Low H2S Yeast Technical Brief

Hydrogen sulfide (H2S) in wine is of significant concern to winemakers. H2S, as well as related defect-causing volatile sulfur compounds (mercaptans and disulfides) — are produced during fermentation by yeast in response to stressful conditions such as low or imbalanced nutrient levels, low temperature, etc. Collectively, these chemicals impart strong sulfurous odors to the wine including notes of “rotten eggs, garlic, and burnt rubber” and, if left untreated, can result in complete spoilage. Traditionally, winemakers have had to expend significant energy, time and resources in remediation efforts such as aeration or the addition of copper — both of which can negatively impact wine quality—but in extreme circumstances winemakers have no choice but to discard wine that is beyond remediation.

Renaissance Yeast’s exclusive H2S-preventing technology is the result of the discovery of a natural trait in yeast that increases sulfur and nitrogen utilization efficiency, thereby completely preventing H2S formation during fermentation. Alternatively, a number of other “low-H2S” yeast strains are also presently commercially available. To compare these other strains to Renaissance Yeast’s H2S-preventing yeast (Vivace, Allegro, Andante and Maestoso), we conducted laboratory-scale fermentations of a typical Chardonnay grape must, and evaluated the yeasts on their overall winemaking performance, as well as their efficiency in mitigating H2S formation during fermentation. More specifically, we measured and compared all of the strains for H2S production, fermentation kinetics, sulfur dioxide (SO2) and acetic acid.

As shown in Figure 1, all of Renaissance Yeast’s H2S preventing yeast strains did not produce H2S at any point during the fermentation (white lead acetate strips). In contrast, the “low-H2S” competitor stains #1 and #3 both produced detectable amounts of H2S during the fermentation (gray/black lead acetate strips).

Figure 1. Renaissance Yeast H2S-preventing yeast strains do not produce H2S during fermentation. Detection of H2S by lead acetate in fermentations of Chardonnay grape must (YAN 200 mg/L).
Figure 2. Renaissance Yeast H2S-preventing yeast strains exhibit fast fermentation kinetics. Fermentation progress in fermentations of Chardonnay grape must (YAN 200 mg/L) was measured by CO2 weight loss.

In terms of fermentation kinetics all of the Renaissance Yeast H2S-preventing yeast strains, as well as competitor #1, finished the fermentation within five days (Figure 2) and had residual sugar less than 1.5 g/L (data not shown). However, competitor strains #2 and #3 performed sluggishly and still had not finished the fermentation even by day seven (residual sugars 5.5 and 10.2 g/L, respectively).

As a result of the relationship between sulfite metabolism and H2S, low H2S yeast strains can often result in excessive SO2 production during fermentation. However, Renaissance Yeast’s H2S-preventing yeast strains are all bred to exhibit normal levels of SO2 production. As seen in Figure 3, the Renaissance Yeast strains and competitors #1 and #3 all produced minimal amounts of SO2 during fermentation, with the Renaissance Yeast Vivace and Maestoso strains actually reducing SO2 slightly, relative to the starting must. In contrast, the low H2S competitor strain #2 produced substantial amounts of SO2 during the fermentation, reaching levels double that of the starting must. Excess volatile acidity (acetic acid) production by yeast can also result from stressful nutrient levels. Therefore, acetic acid production is also a critical consideration when evaluating H2S formation. Figure 4 demonstrates that the Renaissance Yeast H2S-preventing yeast strains all produce normal amounts of acetic acid during fermentation. Conversely, all of the competitor low H2S strains produced significantly more acetic acid, with competitors #2 and #3 producing almost 0.8 g/L. Taken together, these data indicate that — as compared to three competitor’s “low-H2S” strains — all of the Renaissance Yeast H2S-preventing yeast strains provide superior winemaking performance and efficacy in preventing H2S during fermentation. In fact, only the Renaissance Yeast strains offered all of complete H2S prevention, fast fermentation kinetics, normal levels of SO2 and minimal acetic acid production.

Figure 3. Renaissance Yeast H2S-preventing yeast strains do not produce excess SO2. Total SO2 in fermentations of Chardonnay grape must (YAN 200 mg/L) was quantified by the Ripper method.
Figure 4. Renaissance Yeast H2S-preventing yeast strains do not produce excess volatile acidity. Acetic acid levels in fermentations of Chardonnay grape must (YAN 200 mg/L) were quantified by enzymatic assay.