Interpreting the Foam’s Appearance

There are various ways to judge when the foam is at its optimum, from seeing whether the foam will support the weight of a coin or an egg, to seeing how it supports itself, in soft mounds or sharply defined peaks, to seeing whether it clings to the bowl or slips along its surface, whether its surface looks glossy or dry. All these tests tell us how crowded the air bubbles are, and how much lubrication they have between them in the way of liquid from the egg white. And different dishes will define an optimum foam differently. The lightening power of an egg foam depends not just on the foam’s volume, but also on how easily it can be mixed with other ingredients, and how well it can accommodate bubble expansion in the oven. Soufflés and cakes require the lubrication and expansion tolerance of a somewhat underbeaten foam, while in meringues and related pastries volume is less important than shape-holding stiffness.