A tannic wine has an astringency and bitterness that is caused by a high level of tannins. Tannins are a group of chemical compounds found in grape skins, seeds, and stems, and sometimes in the wood barrels wine is fermented in. Tannins are important to the aging of wine. Wines most likely to be described as tannic are red wines. Tannin is about five times higher in red wines than whites. Interestingly, the tannin content of a wine from the same vineyard can double or halve from year to year, depending on the weather.