Nitrogen supplementation affects the pattern of nitrogen
uptake. Ammonium is a preferred yeast nitrogen source, and
when plentiful, it represses the expression of catabolic pathways
by degrading other nitrogenous compounds (7, 8). This mechanism,
called nitrogen catabolite repression (NCR), has recently
been studied during wine fermentations (9). It inhibits the uptake
of arginine and alanine and stimulates the consumption of
branched-chain and aromatic amino acids. Changes in the
nitrogen uptake patterns influence the production of aroma and
spoilage compounds (particularly hydrogen sulfide) and the
amount of urea, the major precursor of the carcinogen ethyl
carbamate