Sulfur and Nitrogen Metabolism

Although metabolism of carbohydrates is obviously critical to the outcome of the wine fermentation, metabolism of other must components is also important. How wine yeasts metabolize sulfur-containing compounds that are present in the must as normal grape constituents is particularly important. Most of the sulphur in grapes is in the form of elemental sulfur, sulfates, or as sulfur-containing amino acids. Since the range of sulfur-containing metabolic end products includes various sulfides, mercaptans, and other volatile compounds, sulfur metabolism can have a profound influence on wine quality.Yeasts can also produce sulfites, which, as already mentioned, have antimicrobial activity. In fact, even if sulfur dioxide or sulfite salts are not intentionally added, wine invariably contains sulfite due to its production by wine yeast.

Grapes usually contain sufficient ammonia, ammonium salts, and free amino acids to support good growth of most wine yeasts. In addition, wine yeasts can synthesize amino acids and purine and pyrimidine nucleotide from ammonia, and, therefore, have no essential requirement for amino nitrogen or pre-formed nucleotides. During the course of the fermentation, total nitrogen decreases. However, nitrogen-deficient grapes can result in an inadequate amount of nitrogen in the must, and, if not supplemented with ammonium salts, can lead to reduced fermentation rates.

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