On the other hand, selection of a suitable strain of yeast for fermentation is very important because there are more than 30 strains of Saccharomyces cerevisiae, which have the potential to produce white wine. Yeast selection entails an understanding of the fermentative potential of the strain and its overall effect relative to the specificity of the wine in question (Grainger & Tattersall, 2005, pp. 64-77).
The third stage of fermentation entails meeting the nitrogen and oxygen needs of the fermenting yeasts and constant aeration of the fermenting juice. Sometimes, assimilable nitrogen levels in juice derived from grapes cultivated in cool climates are sufficient to meet the nitrogen needs of the yeast. However, juices deficient in assimilable nitrogen can also be produced in cool climates especially when the vineyards are supplied with insufficient levels of nitrogen or due to increased summer dryness. As a result, juices with less than 160 mg of assimilable nitrogen require addition of ammonium sulfate during fermentation (Ribereau-Gayon et al., 2006, p. 426). Furthermore, there is the need to maintain temperature at constant levels since untimely changes in temperature can lead to thermal shocks, which can stop the fermentation process.
Conversely, completion of alcoholic fermentation in white winemaking depends on several factors such as the conditions, under which the juice is extracted, the concentration of sugars and assimilable nitrogen in the juice, the strain of yeast present, turbidity, the duration and frequency of aeration, and the fermentation temperature. However, these parameters can be controlled by the winemaker, which implies that a slow fermentation process is caused by carelessness on the part of the operator.
