By Harold McGee
Unlike the protein foams of egg white, egg yolk, and milk, the cream foam is stabilized by fat. Initially, the whisk introduces short-lived air bubbles into the cream. After the first half-minute or so, the bubble walls begin to be stabilized by the de stabilization of the fat globules. As the globules are knocked all around and into each other by the whipping, parts of their protective membranes are stripped away by the shearing action of the whisk, and by the force imbalance in the air bubble walls. The patches of naked fat, which by their nature avoid contact with water, settle in one of two regions in the cream: either facing the air pocket in the bubble walls, or stuck to a patch of naked fat on another globule. The fat globules thus form walls around the air bubbles, and connections between neighboring walls: and so a continuous network develops. This network of solid fat spheres not only holds the air bubbles in place, but also prevents the intervening pockets of fluid from moving very far. And so the foam as a whole takes on a definite, persistent structure.