By Harold McGee
Foam stability depends on the presence in the bubble walls of emulsifier molecules with water-loving and water-avoiding ends; the water-avoiding ends project into the gas while the water-loving ends stay in the liquid, and thus reinforce the gas-liquid interface. In beer, these molecules are mostly medium-sized proteins that come from the malt or from cereal adjuncts, whose proteins are more intact than malt’s and significantly improve head stability. Hop acids also contribute to foam stability, and become concentrated enough in the foam to make it noticeably more bitter than the liquid beneath. Cool-fermented lagers generally give more persistent foams than warm-fermented ales because the latter contain more foam-destabilizing higher alcohols from yeast metabolism.