🍷 Tartaric acid

Tartaric acid is the principal acid of grapes: grape juice at harvest contains roughly 4 to 8 g of it per litre. It accumulates in comparatively few plant species, and it is responsible for much of the freshness and sharpness of wine.

Its low pH helps wine age well and limits spoilage by microbes and oxidation. The pH of a must or wine also affects the colour of red wine and the effectiveness of sulfur dioxide. In warm seasons grapes ripen with less acid and a higher pH, and a winemaker may add tartaric acid to correct this; for red musts the AWRI advises a pH of 3.4 or lower.

As the alcohol level rises during fermentation, and as wine is stored at lower temperatures, tartaric acid precipitates as potassium bitartrate, the salt known as tartar or cream of tartar, forming crystals. Winemakers often chill the wine on purpose (cold stabilisation) so that these crystals form and can be filtered out.

Tartaric acid comes from the grape and stays at a relatively constant level during fermentation. Malic acid, the other main acid of the grape, is converted into lactic acid by lactic acid bacteria in malolactic fermentation.