- Nitrogen is a crucial nutrient required by yeast to successfully complete alcoholic fermentation. In particular, the concentration of yeast assimilable nitrogen (YAN) influences fermentation kinetics and the production of a range of volatile aroma compounds, both desirable and undesirable. YAN is naturally present in the grape, but producers can boost YAN concentration in the vineyard and/or the winery if concentrations are deemed insufficient. In the vineyard, nitrogen can be boosted through the incorporation of organic matter into the soil and/or through the use of fertilizers. In the winery, additions of commercial products containing inorganic and organic nitrogen can be made. While many studies point out the importance of YAN in relation to fermentation kinetics, little is understood about how YAN impacts the sensory properties of wine. In this study, the goal of this was to investigate the impact of boosting YAN in the vineyard or winery on fruit and wine chemistry, fermentation kinetics, and wine sensory properties of Chardonnay (CH) and Pinot noir (PN). Five treatments, including no N addition (Control), winery N addition using either diammonium phosphate (+DAP) or organic N (+Nutriferm), and vineyard N addition either to the soil (+Soil N) or through foliar spray (+Foliar N), were established with four replicates for each variety. CH and PN wines were produced following standard winemaking practices and sensory evaluations were completed using triangle testing and Napping® and Ultraflash profiling (UFP) for aroma and mouthfeel.
Both winery and vineyard supplements raised YAN to sufficient levels for successful fermentation in both the CH and PN. There was variation in the efficiency of the fertilizer applications to raise YAN in CH compared to PN where application through the soil raised YAN more so than foliar application. In the winery, DAP boosted YAN through ammonia N while Nutriferm Arom Plus boosted free amino acid content. No significant differences in time to completion were noted between PN and CH treatments. To investigate how yeast strain and nitrogen source interact to alter fermentation rate and volatile aroma an additional experiment was conducted in CH juice. The same juice from the Control, +DAP, + Nutriferm, and +SoilN treatments were used, but this time there were three different strains of S. cerevisiae (D47, DV10, and CVW5). Nitrogen source had a significant effect on fermentation rate and there was also a significant interaction effect between yeast strain and nitrogen source on fermentation rate.
In addition to increasing YAN, boosting nitrogen in the winery and/or vineyard also altered the concentrations of certain free amino acids. For Chardonnay, the addition of nitrogen in the vineyard consistently boosted arginine, alanine, serine, and glutamine while the addition of Nutriferm Arom Plus consistently boosted threonine compared to other treatments. Additional volatile aroma analysis will be needed to determine if differences in total YAN as well as individual amino acids will impact yeast derived volatile aroma compounds. However, sensory analysis did reveal differences between these wines that could be due to initial differences in juice/must YAN concentration and composition. Nitrogen treatments affected Chardonnay and Pinot noir wine properties, but the extent of impact was dependent on varietal. Panelists could tell apart the CH treatments based on aroma and described these differences using unique terminology. In contrast, panelists could separate the PN treatments based off aroma, but could not successfully describe the differences. These findings suggest that CH producers should carefully consider where and how much YAN they add as fermentation rate and final wine aroma can be altered. However, PN producers have more flexibility in where they add nitrogen as any changes to aroma were very subtle, meaning they can chose both where (vineyard or winery) and how (DAP or organic product) to boost nitrogen based on cost and/or production efficiency rather than worry about potential changes in wine quality.