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 IRC7F. Sensory attributes "sweaty", "vegetal", and "overripe fruit" were more strongly associated with these strains.
Thiols are a group of highly potent aroma compounds found in many Vitis vinifera wines. In berries, thiols occur as precursors formed from a thiol attached to an amino moiety (e.g., cysteine and glutathione). Foliar sulfur and urea supplementation has been found to increase thiol precursors in V. vinifera berries. However, such data are still scarce for interspecific hybrid Vitis varieties, despite a growing interest in these cultivars. In this study, eight thiol precursors, 21 amino acids and glutathione (GSH) of seven white hybrid Vitis varieties collected in Quebec, Canada, were quantified by liquid chromatography with tandem mass spectrometry. Then, the impact of foliar sulfur and urea supplementation on thiol precursors of fully mature berries was tested on the Vitis sp. Vidal, using a complete randomised block design. Among the analysed thiol precursors, only the precursors of 3-mercaptohexan-1-ol (3MH) were detected, with glutathione-3MH (20-711 μg/kg berry fresh weight) as the main precursor in most studied varieties. Vidal samples had the highest concentrations of glutathione-3MH (313 μg/kg berry fresh weight) and GSH (17 mg/L in juice), but a large variability was found among the biological replicates. The amino acids proline, arginine, glutamine and alanine were the main amino acids found in Vidal, Frontenac blanc, Seyval blanc and Saint-Pepin respectively. Correlation analysis showed no significant relationship between thiol precursor and glutathione/amino acid content in berries. Foliar urea and sulfur supplementation had no impact on thiol precursor concentration in Vidal. However, a trend in a higher content of thiol precursors in berries treated with urea and sulfur compared to the control was observed on one of the two experimental sites.
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