What do acidity or freshness mean in wine?

Tartaric acid, malic acid and lactic acid together determine most of the acidity in a glass of wine.

Tartaric acid comes from the grape and is the most stable of the three; it is also the acid in which laboratories often express total acidity. Malic acid also comes from the grape and gives a sharp, green impression. Lactic acid only arises in the cellar and looks softer.

In addition, wine contains smaller amounts of citric acid and succinic acid.

The acidity depends mainly on the ripeness of the grape at harvest.

The longer a grape hangs on the vine, the more the sugar content rises and the malic acid falls. In cooler regions and in higher-lying vineyards this happens more slowly, so more acidity remains at harvest. In warm regions the grape ripens sooner and less acidity remains.

That is why a wine from a cooler area, made from the same variety, often feels fresher than the same grape from a warm region.

Malolactic fermentation converts malic acid into lactic acid and thereby lowers the total acidity.

Lactic acid bacteria take up the malic acid and release lactic acid and carbon dioxide. According to Inagro the wine becomes softer and less acidic as a result. A wine that has gone through this conversion always contains lactic acid; a wine without conversion does not, and the malic acid is usually between 2 and 4 grams per litre.

Most red wines go through this step. With white wines, the winemaker deliberately chooses for or against it.

Total acidity measures how much acid is in the wine; pH measures how strongly that acid acts.

Inagro cites a pH of 3 to 3.5 for white wine, about 2.9 for sparkling wine and 3.3 to 3.8 for red wine. The lower the pH, the sharper the wine feels and the more stable the colour remains.

Two wines can thus have the same total acidity and yet come across differently, because their pH differs.

Within the European rules, wine may be acidified up to at most 2.50 grams per litre and deacidified up to at most 1 gram per litre.

The Rijksdienst voor Ondernemend Nederland converts that 2.50 grams to 33.3 milliequivalents per litre, expressed as tartaric acid. Fresh grapes, grape must and partially fermented must may be deacidified without limit; that limit of 1 gram does apply to wine itself.

In warm years winemakers use the margin to add acidity, in cool years the margin to reduce acidity.