The taste of a wine is mostly a matter of its sourness, or a balance between sour and sweet, and a savory quality that has been attributed to succinic acid and other products of yeast metabolism. Phenolic compounds can sometimes contribute a slight bitterness. The acid content of a wine is important in preventing it from tasting bland or flat; it’s sometimes said to provide the “backbone” for the wine’s overall flavor. White wines are usually around 0.85% acid, red wines 0.55%. Wines that are fermented dry, with no residual sugar, may still have a slight sweetness thanks to the alcohol and glycerol, a sugar-like molecule produced by the yeasts. Fructose and glucose are the predominant sugars in grapes, and they begin to provide a noticeable sweetness when left in wines at levels around 1%. Sweet dessert wines may contain more than 10% sugars. In strong wines, alcohol itself can dominate other sensations with its pungent harshness.