Metabolite changes during Riesling icewine fermentation when yeast micronutrients are used

Icewine is an intensely sweet dessert wine fermented from the juice of naturally frozen grapes. Icewine fermentation poses many challenges such as failure to reach desired ethanol levels and production of high levels of volatile acidity in the form of acetic acid. This study investigated the impact of micronutrient addition (GO-FERMRTM and NATSTEPRTM ) during the rehydration stage of the commercial wine yeast Saccharomyces cerevisiae K1-V1116 during Icewine fermentation. Sterile-filtered and unfiltered Riesling Icewine juice was inoculated with yeast rehydrated under four different conditions: in water only; with GO-FERMRTM; with NATSTEPRTM; or the combination of both micronutrient products in the rehydration water. Using sterile-filtered Icewine juice, yeast rehydration had a positive impact of reducing the rate of acetic acid produced as a function of sugar consumed, reducing the ratio of acetic acid/ethanol and reducing the ratio of acetic acid/glycerol. In the sterile-filtered fermentation, yeast rehydrated with micronutrients generated 9-times less acetic acid per gram of sugar in the first 48 hours compared to yeast rehydrated only with water and resulted in a 17% reduction in acetic acid in the final wine when normalized to sugar consumed. However, the sterile-filtered fermentations likely became stuck due to the overclarification of the juice as evidenced from the low sugar consumption (117 g/L) that could not be completely overcome by the micronutrient treatments (144 g/L sugar consumed) to reach a target ethanol of 10%v/v. Contrary to what was observed in the sterile-filtered treatments, using unfiltered Icewine juice, yeast micronutrient addition had no significant impact of reducing the rate of acetic acid produced as a function of sugar consumed, reducing the ratio of acetic acid/ethanol and reducing the ratio of acetic acid/glycerol. However, in the unfiltered fermentation, micronutrient addition during yeast rehydration caused a reduction in the acetic acid produced as a function of sugar consumed up to 150 g/L sugar consumed. In contrast to the sterile-filtered fermentations, the unfiltered fermentations did not become stuck as evidenced from the higher sugar consumption (147-174g/L). The largest effects of micronutrient addition are evident in the first two days of both sterile and unfiltered fermentations.