Nutritional Status of Grape Juice

The chemical and physical environment of grape juice during fermentation, coupled with competition from indigenous yeast and bacteria, can present significant challenges to the growth of
Saccharomyces cerevisiae
. Individually or collectively, these factors may impact both yeast growth and the conversion rate of sugar to alcohol, leading not only to the formation of objectionable odor- and flavor-active metabolites but, potentially, protracted, incomplete, or “stuck” fermentations as well. Sluggish and stuck fermentations can be described as those where the rate of sugar utilization is extremely slow, especially near the end, and/or where residual fermentable sugar is left in the wine. Such wines create significant management problems. Table wines containing biologically available levels of sugar (>-0.2% w/v glucose + fructose) may undergo spontaneous refermentation. Further, unexpectedly sweet wines may be an unacceptable departure from wine style.