Tannins are found naturally in grape seeds, stems and skins, as well as in wine barrels made from certain woods. They contribute to a wine’s texture, flavor and body, and because they prevent wine from reacting with oxygen, they also act as a natural preservative.
Tannins are present in just about all red wines, since the grapes remain in their skins during fermentation, but they must be added to whites, since those are fermented with the skins removed. Additional grape tannins may be added to red wines to enrich the wine’s flavor and body or to enhance its color.
Tannins create a sharp, astringent effect on the tongue and a dry feeling on the sides of the mouth.