TANNINS (tannic acids) are plant polyphenols that tend to bind with, shrink, or precipitate proteins. Commercially, tannins are used to "tan" or cure animal skin into leather, preserving it. In foods and beverages that contain them, tannins induce astringency or even bitterness.
The primary natural role of tannin is to deter or inhibit insects and bacteria; it may also play a part in regulating plant growth. Tannins are found in many plants and most tissue structures in various concentrations. Tannins do not normally influence plant metabolism, since they are contained in cell vacuoles.
Tannins and tannic acids are responsible for the dry, puckery taste especially noticeable to novice wine drinkers. This effect tends to have less sensory impact when food (depending upon its chemical balance) accompanies the wine. Various tannins or levels of tannin may produce many different sensations of flavor and texture in wine. These effects are much easier to experience than they are to measure.
Many common foods contain tannins, particularly tea, beans (dark skinned), leafy vegetables (dark green), apples, berries, grapes, persimmons, and pomegranates. Tannin modification or decomposition plays a significant role in fruit ripening. Walnuts, and many spices, even chocolate, contains small amounts. This sharing of tannin in their chemical make-up may account for some sensory harmony between these foods and tannic wines.
Tannins are released into wine when the skins of grapes are broken, but can also come from seeds and stems and even leaves if these materials are not separated from the juice before they are broken down by macerating in acid and the increasing heat and alcohol concentration as the grape sugars ferment. When wine is stored in barrels, tannins may also leach into wine from the cells of the wood. Wine that is too low in tannin can be supplemented with an extracted powdered form; too much tannin in wine can be removed using protein-based fining agents.
Darker, smaller, thicker-skinned grapes, such as Cabernet Sauvignon and Syrah, contain higher concentrations of tannin than lighter, larger, thinner-skinned varieties, like Pinot Noir, Grenache, and of course any of the white grape varieties.
As well as many possible levels or various concentrations of tannin there are also different types. Seed and stem tannins tend to be more bitter than skin tannins, for example. The different grain densities of barrel staves made from different oak forests produce a range of flavors. Some tannins (hydrolyzable) can be broken by acids, especially when heated, and some (condensed) cannot, although most tannins are soluble in water and alcohol.
Tannin also has anti-oxidant properties which help to preserve wine as it ages (although additional attributes are required for wine flavors to improve with aging).