In vitro studies on the main beer protein Z4 of Hordeum vulgare concerning heat stability, protease inhibition and gushing

Protein Z4, the most abundant form of serpin protein Z of Hordeum vulgare, is one of the major constituents of beer foam and inhibits the activity of serine proteases. The possible influence of protein Z4 on malt proteases, which impact malt quality, is of interest for brewers. In addition, the persistence of Z4 in the brewing process and the ensuing presence of Z4 in beer matters to brewers because of its impact on foam. In order to analyse the influence of Z4 on protease inhibition and beer gushing, Pichia pastoris cells were heterologously transformed with the Z4 coding gene and the protein was recovered from the supernatant of a transformant's liquid culture. Agar diffusion assays showed that the recombinant Z4 protein had an inhibitory effect on proteases present in barley malt. Heat denaturation of the protein impaired protease inhibition and revealed degradation of the structure of Z4. The effect of Z4 on hydrophobin-induced gushing was analysed by addition of the protein to beers pre-treated with the class 2 hydrophobin FcHyd5p. Results demonstrated that addition of protein Z4 to gushing beer reduces the overflow volume considerably. Heat treatment again had a negative impact on the gushing reduction capacity of Z4. Copyright (c) 2014 The Institute of Brewing & Distilling