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It is well known that sulfur compounds in beer make significant contributions to flavor and aroma. Sulfite plays an important role in flavor stability. Although breeding of bottom‐fermenting yeast strains that produce high levels of SO2 is desirable, it is complicated by the fact that undesirable H2S is produced as an intermediate in the same pathway. We developed a high SO2‐producing bottom‐fermenting yeast strain by integrated metabolome and transcriptome analysis. This analysis revealed that Oacetylhomoserine (OAH) is the rate‐limiting factor for production of SO2 and H2S.
Appropriate genetic modifications were then introduced into a prototype strain to increase metabolic fluxes from aspartate to OAH and from sulfate to SO2, resulting in high SO2 and low H2S production. Spontaneous mutants of an industrial strain resistant to both methionine and threonine analogs were then analyzed for similar metabolic fluxes.
One promising mutant produced much higher levels of SO2 than the parent, but parental levels of H2S. Furthermore, on the other application of CE‐MS to a bottom‐fermenting yeast, we investigated the function of bottom‐fermenting yeast‐specific (BFY) genes that have no significant homology with sequences in the budding yeast. One of the BFY genes, AMI1 encodes a protein with homology to an amidase conserved among plants and fungi.
By metabolome analysis of intracellular compounds followed by differential analysis, we found that the amount of histidine and arginine is increased, and the amount of threonine, lysine, and nicotinic acid is decreased in the Ami1p‐overproducing strain as compared with the control. This suggests that Ami1p may hydrolyze some amides related to amino acid and niacin metabolism in the cell. These results indicate that metabolome analysis is powerful approach to understand the mechanism of cellular metabolisms and breed strains with preferable characteristics for industrial use.