Episode 195: Thiol-releasing Capability of Brewers Yeast

It is well known that hop-forward beer styles exhibit tropical fruit aromas. These aromas are conferred by a range of hop-related volatile compounds, including polyfunctional thiols such as 3-sulfanylhexan-1-ol (3SH) and 4-methyl-4-sulfanylpentan-2-one (4MSP). These compounds are present at relatively high concentrations in hops and are extracted into beer during dry-hopping. Polyfunctional thiols are also abundant as non-volatile glutathione and cysteine conjugates in hops, and to a lesser extent can be found in barley (and therefore, malt). These conjugates are cleaved into amino acids and free thiols by enzymatic activity of yeasts, thus bound precursors may represent an important pool of tropical fruit flavor in beer, particularly in beer styles where dry hopping is minimal. In order to successfully liberate this pool of flavor, yeast with strong carbon-sulfur ß-lyase activity are needed. In wine research it has become evident that only a small number of commercial Saccharomyces cerevisiae starter cultures possess this capacity. High-activity strains may release 10-30x higher concentrations of free thiol from the same amount of available precursor, relative to low-activity strains. This variation has been linked to a range of inactivating mutations in the carbon-sulfur ß-lyase encoding gene, IRC7. In this study we have catalogued several additional mutations in IRC7 that are found in brewing strains of S. cerevisiae. Based upon known mutations that affect wine strains we compare predicted IRC7 activity for various brewing strains, and actual measurements using a model substrate. Furthermore, we show that one of the mutations only found in brewing strains inactivates IRC7. Overall, the data show that brewing S. cerevisiae strains vary widely in their potential to release polyfunctional thiols from conjugated precursors, and that efforts to extract maximum flavor from malt and hops should include consideration of yeast strain.

Special Guests: Chris Curtin and Karen Fortmann.