Ability of Exogenous or Wort Endogenous Enzymes to Release Free Thiols from Hop Cysteinylated and Glutathionylated S-Conjugates

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.

Examining How the Fermentation Medium Influences Thiol Expression and Its Perceived Aroma in Commercial Brewing Yeast Strains

In Saccharomyces, the IRC7 gene encodes for a cysteine S-conjugate β-lyase enzyme which can release polyfunctional thiols from their cysteinylated precursor forms, thereby promoting thiol aroma in beer. This study examined the thiol production of 10 commercial yeast strains in two different media, a hopped yeast extract-peptone-dextrose (YPD) medium and a 100% barley malt wort to explore how differences in yeast strain and medium conditions influence the release of polyfunctional thiols. 3-Sulfanylhexan-1-ol was most affected by medium conditions, and its concentrations were highest in wort fermentations. The higher nitrogen content and pH of the YPD medium relative to the wort fermentations were notable differences, and significant correlations between these variables and the extent of free thiol production were observed. A strong association existed between polyfunctional thiol concentrations and the fermentation-derived, malt, and hop-derived compounds 2-phenylethanol, β-damascenone, and β-ionone. The sensory impressions of thiol character in beer were influenced by the presence of other aromatic compounds such as esters and terpene alcohols, and aroma attributes such as "tropical" were not the most suitable for describing beers brewed with yeasts that fully express homozygous IRC7^F. Sensory attributes "sweaty", "vegetal", and "overripe fruit" were more strongly associated with these strains.

Fruits of their labour: biotransformation reactions of yeasts during brewery fermentation

Abstract

There is a growing appreciation for the role that yeast play in biotransformation of flavour compounds during beverage fermentations. This is particularly the case for brewing due to the continued popularity of aromatic beers produced via the dry-hopping process. Here, we review the current literature pertaining to biotransformation reactions mediated by fermentative yeasts. These reactions are diverse and include the liberation of thiols from cysteine or glutathione-bound adducts, as well as the release of glycosidically bound terpene alcohols. These changes serve generally to increase the fruit and floral aromas in beverages. This is particularly the case for the thiol compounds released via yeast β-lyase activity due to their low flavour thresholds. The role of yeast β-glucosidases in increasing terpene alcohols is less clear, at least with respect to fermentation of brewer’s wort. Yeast acetyl transferase and acetate esterase also have an impact on the quality and perceptibility of flavour compounds. Isomerization and reduction reactions, e.g. the conversion of geraniol (rose) to β-citronellol (citrus), also have potential to alter significantly flavour profiles. A greater understanding of biotransformation reactions is expected to not only facilitate greater control of beverage flavour profiles, but also to allow for more efficient exploitation of raw materials and thereby greater process sustainability.

Key points

• Yeast can alter and boost grape- and hop-derived flavour compounds in wine and beer

• β-lyase activity can release fruit-flavoured thiols with low flavour thresholds

• Floral and citrus-flavoured terpene alcohols can be released or interconverted