Sugars in wine

During the process of fermentation, sugars from wine grapes 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.

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.

The linking of glucose molecules with aglycone, in a process that creates glycosides, also plays a role in the resulting flavor of the wine due to their relation and interactions with phenolic compounds like anthocyanins and terpenoids.

A winemaker that chooses to halt fermentation (either by temperature control or the addition of brandy spirits in the process of fortification) will be left with a wine that is high in fructose and notable residual sugars.

This is because the unfermented grape must will still have roughly equal parts of fructose and the less sweet tasting glucose.

The sugars in grapes are stored in the pulp along with water, organic acids and other compounds
In wines like Port , the addition of neutral grape spirits stuns the yeast and halts fermentation, leaving a wine with a higher proportion of fructose sugars and creating a sweet wine.