Sulfate Metabolism during Fermentation

Overall sulfur balance and sulfate metabolism during fermentation of brewery wort by ale and lager yeast have been studied by [35S]sulfate labeling experiments. Ale yeast derived 40 and 27% of the total sulfur assimilated during stirred flask and unstirred column fermentations, respectively, from wort sulfate. The corresponding values for lager yeast were 41 and 21%. The contributions of sulfate to H2S evolved in the fermentor gases, and to medium SO2 formed during fermentation, have been examined. Ale fermentations produced < μg S of fermentor gas H2S/I. medium and <0.2 μg S of medium SO2/I. medium. Stirred flask lager fermentations evolved an average of 62 μg H2S/I. of medium, 72% of which was derived from sulfate. Sulfate-derived H2S constituted 64% of the total H2S evolved during unstirred column fermentations (130 μg H2S/I. medium). The effects of individually added amino acids (L-methionine, L-threonine, and L-cysteine) and heavy metals (Fe, Cu, and Zn) have been studied and are discussed in terms of literature results and yeast sulfur metabolism. The total bound SO2 levels averaged 0.17 μgS/g and 11 μgS/g in beer from stirred flask and unstirred column lager fermentations, respectively. Radiochemical tracer experiments showed that all of the SO2 formed in the latter experiments was derived from wort sulfate.