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On the Behaviour of Dimethyl Sulfoxide in the Brewing Process and its Role as Dimethyl Sulfide Precursor in Beer
M. Baldus, R. Klie, M. Biermann, P. Kreuschner, M. Hutzler and F.-J. Methner

Dimethyl sulfide (DMS) has a considerable impact on the aroma of beer and may lead to undesirable flavour impressions. The undoubdted role of S-methyl methionine (SMM), as the thermal precursor of DMS has been elaborately elucidated in the literature. DMS can also be generated via reduction of dimethyl sulfoxide (DMSO) during fermentation. However, there are uncertainties regarding the role of DMSO as DMS precursor and its contribution to final DMS levels in beer. The behaviour of DMSO in the brewing process has not been investigated in detail. Also, the extent to which different yeast strains reduce DMSO is mostly unknown. In this work the behaviour of DMS and its precursor SMM and DMSO was investigated throughout the brewing process. The main focus was to ascertain DMSO reduction during fermentation by lager and top-fermenting yeast. During mashing, SMM and DMSO were extracted rapidly owing to their high water solubility, whereas SMM was extracted faster. In the further course of mashing SMM and DMSO levels remained approximately constant. DMS was found to be evaporated steadily in open mashing systems and was not subject of significant oxidation to DMSO, even in a closed mashing sytem. During wort boiling SMM was degraded in a 1st order mechanism (k=0.021 min-1) whereas the generated DMS was evaporated subsequently. The levels of DMSO increased linearly with increasing evaporation of water but where not affected when boiling was conducted with a rectification column. During wort heat holding in hermetically closed systems ~15 % of the accumulated DMS was oxidized to DMSO. During fermentation significant DMS formation was observed. DMSO reduction was higher in top fermenting Saccharomyces cerevisiae yeast (TUM 149) than in Saccharomyces pastorianus lager yeast (TUM 34/70) but was not correlated with genetic yeast diversity (domestication clusters). This work demonstrates that DMSO reduction during fermentation significantly contributed to the levels of DMS in beer. Therefore we suggest that DMSO should be recognized and assessed as DMS precursor by maltsters and brewers.

Descriptors: dimethyl sulfide, dimethyl sulfoxide, S-methyl methionine, DMS precursor, fermentation, Saccharomyces pastorianus/cerevisiae

BrewingScience, 71 (January/February 2018), pp. 1-11