Tannins are naturally found in the skins of grapes and it is what gives grapes their bitter and astringent flavor which fades as the fruit ripens. Wine tannin is often added to non-grape wines to add astringency, dryness, and structure to the body of a wine. Wine tannin can also be added in smaller amounts to grape wines to boost these characteristics. Use liquid tannin instead of powder when very small amounts are required. 4 oz of liquid tannin.