Fermentation
Inoculation of yeast is the second step in the fermentation process during making of white wine. It requires concentrated sulphates, typically with the concentration of 10 g/hl, to be added into the juice. Such approach allows stimulating the participating fermentors and inhibiting the action of spoilage microflora. In the next step, the starter juice has to go through the process of spontaneous fermentation, before it can be added into the tanks, which have been freshly filled, maintaining the concentration of 2-5%. The fermentation process dynamics in this case is largely dependent on the inherent strain of the yeast.
Moreover, yeast of low concentration may decelerate fermentation, impeding inoculation. Therefore, special attention should be paid to selecting the appropriate yeast strain among the 30 strains of Saccharomyces cerevisiae, which are suitable for fermentation in white wine production. Only full consideration of the effect, the strain has on the particular wine under discussion, and its fermentative potential may lead to the appropriate yeast choice (Grainger & Tattersall, 2005, pp. 64-77).
Next step of the process involves continuous aeration of the juice, in order to provide the yeast with the necessary amount of nitrogen and oxygen for efficient fermentation. In some cases, nitrogen level in juice meets the requirements of the yeast fermentation, as the assimilable level of nitrogen in grapes, grown in cool environment usually does not require addition of extra nitrogen. However, if summer droughts resulted in a decrease of assimiable nitrogen in the juice below 160mg, ammonium sulphate should be added during the process of fermentation (Ribereau-Gayon et al., 2006, p. 426). Moreover, the temperature level during fermentation should be kept constant, in order to avoid thermal shocks, which could terminate fermentation.
According to Ribereau-Gayon et al. (2000, p. 432), alcoholic fermentation in white wine making does not usually last more than 12 days. Its completion depends on a number of aspects such as sugar concentration, assimilable nitrogen level, turbidity, efficient aeration, maintained temperature, yeast strain and the conditions of juice extraction. Since most of these factors may be controlled, long fermentation is usually the fault of the winemaker’s operation processes. Slow fermentation may result from a high concentration of reducing sugar, which is above the level of 2g/l, considered by Ribereau-Gayon et al. (2000, p. 432) .
