A SCIENTIFIC BASIS FOR BEER FOAM FORMATION AND CLING

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Abstract
The most complex types of nitrogen compound in beer (here termed “proteoses”) are, when not combined with polyphenols, highly surface active and hence concentrate in the foam. With unhopped beer, the foam is of the “liquid-viscous” type, which eventually drains and collapses leaving no residual solids. The iso-α-acids in hopped beers are also surface-active, so they too are concentrated in the foam. At the corresponding concentration in bulk solution the solubility limits of some isohumulates* are exceeded. Thus we believe that solids are similarly formed in the bubble films, so that these are reinforced and stiffened to give “cling”. One consequence of the increased concentration of iso-α-acids in the foam is that there can be precipitation of iron, nickel, cobalt and zinc isohumulates. This results in concentration of these metals in foam. The most significant finding is that the concentration of both proteoses and iso-α-acids in the foam exceeds the bulk solubility limits and results in the formation of proteose-isohumulate salts which make the main contribution to the solid reinforcement of bubble films. These findings make it possible to explain satisfactorily “lacing” or “cling” and also isohumulone losses (a) during boiling and trub separation, (b) during post-fermentation bittering, and (c) during any subsequent foam formation. An explanation is also provided why losses of bitterness and of head retention are linked. Consequently, linked recovery of both bitterness and head retention can occur, as has been found is some recent fermentation improvements, such as continuous fermentation.

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