Harold McGee

Milk Foams

Appears in
On Food and Cooking
On Food and Cooking

By Harold McGee

Published 2004

  • About
A foam is a portion of liquid filled with air bubbles, a moist, light mass that holds its shape. A meringue is a foam of egg whites, and whipped cream is a foam of cream. Milk foams are more fragile than egg foams and whipped cream, and are generally made immediately before serving, usually as a topping for coffee drinks. They prevent a skin from forming on the drink, and keep it hot by insulating it and preventing evaporative cooling.

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.