Résumé
Polyfunctional thiols (PFTs) present in beers or in wines are known to be released by yeast, during fermentation, from bound forms originally found in raw materials. In the brewing field, huge amounts of S-conjugates have been evidenced in several dual-purpose hop varieties. Malt, however, being the major raw material of beer, could also be a significant contributor of PFTs to beer (cysteinylated, Cys- and glutathionylated, G- precursors of 3SHol already identified in a few samples of barley and malt).A large panel of barley malts from 2 to 1500 EBC and 5 other malted cereals were here screened to characterize their thiol precursor profile (SIDA-LC-MS/MS quantitation of G- and Cys- as well as dipeptidic bound CysGly- and γGluCys- forms of 3SHol). First, we confirmed that G-3SHol was ubiquitous reaching up to 320 µg/kg in some samples, whereas Cys-3SHol remained at trace level up to 13 µg/kg. Moreover, for the first time, dipeptidic bound forms of 3SHol were evidenced in malt (up to 10 and 11 µg/kg for CysGly-3SHol and γGluCys-3SHol, respectively). In pale malts, the level of the CysGly- form was shown to be proportional to that of G-3SHol (in situ γGT during the malting process). This appeared to be no longer true for special malts (ranging from 5 to 45 °EBC), whose CysGly-3SHol level correlated instead with Cys-3SHol (suspected thermal conversion from the dipeptide conjugate). As for γGluCys-3SHol, it was only found in the special barley malts (indicating another thermal conversion from G-3SHol) and in malted rye, spelt and wheat. No precursors at all were found in roasted malt.The fate of S-conjugates (enzymatic and chemical interconversions) through laboratory-scale mashing trials was then evaluated as a function of malt-water ratio and applied temperature. A two-step catabolic pathway from G-3SHol to, first, CysGly-3SHol, then to Cys-3SHol (up to 100 µg/kg) was evidenced at room temperature. The kinetic slowed down for lower malt-water ratio while more CysGly-3SHol was measured at 55 °C (better γGT activity suspected). Even at 80 °C, the dipeptide is still regenerated, suggesting in situ formation from LOX-derived hexenal.