Found in grape skins, pips and stalks, tannins are harsh, bitter compounds which if present in large amounts make a wine difficult to drink as they leave a dry, puckered sensation in the mouth. The amount of tannin can be increased by enhancing extraction, achieved by prolonging the cuvaison. Tannins may also enter the wine from oak barrels. Tannic wines are generally destined for ageing, the tannins polymerising to form sediment with time.