Flavor perceived on the tongue and caused by the acidity of a wine or beer. Some acidity is as essential part of the "backbone" or "structure" of a wine, providing a crispness of flavor important in wine. Acidity also contributes to the aging potential of wine. White wines are generally slightly more sour or acidic than reds, often balanced by residual sugar. Red wines are generally more tannic. Sourness is also associated with some unique beer styles, especially Belgian Lambics.