Malt and Hop as Sources of Thiol S-Conjugates: Thiol-Releasing Property of Lager Yeast during Fermentation

The contribution of polyfunctional thiols (PFTs) to the overall flavor of a lager beer has been well documented, but their origin remains unclear. In comparison with Saccharomyces cerevisiae strains previously investigated (same conditions: 7 days at 24 °C, 3 days at 4 °C), we examined how Saccharomyces pastorianus yeasts are able to produce these PFTs from cysteinylated (Cys-) and glutathionylated (G-) conjugates. Up to 0.35% release was observed from G-conjugates against less than 0.08% for all yeasts studied so far. Lowering the wort nitrogen level and the fermentation temperature (12 °C) strongly increased the release efficiency and the ester/alcohol ratio from Cys-conjugates. However, it did not improve the release from G-conjugates and even prevented the yeast from producing any acetate. Although poor in free PFTs and their Cys-precursors, both Saaz hop and pale malt (classical ingredients for premium lager beers) confirmed to be significant sources of G-conjugates (especially those of 3-sulfanylhexanol and 3-sulfanyl-4-methylpentanol).