Validation of Bismuth-Containing Indicator Media for Predicting H₂S-Producing Potential of Saccharomyces cerevisiae Wine Yeasts Under Enological Conditions

Abstract

Hydrogen sulfide (H₂S) production by strains of Saccharomyces cerevisiae was compared using bismuth-containing indicator media and fermentations in model grape juice. Discoloration of yeast colonies on indicator media resulting from the production of H₂S was essentially independent of nitrogen, sulfite, and glucose levels or the availability of oxygen. Discoloration on bismuth-containing test media did not correlate with H₂S production in a complete chemically-defined grape juice medium in which only trace amounts of H₂S were produced. By comparison, discoloration on bismuth-containing test media correlated with H₂S production developing after nitrogen depletion in a nitrogen-limited liquid medium. It is proposed, therefore, that intensity of discoloration on bismuth-containing indicator media is an indication of the maximum genetically determined sulfite reductase activity for a given strain. While a high sulfite reductase activity does not necessarily predispose a strain to excessive H₂S production in complete media, the indicator-media test does appear to predict a high degree of H₂S production following nitrogen starvation in liquid media. This screening protocol is therefore valid for assigning the H₂S-producing potential of a strain under permissive conditions. In this way, strains identified as high H₂S producers can be avoided for the fermentation of low-nitrogen grape juices or where nitrogen supplements can not be made.