Redox Control of a Chardonnay Fermentation to Limit the Conversion of Elemental Sulfur to Hydrogen Sulfide

<h3>Abstract</h3> <h3>Background and goals</h3> The formation of hydrogen sulfide (H₂S) during wine fermentations that contain residual elemental sulfur continues to be a significant and chronic fermentation problem occurring in wineries worldwide, requiring additional treatments that often contribute to long-term wine instability and/or the loss of wine quality and commercial value. This study investigated the possibility of preventing H₂S formation during wine fermentation by controlling the redox potential during fermentation. <h3>Methods and key findings</h3> Triplicate juice volumes were fermented with the same yeast strain, nutrient conditions, and temperature. Three fermentations were controlled so that the redox potential during fermentation would not fall below +150 mV (silver/silver chloride [Ag/AgCl] reference). Three others were fermented without any intervention. The controlled redox potential fermentations had 10% of the H₂S found in the uncontrolled cases at peak formation, and 2% of that found in the uncontrolled cases at the end of fermentation. The controlled redox potential replicates also completed fermentation 2 days earlier. The wines from the controlled redox fermentations had a glutathione content of 552.4 mg/L, compared to 252.4 mg/L in the reference fermentations. <h3>Conclusions and significance</h3> The outcome of this experiment is the first demonstration of effectively limiting the conversion of elemental sulfur into H₂S by regulating the redox potential throughout fermentation. It also confirms previous results that this control action can lead to earlier completion of fermentation by modifying the fermentation ability of nongrowing cells. It demonstrates the interaction between the redox potential of the juice and yeast metabolism that resulted in the formation of glutathione under the controlled redox condition.