat causes the whole thing to collapse. That isn’t true, however it is true that fat makes egg foams unstable, prone to collapsing, and thus we arrive at the great hazard of whole egg foam batters. Agitate them, knock them around too much, and the bubbles in that unstable foam start to pop. Get enough of that popping going on and the entire basis of your leavening strategy goes out the proverbial window. Hence a familiarity with gentle mixing methods like folding comes in quite handy. Being careful to lay the finished batter gently into the pan, spreading it delicately with your spatula, and refraining from knocking the pan on surfaces as you put it into the oven…all of this works toward the greater good of a light and fluffy génoise.