Tannins are a naturally occurring group of compounds called polyphenolics, that are important in wine, in particular red wines, and which are found in plants, seeds, wood, leaves and fruit skins. All wines have tannins to a greater or lesser degree, but they are most commonly found in red wine (white wines, are not normally fermented with their skins or seeds, and so the extraction of tannins is greatly reduced). Tannins prevent oxidation and play a part in a red wine’s structure, complexity and aging potential. Tannins can also be extracted from the oak barrels in which wines are aged and can impart strong, complex flavors into wines.
In the winemaking process, the wine maker will more often than not try to extract the softer tannins found in the grape skins during maceration rather than the more astringent ones found in the grape seed.
A young wine described as tannic will generally be astringent in the mouth, while older or better-integrated wines will be smoother and riper. This is because tannins react with the pigments and acids in the wine to form new compounds, and while this ultimately results in the gradual reduction of a red wine’s color, the tannins will lose their bitterness, while the aromatic qualities of the wine are enhanced and any harshness should disappear.