Yeast Assimilable Nitrogen (YAN) Optimization for Cool-Climate Riesling
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Authors: Camila Tahim and Anna Katharine Mansfield
AJEV Papers in Press, December 2018. DOI 10.5344/ajev.2018.17087.
Summary by Janet van Zoeren
- Yeast assimilable nitrogen (YAN) levels in New York Riesling are often below the recommended levels of 140 mg/L
- In wine made from New York-grown Riesling grapes, supplemental nitrogen may not be necessary to complete fermentation. This is contrary to findings from other regions.
- Unnecessary nitrogen supplementation may lead to increased off-flavors in the wine.
- Greater than 250 mg nitrogen/liter in Riesling must in not recommended.
Background. Yeast Assimilable Nitrogen (YAN) is a necessary ingredient in order for yeast to be able to fully ferment grape must into wine. Previous wisdom has been that full fermentation requires a YAN concentration above 140 mg/L, with optimal levels ranging from 200 to 350 mg/L depending on sugar concentration, yeast strain, and other factors. YAN naturally present in grape juice is generally below these levels. For that reason, typical industry practice is to supplement nitrogen. Furthermore, nitrogen supplementation has been shown to change the concentrations of esters, alcohols and monoterpenes through complex pathways, which are not fully understood. Some of these changes may promote desirable aromas, while others may lead to off-flavors.
Previous studies on the effect of nitrogen supplementation have used juice from grapes grown in warm regions, and the results of those studies may not be transferrable to grapes grown in cooler climates. Tahim and Mansfield looked at the effects of supplementing nitrogen to New York-grown Riesling juice, in order to determine optimal YAN concentrations for wine made in cool-climate regions.
Experiment. All trials used a Riesling (V. vinifera) juice at 20 Brix, grown in the Cornell University research vineyards in the Finger Lakes region of New York. Fruit was pressed and prepared for fermentation using standard processing methods.
Four YAN concentration treatments were used: (1) 130 mg N/L (control – the naturally occurring amount), (2) 180 mg N/L, (3) 250 mg N/L, and (4) 300 mg N/L. Beyond 130 mg/L, the higher YAN concentrations were obtained through the addition of diammonium phosphate. Each of these four treatments was fermented to completion by each of three yeast strains: EC1118 (Lallemand), W15 (Lallemand), and Cote des Blancs (Red Star).
The effects of these treatments on four outcomes was tested: (1) time to complete fermentation, (2) ethanol and residual sugar concentrations, (3) wine chemistry, and (4) preference based on a sensory panel.
Results. With one yeast strain (EC1118), nitrogen supplementation decreased fermentation time. However, with the other two yeast strains, increased YAN had little to no effect on time to fermentation completion. All yeast strains were able to consume more than 130 mg/L YAN when it was available.
130 mg N/L is sufficient to ferment to completion. Initial YAN concentration had no effect on wine ethanol concentrations, indicating that, for all yeast strains, 130 mg N/L was sufficient for compete fermentation. For one yeast strain (Cote des Blancs), residual sugar decreased in the treatments with the highest YAN concentrations, although that did not translate to a difference in ethanol. In the other two yeast strains, increased YAN led to increased titratable acidity.
Higher alcohols decreased and ester concentrations increased following YAN supplementation. The two higher alcohols evaluated were 2-phenylethanol and 1-hexanol. Both of these were found in lower concentrations in the YAN supplementation treatments, in at least one yeast strain. The concentration of some esters (in particular 3-methyl butyl acetate) increased with nitrogen supplementation. The concentrations of several desirable aromatic compounds (linalool and norisoprenoid) were not affected by YAN supplementation.
When fermented by EC1118 yeasts, nitrogen supplementation led to decreased flavor preference. For the EC1118 fermented yeasts, the most preferred wine was the control (130 mg N/L), with preference decreasing as YAN increased. There was no change in preference based on YAN supplementation for wines fermented by the other two yeasts.
Conclusions and practical considerations. The study suggests that supplemental nitrogen as low as 130mg/L may be adequate for wines made from cool-climate Riesling grapes to complete fermentation. Furthermore, in one case, nitrogen supplementation made the wine less preferred in a taste panel. In general, winemakers in cool-climate regions should use nitrogen supplementation sparingly, as we show no benefit for increasing YAN above 250 mg/L, and some risk in doing so.
Janet van Zoeren is an extension support specialist with the statewide viticulture extension program, in the Section of Horticulture, based at Cornell AgriTech in Geneva, NY.