Different acidities of wine. In the mouth it is felt on the sides of the tongue, but the acidity which one finds in a wine is the result of a group of acidic substances, some from the grape and others from the fermentation process. These are fundamental to the balance of the wine, not merely from the point of view of taste but also to help it keep over time. In brief, the acidity is the fuel which allows the wine to go a long way.
The total acidity of a wine includes all the volatile substances as well as the fixed substances. It is much more useful to refer to the pH of a wine, which expresses the real acidity of a substance, giving its real level. The pH is measured on a scale of 0 to 14 and the lower the figure the higher the acidity.
The pH figure, then, will be lower in fresh, fragrant wines where the grapes were harvested early to keep a high level of acidity. In more important, long-lived wines the pH will be higher (so the acidity will be lower) but should be well balanced and in harmony with the structure of the wine.
For these wines it is important that the acid substances are less aggressive: this is why the alcoholic fermentation is followed by a malolactic fermentation which changes the malic acid into lactic acid, which is weaker and gives the wine more smoothness.