Yeast Assimilable Nitrogen In Grapes And Its Relationship To The Origin Of Aroma Compounds

Abstract

Wine is a complex mixture of over 100 volatile compounds produced via various biochemical pathways, some in the grapes, and others de novo through yeast metabolism. Nitrogen is associated with the production of alcohols and esters during fermentation, and is often the limiting metabolic factor for fermentation. The biochemical pathways utilized by yeast to produce aroma compounds are affected by the concentration and source of nitrogen. This dissertation investigates the development of nitrogenous compounds in grape berries during ripening, methods to predict concentrations before harvest, the use of isotope ratio mass spectrometry (IRMS) to investigate the origins of fermentative aroma compounds, and explores the way taste physiology affects wine perception. In the first study of yeast assimilable nitrogen, 60 sites across the Finger Lakes region of New York State were sampled during a three-year period (2010 through 2012) and regression models were developed to predict YAN two weeks before harvest. The second study on YAN expanded the investigation to YAN in grape berries from veraison to harvest in Cabernet Franc, Chardonnay, Merlot, Noiret, Pinot noir, Riesling, and Traminette cultivars across New York State. Chronic deficiency (YAN concentrations less than 140 mg/L) was observed in Cabernet Franc, Riesling, and Traminette, while Chardonnay and Pinot noir were consistently above 200 mg/L. Population distributions and regression models developed from this work can be used to decrease the amount of supplemental nitrogen added to the must prophylactically, while minimizing the chance for over or under supplementation. The next portion of the research investigated the development of an IRMS method to determine what portion of aroma compounds are produced from yeast metabolism of sugar verses other grape precursors. The study demonstrated that most of the aroma compounds derived their carbon from sugar metabolism, while 1-hexanol derived most of its carbon from grape sources other than hexoses. Finally, in a preliminary experiment, inexperienced PROP tasters reported being significantly less confident when choosing a wine than non-tasters. However, a follow-up experiment with more experienced wine consumers showed no significant differences in confidence or influence of external cues on panelists with different taste phenotypes.