Yeast genes involved in sulfur and nitrogen metabolism affect the production of volatile thiols from Sauvignon Blanc musts

Abstract

Two volatile thiols, 3-mercaptohexan-1-ol (3MH), and 3-mercaptohexyl-acetate (3MHA), reminiscent of grapefruit and passion fruit respectively, are critical varietal aroma compounds in Sauvignon Blanc (SB) wines. These aromatic thiols are not present in the grape juice but are synthesized and released by the yeast during alcoholic fermentation. Single deletion mutants of 67 candidate genes in a laboratory strain of Saccharomyces cerevisiae were screened using gas chromatography mass spectrometry for their thiol production after fermentation of SB grape juice. None of the deletions abolished production of the two volatile thiols. However, deletion of 17 genes caused increases or decreases in production by as much as twofold. These 17 genes, mostly related to sulfur and nitrogen metabolism in yeast, may act by altering the regulation of the pathway(s) of thiol production or altering substrate supply. Deleting subsets of these genes in a wine yeast strain gave similar results to the laboratory strain for sulfur pathway genes but showed strain differences for genes involved in nitrogen metabolism. The addition of two nitrogen sources, urea and di-ammonium phosphate, as well as two sulfur compounds, cysteine and S-ethyl-L-cysteine, increased 3MH and 3MHA concentrations in the final wines. Collectively these results suggest that sulfur and nitrogen metabolism are important in regulating the synthesis of 3MH and 3MHA during yeast fermentation of grape juice.

Inactivating Mutations in Irc7p Are Common in Wine Yeasts, Attenuating Carbon-Sulfur β-Lyase Activity and Volatile Sulfur Compound Production

“…It is important to note that most functional studies have been conducted by deletion of IRC7 S in different S. cerevisiae laboratory strain backgrounds (33)(34)(35). Even though we, and others, have shown that the Irc7p S has no activity toward substrates such as cysteine or Cys-4-MMP, the truncated protein and/or the genomic locus itself may be functionally important in yeast.…”

“…The biological role of IRC7 is not well established, as many functional studies have been conducted in strain backgrounds containing the inactive version of IRC7 (33)(34)(35). While IRC7 was originally suggested to be involved in DNA metabolism since the null mutant causes increased rates of recombination between homologous chromosomes (33), its role as cysteine desulfhydrase also points out to an involvement in the regulation of cysteine homeostasis (32).…”

Impact of Nutrient Availability on the Fermentation and Production of Aroma Compounds Under Sequential Inoculation With M. pulcherrima and S. cerevisiae

“…Nitrogen availability during fermentation has an impact on the release of varietal thiols, but the effect is not well characterized. Thus, opposite effects of nitrogen addition (as ammonium) have been reported, either increasing (Harsch and Gardner, 2013) or decreasing the release of thiols (Subileau et al, 2008) depending on the study.…”

Thermal resistance of Saccharomyces yeast ascospores in beers

“…In addition the following strains from the School of Biology Sciences of the University of Auckland were used because of their good sporulation: BC186 (natural isolate from oak trees), SK1 (= NCYC 3265, lab strain isolated from soil; Liti et al, 2009), Zymaflore F15 (commercial wine yeast; Harsch and Gardner, 2013), and Lesaffre Ethanol Red ® (industrial fermentation).…”