Tannin is a substance found natural

Tannin is a substance found naturally in many different plants, most notably in grapes, tea leaves, and oak. The word comes from the historical practice of extracting this compound from oak bark to tan leather, although in the modern world, synthetic forms or other chemicals are usually used instead.

This substance is found in the skins, seeds, and stems of grapes. Wine that is fermented while still in contact with these parts of the grape — red wine — absorbs some of the tannin, which lends a distinct character to the wine. Because the tannin in the seeds of grapes is particularly harsh, wine is usually crushed rather than pressed, to minimize its absorption. Many wines also absorb some from the oak barrels they are aged in, which lends additional flavor to the wine.

Tannin is crucially important to the complexity of a wine and how well it will age over time. Wines that have high amounts tend to age best for long periods of time, such as those made from the Cabernet Sauvignon, Nebbiolo, and Syrah grapes. When young, these wines often seem searing, creating a sensation of dryness in the mouth, particularly the roof of the mouth. The most tannic of wines cause the mouth to pucker, which is not necessarily a pleasant experience.