By Harold McGee
Milk owes its foaming power to its proteins, which collect in a thin layer around the pockets of air, isolate them, and prevent the water’s strong cohesive forces from popping the bubbles. Egg foams are also stabilized by proteins, while the foam formed by whipping cream is stabilized by fat. Milk foams are more fragile and short-lived than egg foams because milk’s proteins are sparse—just 3% of the milk’s weight, where egg white is 10% protein—and two-thirds of the milk proteins are resistant to being unfolded and coagulated into a solid network, while most of the egg proteins readily do so. However, heat around 160°F/70°C does unfold the whey proteins (barely 1% of milk’s weight). And if they unfold at the air-water boundary of a bubble wall, then the force imbalance does cause the proteins to bond to each other and briefly stabilize the foam.