The role of
yeast in winemaking is the most important element that distinguishes
wine from
grape juice. In the ,
yeast converts the
sugars of wine grapes into
alcohol and
carbon dioxide through the process of
fermentation. The more
sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to
dryness. Sometimes winemakers will stop fermentation early in order to leave some
residual sugars and sweetness in the wine such as with
dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile
filtering the wine to remove the yeast or
fortification with
brandy to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients, and the wine has not yet reached dryness this is considered a
stuck fermentation.