Bell and Henschke (2005) state that both the form and amount of yeast-assimilable nitrogen (YAN) have implications for quality. Low must YAN leads to low yeast populations and poor fermentation vigor, increased risk of sluggish fermentations and production of thiols and higher alcohols, and low production of esters and long chain fatty acids. High must YAN increases biomass production, greater fermentation vigor, increased production of volatile acidity, the possibility of protein hazes due to higher levels of protein and microbial instability. Injudicious use of diammonium phosphate supplements often results in excessive levels of residual nitrogen, leading to microbial instability and ethyl carbamate (and phosphate in the case of DAP) accumulation in wine (Pretorius, 2000). The degree of supplementation of inorganic nitrogen in grape juice is usually regulated. This implies that knowledge of the nitrogen content of grape juice and the requirement for nitrogen by yeast are important considerations for optimal fermentation performance and the production of wines that comply with the demands of regulatory authorities and consumers. Urea, short peptides and nitrogenous bases (excluding thymine) can also be degraded by Saccharomyces. Nitrates, nitrite and most of the other organic amines cannot be utilized (Jiranek et al., 1995). For more discussion on metabolism of amino acids by brewers’ yeast in beer production, see Section