Impact of Fermentation Rate Changes on Hydrogen Sulfide Concentration in

The correlation between alcoholic fermentation rate, measured as carbon dioxide (CO2) evolution, and the rate of hydrogen sulfide (H2S) formation during wine production has a significant impact on the H2S content of a finished wine. Both rates and the resulting concentration peaks in fermentor headspace H2S are strongly impacted by yeast assimilable nitrogenous compounds in the grape juice. We have conducted a series of model fermentations in temperature-controlled and stirred fermentors using a complex model juice with a defined combination of ammonium ions and/or amino acids. Fermentation rate was measured indirectly by weighing the fermentors on a laboratory scale. This assumes that once CO2 saturation of the juice is reached, weight loss corresponds to CO2 evolution which in return is proportional to ethanol formation. H2S production was measured using a calibrated transparent tube packed with color-indicating metal acetate. The tube was inserted into a fermentor port instead of a regular gas lock, and provided quantitative trapping of H2S formed over time. Fermentation rates for CO2 and H2S as well as the relative ratios between them were calculated. The fermentations confirmed observations that high concentrations of yeast assimilable nitrogen do not necessarily protect against elevated H2S formation. High initial concentrations of ammonium ions via addition of diammonium phosphate can cause a higher evolution of H2S as in a non-supplemented but non-deficient juice. We observed that the availability of yeast assimilable amino acids, particularly arginine, can results in a more evenly distributed CO2 production throughout the alcoholic fermentation. In addition, relative maximum H2S evolution rates can be observed earlier in the fermentation, and CO2 produced during the remainder of the fermentation may sufficiently strip out initial sulfides.