Evidence of Enzymatic and Chemical Interconversions of Barley Malt 3-Sulfanylhexanol Conjugates during Mashing

Recent studies have highlighted in malt the occurrence of the glutathionylated precursor of 3-sulfanylhexanol (G-3SHol) at concentrations reaching hundreds of μg/kg. Here, SIDA-LC-MS/MS was used to investigate the potential conversion of G-3SHol to its dipeptide and cysteinyl analogues during mashing. At 45 and 55 °C, malt γGT and carboxypeptidase activities quickly degrade G-3SHol (up to 90% loss), first to the cysteinylglycine conjugate and then to the cysteine conjugate (up to 205% increase). No γ-glutamylcysteine S-conjugate formation is observed. At 80 °C, despite enzyme inactivation, the G-3SHol level decreases steadily because of suspected imine formation with wort aldehydes at pH 5.5. More surprisingly, CysGly-3SHol is still generated at 80 °C. This indicates the presence in the wort of as yet unidentified precursors.

Modulation of the Sulfanylalkyl Acetate/Alcohol Ratio and Free Thiol Release from Cysteinylated and/or Glutathionylated Sulfanylalkyl Alcohols in Beer under Different Fermentation Conditions

The occurrence of a substantial pool of cysteinylated and glutathionylated forms of polyfunctional thiols has been evidenced for several dual-purpose hop varieties, and so is the ability of Saccharomyces cerevisiae yeast to release free thiols from these forms through fermentation. The present work aimed to investigate the effect of temperature, wort density, maturation time, and strain on the efficiency of free thiol release by S. cerevisiae yeasts. Model media at 12, 15, or 17°P were spiked with three cysteinylated (Cys-) or three glutathionylated (G-) sulfanylalkyl alcohols (Cys- or G-3-sulfanylpentan-1-ol, 3-sulfanyl-4-methylpentan-1-ol, and 3-sulfanylhexan-1-ol), fermented for 7 days at 18, 24, and 28 °C, and kept at 4 °C for varying number of days. The released sulfanylalkyl alcohols and their corresponding acetates were extracted with a Ag-ion SPE cartridge and analyzed by gas chromatography-pulsed-flame photometric detection. The wort density and yeast strain greatly affected the acetate/alcohol ratio. This ratio varied from 1 to 80% according to the yeast strain and was at its highest at 17°P and 24 °C. Maturation appeared as the crucial step for free thiol excretion from yeast cells (no thiol was recovered in the fermented worts without maturation). Among the five yeasts tested, the yeast strain SafAle K-97 released the highest level of sulfanylalkyl alcohols into the medium (up to 0.45% of the added cysteinylated adducts and 0.08% of the glutathionylated adducts), whereas S-33 or S-04 should be preferred when release of esters is sought out (release efficiencies up to 0.35% from cysteinylated adducts and 0.02% from glutathionylated adducts are observed if both the alcohol and its acetate are considered).

Spotlight on release mechanisms of volatile thiols in beverages

Volatile thiols are very strong-smelling molecules that can impact the aroma of numerous beverages. Several thiols and thiol precursors have been reported previously in different plants used as raw material for beverages, some of which are fermented. We focused on thiols in beverages and their release mechanisms from precursors during processing. Volatile thiols in beverages can be classified aslow molecular weight volatile thiols (e.g. H₂S, methanethiol) which impact the smell negatively, and volatile thiols with higher boiling points that contribute positively to the aroma profile. The first part of this review is devoted to volatile thiols, without considering small malodorous molecules. The second part deals with thiol precursors and the different release mechanisms induced by processing (e.g. extraction, roasting or fermentation) and by the growing methods (e.g. viticulture), which can impact on amounts of thiols and their precursors.