foaming properties of proteins isolated from barley

Albumin and hordein protein fractions have been isolated from barley by extraction with salt and aqueous ethanol respectively. Both produce stable foams as assessed by a shaking procedure, and in each case foaming was enhanced by denaturation of the proteins. Foam power was increased rather more for albumin than for hordein, especially by heat, and this was accompanied by an increase in hydrophobic character as assessed by the fluorochrome 1-anilino-8-napthalenesulphonic acid. Iso-α-acids and reduced iso-α-acids increased the foam stability of both albumin and hordein, with there being a proportionately greater increase for the undenatured proteins. Albumin is resistant to hydrolysis by the cysteine proteinases ficin and papain but is hydrolysed by the serine proteinase trypsin. In the latter instance, hydrolysis is accompanied by a loss of foam stability. By contrast, hordein is resistant to digestion by trypsin but is susceptible to the action of ficin and papain. In these instances, limited proteolysis leads to a substantial enhancement of foam stability from hordein, though prolonged proteolysis is detrimental. Proteinase A from yeast hydrolyses both protein classes and lowers their ability to give stable foams. It is concluded that polypeptides derived from both albumin and hordein could make a contribution to the foam potential of beer.