The generation and stabilization of a foam is subject to the same thermodynamic energy considerations as emulsions; lowering the interfacial (surface) tension favors foam formation. Small surfactant molecules dissolved in the aqueous phase will promote foaming, and the stability of the foam is dependent on the maintenance of a film of water between air bubbles. The lipophilic portion of the surfactant enters the gas phase (rather than an oil phase) but in all other respects the situation is analogous to that where both phases are liquid (ie, emulsions).
Proteins are amphiphilic molecules in that many amino acids (eg, leucine, isoleucine, and valine) contain hydrophobic (lipophilic) side chains while others (glutamic, as-partic, lysine, and arginine) have ionic, hence hydrophilic, side chains. Normally, proteins such as egg albumin in solution are folded in such a way that the hydrophobic side chains are buried in the interior of the molecule in a nonpolar environment, while the hydrophilic side chains are on the surface of the molecule and interact with the polar aqueous environment. Introduction of air bubbles into the solution presents a new possibility for the lowest energy state of the protein, namely for it to unfold with the hydrophobic side chains entering the air phase and the hydrophilic chains remaining in the water phase. The portion of the proteins located in the aqueous phase hold water, preventing it from draining away from this region, hence stabilizing the foam. Whipping aids enhance the ability of the protein to unfold at the air-water interface; the energetics of protein unfolding become more favorable and the ease of foam formation increases. Solutes that increase the viscosity of the water phase, for example, gums, slow the rate of draining and thus increase foam stability.
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