This is prohibited in zone A, but some acidification was permitted in 2003.
In the warmer viticultural areas (e.g. Australia and the Languedoc) the natural acidity of the crop tends to be low, so acidification is approved.
Acidification can improve the flavour, balance and colour of wine, but also decreases the pH, thus increasing the effectiveness of sulphur dioxide and inhibiting the growth of spoilage micro-organisms.
In the EU, tartaric acid is generally used for acidification by adding pre-fermentation, but citric acid can also be used (up to 1 g/l as tartaric) in order to increase wine stability. However, citric must never be added pre-fermentation, as it can be broken down into acetic acid and other off-flavours/aromas by yeast and bacteria.
Legal limits in Europe for the addition of tartaric acid are 150 g/hl in musts and 250 g/hl in wine.