Interactions between polypeptides derived from barley and other beer components in model foam systems

The impact of a range of variables on the foaming properties of hordein- and albumin-derived fractions from barley has been investigated. When foamed using nitrogen, the hordein-derived polypeptides produce the most stable foams. However, when carbon dioxide was the foaming agent, the previously reported observations were repeated, namely that proteolysis weakens the foaming capabilities of albumin, whereas partial hydrolysis enhances the stability of foams derived from hordein. At lower levels of bitterness (iso-(alpha)-acid) addition there is a reduction in foam stability of albumin-derived polypeptides, although hordein-derived polypeptides can withstand this destabilising effect. Higher bitterness levels in both instances lead to a stabilisation of foam. It is likely that this involves ionic interactions between the acid anion (pK of iso-(alpha)-acids is ca 3.1) and divalent cations, because higher pHs (in the range 3.8-4.6) and the addition of zinc lead to an enhancement of foam stability, particularly of the albumin-derived fractions. The foaming of the latter, however, is significantly impaired by the presence of ethanol, whereas hordein-based foams are enhanced by increasing concentrations of ethanol.