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

Abstract

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.

Microbial β C-S Lyases: Enzymes with Multifaceted Roles in Flavor Generation

“…These precursors are actually present in various plant-based foods additionally to Allium species such as Brassica (including cauliflower and broccoli), asparagus, bell peppers, and fruits (including passion fruit, guava, and durian). They are also present in beverages like tea, coffee, wine, sake, and beer due to their presence both in hop and in some malts [ 16 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ]. Cysteine conjugates can be formed during cooking through Maillard reactions between cysteine and sugars.…”

“…Concerning malts, evidence was shown of enzymatic and chemical interconversions between 3-sulfanyl-hexanol conjugates during mashing [ 62 ]. In addition to cysteine S-conjugates, other sulfur precursors exist as glutathione, γ-glutamyl-cysteine, and cysteine-glycine states, while their biotransformation by β-CSLs is, to our knowledge, not well documented.…”

“…While the release of polyfunctional thiols is desirable for improving beer flavor, it has been shown that the presence of polyphenols and an acidic environment can hamper the activity of C-S lyase during fermentation [ 84 ]. Current research aims to address these challenges and determine optimized processes for increasing thiol compounds in beer [ 62 ]. Alternative methods include adding fermented beer powder made from Sauvignon Blanc grape skins rich in precursors (e.g., Phantasm™ Powder).…”