Fine wines are known for their complex tastes and bouquets. Among the many components that contribute to these qualities are the tannins found in wine. In particular, wine tannin contributes bitterness and astringency to a wine. Dry wines, in particular, tend to have a high tannin content.
Tannin is a naturally occurring chemical found in many plants. Specifically, it is a polyphenol and is concentrated in the skins of some fruits as well as twigs seeds and other parts of some plants. Grape skins are particularly rich in tannin. Red wine generally has higher concentrations of tannin that white wine, simply because of the way wines are made.
During the early fermentation process, most red wines are allowed to sit for an extended time with the grape skins, allowing for more tannin to be released to the new wine. Most white wines are fermented without contact with grape skins, hence they tend to have less tannin.