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January 2016 WINES&VINES 119 PRACTICAL WINERY & VINEYARD WINEMAKING compounds involved in the aroma matrix of wine: higher alcohol and acetates. Yeast metabolism also influences the expression/ creation or preservation of certain aroma precursors of an amino nature (cysteinylated precursors or glutathionylated precursors of AVOIDING STUCK FERMENTATION WITH PROPER NUTRITION A ssimilable nitrogen is es- sential for protein synthe- sis in yeasts. It is usually the main limiting nutrient and the major reason for slow fermenta- tion in standardized conditions. There is a relationship be- tween the assimilable nitrogen concentration in the must and the maximum fermentation rate. Ammonia nitrogen and free amino acids are the main nitrogen sources even though some small peptides can also be assimilated. Assimilable nitrogen is consid- ered deficient when it is less than 150 mg/L. In such cases nitrogen supplementation is very efficient at about one-third of alcoholic fermentation. When nitrogen is added too early, the cell population and the fermen- tation rate during the first half of fermentation are increased, but the cell viability at the end may be decreased. To reduce the risk of stuck fermentation, add nitrogen at the beginning of the station- ary phase (after 30% to 40% of fermentation) and combine this addition with oxygenation or to add a complex product containing other nutrients, such as lipids. Ammonia nitrogen can now be replaced by organic nitrogen, with a better effect on fermentation kinetics. —Dr. Jean-Marie Sablayrolles IMPACT OF INORGANIC AND ORGANIC NUTRITION ON ALCOHOLIC FERMENTATION Fermentation kinetics are shown for a Viognier fermented at 18° C with a sugar concentration of 215 g/L and initial YAN less than 100 mg/L. Addi- tions of 16 mg/L of YAN in two different forms—inorganic nitrogen (DAP) and organic nitrogen (organic nutrient)—were made at two stages of fermentation: 8 mg/L of YAN at the begin- ning of alcoholic fermenta- tion and 8 mg/L of YAN at one-third of alcoholic fermentation). The enlarged section represents the end of the fermentation.