1a.Objectives (from AD-416):
1. Evaluate relationships between yeast assimilable nitrogen (YAN) and °Brix towards fermentation characteristics of musts.
2. Determine the impact of excessive nutrient supplementation of musts on the amount of residual nitrogen in wines and subsequent risk of spoilage by Brettanomyces.
1b.Approach (from AD-416):
Relationship between must yeast assimilable nitrogen (YAN) and °Brix for a healthy fermentation will be determined by using synthetic grape juice medium containing variable amounts of YAN and sugar. Impact of excessive nutrient supplementation of musts on the amount of residual nitrogen in wines and Brettanomyces (unwanted yeast in winemaking) growth pattern will be monitored by our laboratory's established methods. Undesirable volatile compounds that are indicators for Brettanomyces contamination and growth will also be monitored.
This research was conduction in support of NP 305 objective 1 "Determine effects of water management on wine grape productivity and fruit maturity" of the parent project. The relationship between must sugar and the amount of YAN required to complete fermentation without off-odors (e.g., H2S) needs to be re-evaluated as must sugar concentration did not appear to affect the amount of nitrogen required to complete fermentation in musts containing 230 to 295 g/L sugar. In fact, 150 mg N/L was sufficient for complete fermentations in musts containing a variety of sugar concentrations. This research also illustrated that excessive nitrogen supplementation may not prevent stuck fermentation in musts containing more than 270 g/L sugar.
Excessive nitrogen supplementation yields higher residual concentrations of nitrogen-containing compounds (i.e., amino acids) which could, theoretically, encourage subsequent spoilage by other microorganisms such as Brettanomyces. However, Brettanomyces was not affected in these wines compared to unsupplemented musts/wines as the amount of nitrogen needed for growth was =6 mg N/mL YAN. On one hand, this is good news for the winemaker because it suggests that excessive quantities of nitrogen may be added prior to alcoholic fermentation without increasing the probability of a Brettanomyces infection. On the other hand, this is unfortunate because Brettanomyces growth occurred in wines containing only trace amounts of nitrogen. This research suggests that ethanol, rather than nutrient availability, may be a major factor influencing the ability of Brettanomyces to grow in wines.