A wine's acidity should be detectable as a sharpness in the mouth, particularly around the front sides of the tongue. It should be neither too obvious nor absent. It provides a refreshing sensation in white wines, and balance in reds. Its absence makes a wine dull and 'flabby' - a defect in any wine, but a disaster in sweet wines which to me become undrinkable without balancing acidity. Too much acidity can make a wine difficult to drink. There are many acids in a wine, but the principle ones are acetic, malic, tartaric, lactic, citric and carbonic acid.