Grapes contain both inorganic and organic sources of nitrogen.Total nitrogen concentrations in grapes (or musts) range from about 0.2 g/L to 0.4 g/L.The ammonium nitrogen is less than 0.1 g/L. Despite their relatively low concentration in juice, the nitrogen content of most musts is generally adequate for rapid growth of yeasts. In fact, the primary role of nitrogen in wine appears to be as a nutrient source for the yeasts, rather than affecting any of the organoleptic or other properties of the wine, per se. Moreover, wine yeasts can assimilate free ammonia into amino acids and can, therefore, use ammonia directly as a source of nitrogen. Nonetheless, nitrogen deficiency can occasionally occur, and is one of the main causes for sluggish or "stuck" fermentations. Some wine makers, therefore, routinely add ammonium salts to the must, in the form of a yeast food (especially for those occasions when the grapes are deficient in nitrogen).
The main organic nitrogen-containing compounds are amino acids, amides, amines, and proteins.The free amino acids are not only synthesized into proteins (following transamination reactions), but several can also be used as an energy source.Among the proteins found in must, the most important are the enzymes. Some grape enzymes serve useful functions, whereas others can present problems. The pectinases and other hydrolases that enhance extraction of juice from the grapes during crushing are especially important. In contrast, phenol oxidases participate in the well-known enzymatic browning reaction. In the presence of oxygen, these enzymes form undesirable brown pigments that seriously discolor the wine. This reaction, however, is effectively inhibited by sulfur dioxide.
In recent years, the presence of potential bioactive nitrogen-containing compounds in wine has attracted concern and interest. When the amino acids histidine and tyrosine are decar-boxylated by decarboxylase enzymes produced by wine bacteria, the amines histamine and tyra-mine are formed. Depending on the dose and individual sensitivity, these biogenic amines can cause headaches, nausea, and allergic-type reactions. Another compound, ethyl carbamate, can be formed via a chemical reaction between urea and ethanol. Ethyl carbamate is suspected of being a carcinogen,and its presence in wine is increased by heating steps, such as pasteurization, and by high urea concentrations.
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