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January 2016 WINES&VINES 117 PRACTICAL WINERY & VINEYARD WINEMAKING nitrogen found in the yeast pro- teins, peptides, tripeptides and free amino acids is available for the fermenting yeast cells to use during alcoholic fermentation. For example, Fermaid O (100% yeast autolysate derived from enologi- cal S. cerevisiae) is composed ex- clusively of organic nitrogen. When is the best time to add nitrogen? Extensive studies have deter- mined that the best time to add nitrogen is one-third of the way through fermentation, when the yeast population has reached its peak (see "Timing of Organic Ni- trogen Addition"). A single addition of inorganic nitrogen at the beginning of fer- mentation is not recommended, as it will lead to a very high yeast population, a sudden increase in fermentation speed that is accom- panied by an exothermic reaction (heat production) and high nitro- gen depletion. This results in slug- gish and stuck fermentation since the yeasts will be left without ad- equate nitrogen. Sluggish or stuck fermentations can occur with a single addition of DAP (30 g/hl equivalent to 63 mg/L of YAN) at the onset of the fermentation. Although it has been shown that the best time to add nitrogen is at about one-third of alcoholic fermentation, when organic nitro- gen is added at the beginning of fermentation and at one-third of fermentation, the use of organic nitrogen is slower and more con- trolled. Consequently, the fermen- tation is more even—with no heat peaks and better temperature control—and the alcoholic fer- mentation goes to completion (see "Impact of Organic Nutrition on Alcoholic Fermentation"). Effect of nitrogen on fermentation kinetics YAN content is one of the most important regulators of fermen- tation speed; it impacts yeast biomass at the beginning of fer- mentation and sugar transport kinetics during fermentation. As soon as must has a nitrogen defi- ciency at the end of the growth phase, there is a decrease in pro- tein synthesis and sugar transport activity. YAN addition to nitrogen- deficient must leads to a signifi- cant decrease in fermentation length by reactivating protein synthesis and increasing sugar transport speed, which results in an increase in the fermentation rate. There are many studies de- scribing the difference in effi- ciency between organic and inorganic nitrogen additions with respect to kinetics. We compared the efficiency of both sources (DAP for inorganic nitrogen and TIMING OF ORGANIC NITROGEN ADDITION About one-third of the way through fermentation is the optimum time to supple- ment the fermenting must with appropriate nutrition (YAN, oxygen and sterols sources). Source: A. Julien, 2015 The references for this article are available online at winesandvines.com Growth phase Stationary phase 1/3 Alcoholic fermentation 300 200 100 0 0 24 48 72 96 Time (hours) Nitrogen (mg/L) Total sugars (g/L) Population (cfu/mL)