Manipulating Soil Moisture and Nitrogen Availability Part II. Fermentable Nitrogen Content and Must and Wine Composition

An approximation of the total yeast fermentable nitrogen content in juice or must is taken as\ud the sum of the nitrogen available from ammonia and the alpha-amino acids present (Bisson\ud 1991 ; Dukes and Butzke 1998 ; Jiranek, Langridge, and Henshcke 1995). Recommended\ud levels of fermentable nitrogen needed by yeast for healthy fermentations are reported to vary\ud from as low as 140 mg (N)/L to as high as 500 mg (N)/L or more (Butzke, 1998 ; Spayd,\ud 1998) . If assimilable nitrogen levels are too low, fermentations may be slow and may stick,\ud producing wines with undesirable levels of residual sugar. Problem fermentations are also\ud sometimes accompanied by production of hydrogen sulfide and other reduced sulfur odors\ud (Kunkee 1991 ; Jiranek, Langridge, and Henshcke 1995). Other undesirable flavors have been\ud described in wines produced under drought and stressed conditions, including those related to\ud atypical aging (UTA) syndrome in white wines (Sponholz, 2000). Winemakers often add\ud supplements to juice and fermenting wines to balance perceived nutritional deficiencies\ud (Montiero and Bisson, 1992; Bisson, 1999). It is also thought that variations in climate, soil\ud type, and cultivation, soil moisture, and fertilizer practices may have an impact of juice and\ud must composition and nutrition (Butzke, 1998; Ingledew, 1985)