Effect of Yeast Inoculation Rate, Acclimatization, and Nutrient Addition on Icewine Fermentation

Abstract

Fermentations of highly concentrated icewine juice (35 to 42 Brix) are often sluggish, taking months to reach the desired ethanol level, and usually have high levels of volatile acidity. Two yeast inoculum levels using commercially available strain K1-V1116 were investigated in sterile-filtered icewine juice: 0.2 g of active dried wine yeast/L and 0.5 g of active dried wine yeast/L. The fermentation kinetics of inoculating these levels directly into icewine juice after yeast rehydration, or conditioning these cells to the concentrated juice using a stepwise acclimatization procedure after rehydration before inoculation, were compared. The effect of adding a yeast micronutrient supplement during yeast rehydration was also assessed. Yeast inoculated at 0.2 g/L stopped fermenting before the required ethanol level was achieved regardless of the inoculation procedure, producing only 62.2 g/L (7.8% v/v) and 64.4 g/L (8.1% v/v) ethanol for the direct and stepwise acclimatized inoculations, respectively. At 0.5 g/L, the stepwise acclimatized cells fermented the most sugar, producing 95.5 g/L (12.0% v/v) ethanol, whereas the direct inoculum produced 82.0 g/L (10.5% v/v) ethanol. The addition of the yeast nutrient during yeast rehydration increased the rate of biomass accumulation, reduced the fermentation time, reduced the ethanol concentration in the icewines, and reduced the rate of acetic acid produced as a function of sugar consumed. There was no difference in acetic acid concentration in the final wines across all treatments.