Urea Accumulation in Fermenting Grape Juice

Abstract

Previously identified factors of yeast strain, arginine and high amino acid concentration that enhanced the accumulation of urea in fermenting grape juice were verified. It was determined that aeration of the ferment was required, and its timing important, for a significant accumulation of urea in wine. Continuous aeration caused a rapid initial accumulation of urea in the fermenting juice during the yeast growth phase, followed by a rapid and complete utilization of the urea, resulting in a minimal concentration in the final wine. Limited aerobic conditions (initial-aerobic followed by anaerobic fermentation) caused a similar rapid initial accumulation of urea, but the urea was then only partially utilized during the latter stage of fermentation. This resulted in a high concentration of urea in the final wine: the concentration being higher in a wine made from a nitrogen-rich Chardonnay juice than in that from a lower-nitrogen Rhine Riesling juice. Only very low concentrations of urea accumulated in the wine when anaerobic ferments were aerated during the latter stage of fermentation. Supplementing the ferment with excess ammonia (1 g/L) prevented accumulation of a high concentration of urea even in the presence of excess arginine and limited aeration.