Which substance is known to stabilize an egg white foam?

An acid is indeed known to stabilize egg white foam. When egg whites are beaten, the proteins unfold and then create a network that traps air, forming a stable foam. The addition of an acid, such as cream of tartar or lemon juice, can help this process by lowering the pH of the egg whites, which strengthens the protein structure and promotes the stability of the foam. This enhancement is crucial during baking and cooking, as it allows the foam to maintain its volume and aeration, yielding better textures in meringues, soufflés, and angel food cakes.

In contrast, water is not effective in stabilizing egg white foam; it can actually dilute the proteins and hinder their ability to form a stable structure. Flour is primarily used for providing structure in baked goods, but it does not play a role in stabilizing egg whites. Milk contains water and fats that can interfere with the formation of a good foam and is not typically used in this context either. Thus, the role of an acid in strengthening the protein structure of egg whites is key to achieving a stable and well-aerated foam.