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

Abstract

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 βionone. 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.

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

“…This might be the first evidence of in situ γGT acting during the malting process . The high CysGly-3SHol content observed in wort by Molitor et al (109 μg/L in a 12°P wort) further suggests that γGT is still active during the mashing step …”

“…As already mentioned, many 3SHol S -conjugates have been identified in malt . First, their fates at room temperature (25 °C) were investigated here for 24 h in water.…”

“…As already mentioned, many 3SHol S-conjugates have been identified in malt. 21 First, their fates at room temperature (25 °C) were investigated here for 24 h in water. After only 1 h, the Pilsen malt wort (sample A, 3°EBC; malt/water ratio 1/ 10) already displayed a loss of 28% of the G-3SHol and an increase in CysGly-3SHol from 0.3 to 14.3 μg/L.…”

“…11 The high CysGly-3SHol content observed in wort by Molitor et al (109 μg/L in a 12°P wort) further suggests that γGT is still active during the mashing step. 21 In order to evidence potential actors capable of converting the different bound forms of PFTs to better yeast nutrients, the aim here was to identify all enzymatic and chemical interconversions of malt 3-sulfanylhexanol S-conjugates during mashing (these compounds having been identified as the most abundant and ubiquitous thiol precursors in all the studied malts). 11 In lab-scale mashing trials, the influences of the malt/ water ratio, the temperature applied, the sample color (as a result of the kilning process), and acidity were studied.…”

“…The resulting glutathionylated (G-) conjugate can be further metabolized through γ-glutamyltransferase (γGT) or carboxypeptidase activity to two intermediate dipeptide conjugates, the cysteinylglycine (CysGly-) and γ-glutamylcysteine (γGluCys-) conjugates (routes A and B in Figure , respectively), and ultimately to the cysteinylated (Cys-) conjugate. Although it was initially suggested that plant cells tend to use route B and animal and yeast cells essentially route A, − both dipeptides have been identified in plants such as grapes and hops. − Depending on the efficiency of the corresponding enzymatic activities, different proportions of all four S -conjugates are found in these matrices …”