Unusual Profile of Thiol Precursors in Special Malts: First Evidence of Chemical Glutathione-/γGluCys- and CysGly-/Cys- Conversions

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). Forty-two barley malts from 2 to 1500 EBC and five other malted cereals were screened to characterize their thiol precursor profile (G- and Cys- as well as dipeptidic bound CysGly- and gamma GluCys- forms of 3SHol). First, it was confirmed that G-3SHol was ubiquitous reaching up to 320 mu g/kg in some samples, whereas Cys-3SHol remained at a trace level of up to 13 mu g/ kg. Moreover, for the first time, dipeptidic bound forms of 3SHol were evidenced in malt (up to 10 and 11 mu g/kg for CysGly-3SHol and gamma GluCys-3SHol, respectively). In pale malts, the level of the CysGly- form was shown to be proportional to that of G-3SHol (in situ gamma GT during the malting process). This appeared to be no longer true for special malts (ranging from 5 to 45 degrees EBC), whose CysGly-3SHol level correlated instead with Cys-3SHol (suspected chemical conversion from the dipeptide conjugate). As for gamma GluCys-3SHol, it was only found in the special barley malts (indicating another chemical break of the Cys-Gly bond, here on G-3SHol) and in malted rye, spelt, and wheat. No precursors were found in roasted malt.

Automated summary

Polyfunctional thiols (PFTs) are released by yeast during fermentation, and both beers and wines contain these compounds. This study focused on the thiol precursors present in barley malt and other malted cereals. The results showed that G-3SHol was commonly found in all samples, while Cys-3SHol was only present in trace amounts. Dipeptidic bound forms of 3SHol were also identified for the first time in malt. The findings provide valuable insights into the contribution of malt to the thiol content in beer. Rating: 8/10.