Tannin

Tannic acid, essential for good aging qualities and balance, gives most wines their "zest" or "bite." Tannin is found naturally in the stems, skins and pips (seeds) of most red and dark fruit such as grapes, elderberries, sloes, apples, and plums, but also in pear skins, oak leaves, and dark tea leaves. Most grains, roots and flowers used in brewing and winemaking lack any or sufficient tannin, so must be supplemented with grape tannin or tannin from another source. Wines containing too much tannin can be ameliorated by adding a little sugar or glycerin, fined with gelatin, or blended with another, softer wine.