Proteins, polypeptides and amino acids

The presence of polypeptide material in beer is important for the contribution it makes to foam (Bamforth 1985b). In the processes of malting and brewing, the native proteins of barley undergo considerable degradation and denaturation, such that those present in the nished beer bear little resemblance to those found in the barley kernel. While polypeptides can be bene cial for foaming, they are detrimental in another respect: they can crosslink with polyphenols to form hazes (McMurrough & Delcour 1994).
The amino acids in beer provide no real bene t to the beer. If present in excess, they potentiate infection of a product by acting as nitrogen sources for spoilage microorgan­isms. This is why brewers seek to optimise the level of amino acids in wort, so that the yeast uses up all that is readily assimilable.
Lipids
Barley contains about 3% w/w lipid, most of it congregated in the living tissues (embryo and aleurone) (Anness & Reed 1985). Very little lipid, however, survives into beer, making this beverage essentially a fat-free food. This is just as well, from an aesthetic point of view, because lipids are very bad news for beer foam (Bamforth 1985b).
The other adverse in uence of lipids is through their ability to act as precursors of stale avours in beer (Drost et al. 1971). The unsaturated fatty acids, such as linoleic acid, may get a good press for their health-giving properties; however, they can be oxidised, ultimately to yield carbonyl compounds that afford aged character to beer. For this reason many brewers try to ensure that as little lipid as possible survives the brewing process and therefore they are meticulous about eliminating solid material at all stages, because the insoluble lipid associates with solids.