· The conversion of must into wine from , yeasts in the absence of air (oxygen) is the known alcoholic fermentation. · In essence ,, a high concentration of sugars (glucose and fructose) is converted through a series of metabolic steps (glycolysis of , alcoholic fermentation) into carbon dioxide and , ethanol alongside a variety of by-products formed at relatively low concentrations.
Alcoholic fermentation is carried out primarily by yeasts of the genus Saccharomyces and in particular by strains of cerevisiae (Fleet and Heard 1993, Henschke 1997b, Lambrechts and Pretorius 2000). * These yeasts are either found naturally in the bark of , rails or in the harvesting and processing equipment at , winery or added as a starter crop.
As the initial population of yeasts (1-5 x 106 · Cells/mL) inoculated in the must are usually not sufficient to complete , fermentation must be multiplied so that a sufficient fermentation rate can be achieved.
* But ,'s growth depends on the availability of appropriate sources of nutrients in addition to sugars and especially nitrogen (Henschke and Jiranek 1993, Albers et al. 1998, Magasanik and Kaiser 2002). · Nitrogen is an important trigger for alcoholic fermentation , as by reducing , the fermentation rate can substantially decrease , and even stop (Lagunas 1982, Salmon 1989, Manginot et al. 1997).
· Grapes usually provide the necessary nutrients for , yeasts and represent the primary source of endogenous , nitrogen unless supplementation is added by the winemaker. · The decision on the addition of a nitrogen supplement to the must should be made on the basis of the measurement of the assimilable by the yeasts before fermentation ,, the winemaking conditions (e.g., strain of dough , temperature , aeration) and the type of wine (Henschke 1997a).
Source: Bell S-J and P.A. Henschke , 2005. Implications of nitrogen nutrition for grapes , fermentation and wine. Australian Journal of Grape and Wine Research 11 , 242-295.
