The physics and chemistry of beer foam: a review

The assurance of a stable and appealing foam on beer requires an understanding and application of several physical and chemical principles and these are reviewed in this paper. In terms of physics, it is essential that the foam is produced efficiently, the presence of nucleation sites being important, with major determining factors being temperature and carbonation level. The principle driving force for foam instability is disproportionation, wherein bubble collapse is caused by the passage of carbon dioxide from smaller to larger bubbles. As nitrogen does not readily make this passage, this gas allows for much more stable foams. The stability of foam depends on the balance of foam-positive foaming components (polypeptides, hop bitter acids, metal ions, melanoidins) over foam-negative entities (ethanol, lipids, detergents). The principal foam-stabilizing proteins contributed by malted barley are Protein Z4 and Lipid Transfer Protein (LTP1), but the fragments of hordein produced by proteolysis in malting and perhaps mashing have a greater ability to enter into the foam but display less ability to stabilize the bubbles. They compete with Protein Z and LTP1 which display less foamability but greater foam stability. Carbohydrate complexed with protein may have a significant role to play, and this is potentially the reason for the benefits to foam afforded by wheat. Kinetic models analogous to those employed in the study of enzyme kinetics are useful in terms of interpreting foaming systems, as are model systems.