Abstract
Hop is known to bring methanesulfenic acid to wort, the precursor of an onion-like odorant in beer: dimethyltrisulfide. Recently, 2-mercapto-3-methylbutanol was also suspected to be responsible of some onion off-flavors in beer. In model media, 2-mercapto-3-methylbutanol, 3-mercapto-3-methylbutanol and 3-methyl-2-butene-1-thiol were shown to be synthesized from 3-methyl-2-buten-1-ol and hydrogen sulfide (radical anti-Markovnikov addition, electrophilic Markovnikov addition and nucleophilic substitution, respectively). The present work shows that these mechanisms also occur in pilot-scale beer productions by the presence of hydrogen sulfide excreted by yeast. 1.8 μg/L of 2-mercapto-3-methylbutanol was detected after fermentation of a wort spiked with 10 mg/L of 3-methyl-2-buten-1-ol. A strong onion-off-flavor was also detected in the fresh beer. Sensorial analyses were performed on purified 2-mercapto-3-methylbutanol (radical addition favored by adding hydrogen peroxide and catalytic traces of FeS). Its typical freshly-cut onion flavor revealed very different from the “onion soup” descriptor currently used to describe dimethyltrisulfide.
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