Polyfunctional thiols are known to have a strong impact on the overall aroma of many fermented foods. Surprisingly, very little data is available on their occurrence in beer. A specific extraction with p-hydroxymercuribenzoic acid was performed on four different fresh light-protected lager beers. gas chromatography-olfactometry, gas chromatography-mass spectrometry, and gas chromatography-pulsed-flame photometer detector analyses of the extracts revealed the presence of more than 10 polyfunctional thiols. All of them were absent from wort, suggesting a key role of the H(2)S excreted by yeasts. 3-Methyl-2-buten-1-thiol, 2-mercapto-3-methylbutanol, 3-mercapto-3-methylbutanol seem to be created from hop allylic alcohols via four different mechanisms: nucleophilic substitution, addition-elimination, and radical anti-Markovnikov or electrophilic Markovnikov additions. 1,4 Addition of hydrogen sulfide to wort alpha,beta-unsaturated aldehydes or ketones may explain the synthesis of 1-mercapto-3-pentanol, 3-mercaptohexanol, and 4-mercapto-4-methyl-2-pentanone through fermentation. Finally, 2-mercaptoethanol, 3-mercaptopropanol, and their corresponding acetates may derive from Ehrlich degradation of sulfur amino acids, while 2-methyl-3-furanthiol should be logically issued from Maillard reactions.

Polyfunctional thiols are contributors to the hop varietal aroma of beer. Besides free thiols, a cysteine-S-conjugate has recently been shown to be an additional component of the thiol potential of hop. Such cysteine adducts investigated here in four hop cultivars and in different hop forms. Hop hydroalcoholic extracts were purified on a cation exchanger and subjected to apotryptophanase β-lyase activity. The Cascade hop variety exhibited the highest bound 3-sulphanylhexan-1-ol (grapefruit-like) potential, while both Tomahawk and Nelson Sauvin cultivars were confirmed to be important sources of bound 3-methyl-2-butene-1-thiol (skunky-like), 3-sulphanylpentan-1-ol and 4-sulphanyl-4-methylpentan-2-one (box-tree-like). Surprisingly, hop CO 2 extracts proved to contain cysteine conjugates. Although related, the concentrations of cysteine-bound thiols in hop are not strictly correlated to the amounts of free volatiles found in the derived beers.

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Sulphanylalkyl alcohols and their corresponding acetates were investigated in 14 commercial Belgian beers. Although it was the major peak at the pulsed-flame photometric detector, the empyreumatic 2-sulphanylethyl acetate was found at concentrations below its individual odour threshold, estimated at 40 mg/L (0-4 mg/L in most fresh beers, 5-12 mg/L in three fresh high-bitter beers). Both the Ehrlich pathway and hop constituents contribute to this content. In 11 of the investigated samples, synthesis of 2SE-A and 3-sulphanylpropyl acetate (roasted, burned) continued during the first three months of storage. Although below their individual thresholds, these compounds might interact by synergy with other aged flavours. As yeast was absent from most of the investigated bottles, chemical degradation of precursors is suspected.

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