A Parallel Analysis of H2S and SO2 Formation by Brewing Yeast in Response to Sulfur-Containing Amino Acids and Ammonium Ions

Abstract

Volatile sulfur compounds (H2S and SO2) have a profound effect on the sensory quality of beers, but are one of the more difficult variables to control, even in modern breweries, because of the microvariation in malt quality, yeast strain, and fermentation conditions. Research into H2S production of yeast has been relatively extensive compared with that of SO2. A high through-put method was developed that could detect the formation of these compounds in the industrial setting. This technique allowed for the screening of industrial yeast strains for their H2S and SO2 formation profiles and evaluation of fermentation conditions. An investigation into the relationship between H2S and SO2 production showed that both compounds were produced in greater quantities by yeast when grown in the presence of increasing concentrations of cysteine. Methionine repressed the cysteine-induced increase in the H2S production but had no effect on the formation of SO2. Differences were also seen in H2S compared with SO2 production in response to nitrogen levels in wort. Previous observations were confirmed that nitrogen starvation increased the H2S production, but our results showed that the SO2 production was not affected by this condition. In fact, whereas increasing nitrogen levels relieved cysteine-induced H2S production, these higher nitrogen levels actually increased the amount of SO2 produced by yeast. We have shown that, although biochemically H2S and SO2 production are closely linked, environmental conditions can have different effects on their rate of formation. These results provide insights into possible opportunities to modulate the levels of H2S and SO2 in industrial fermentations.