The
sugars in wine grapes are what make
winemaking possible. During the process of
fermentation, sugars are broken down and converted by
yeast into alcohol (
ethanol) and
carbon dioxide. Grapes accumulate sugars as they grow on the
grapevine through the
translocation of
sucrose molecules that are produced by
photosynthesis from the leaves. During
ripening the sucrose molecules are
hydrolyzed (separated) by the enzyme
invertase into
glucose and
fructose. By the time of
harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-
carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable with sugars like the five-carbon
arabinose,
rhamnose and
xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, no wine is ever fermented completely "
dry" (meaning without any
residual sugar). Sugar's role in dictating the final
alcohol content of the wine (and such its resulting
body and "mouth-feel") sometimes encourages winemakers to add sugar (usually
sucrose) during winemaking in a process known as
chaptalization solely in order to boost the alcohol content - chaptalization does not increase the
sweetness of a wine.