Identification of Yeast Strain Genetic Factors in the Formation of Volatile Sulfur
One of the greatest yet least understood variable influencing hydrogen sulfide formation by yeast during wine production is differences in genetic background. The overall goal of this research program is to define and categorize the pattern of strain differences in mRNA and protein profiles correlated with low and high H2 S production. Hydrogen sulfide formation was evaluated in 15 different yeast strains, 6 commercial yeasts and 9 natural winery isolates, in three different media at several different nitrogen concentrations. Two media were synthetic grape juice formulations and the third was a Pinot noir juice. All 15 strains increased hydrogen sulfide production upon H2S limitation, but the magnitude of the effect varied greatly (from 100 fold to a less than 50%increase with a 10 fold decrease in nitrogen concentration). Other juice parameters thought to impact hydrogen sulfide formation were also evaluated at levels at which they are normally found in wines: metal ions, sulfur dioxide, glutathione. None of these compounds had a significant effect on H2S formation in any of the 15 strains at these concentrations. The effect of the amino acids threonine, methionine and cysteine were also evaluated. Cysteine at concentrations found in juice had no effect on H2S production in any of the strains evaluated. Threonine, at the high end of the concentration range at which it is found in juice generally increased hydrogen sulfide production in all strains tested, although the magnitude of the effect varied. The effect of methionine was quite variable. Methionine inhibited hydrogen sulfide formation in several strains, but had no effect in the others. In general, strains showed similar responses but differed dramatically in the magnitude of the response to juice conditions. In order to further clarify the genetic basis of this difference in response, mRNA and protein profiling is being conducted. Strains display dramatic changes in both mRNA and protein patterns upon nitrogen limitation, however, there are great differences in protein profiles between different strains grown under nutrient rich conditions. The genome-wide analytical technologies will allow a detailed description of strain differences.