Acidity: the acids in wine and what each does

Acidity is what stops wine tasting flabby, and it is the single most important structural component in white wine — the equivalent of what tannin does for reds.

Unlike tannin, it is a taste rather than a texture: sourness is detected by taste receptors, and it also provokes salivation, which is why a high-acid wine makes the mouth water.

The acids are not interchangeable. Which ones are present, and in what proportion, changes the character as much as the total quantity does.

Tartaric acid is the principal acid in grapes and is unusual in being rare in other fruit. It is relatively stable through fermentation and is the main contributor to a wine’s pH. It can precipitate as potassium bitartrate crystals when the wine gets cold, which is what cold stabilisation is for.

Malic acid is the second major grape acid and is the same acid that makes a green apple sharp. It falls during ripening as the vine metabolises it, and it falls faster in warm conditions — which is a large part of why warm-region fruit arrives with lower acidity.

Lactic acid is not present in the grape. It appears when lactic acid bacteria convert malic acid during malolactic fermentation, and it is a weaker, softer-tasting acid. The conversion lowers total acidity and raises pH.

Acetic acid is the acid in vinegar and is produced by acetic acid bacteria and by some yeasts. Every wine contains a small amount; above roughly 0.6 to 0.9 g/L most tasters begin to notice it, and it is then treated as volatile acidity, a fault.

Citric acid is present in small quantities and may be added in some jurisdictions, within limits, though it is microbiologically less stable than the others.

Acidity is one of the most reliable predictors of how a wine will sit with food, because it cuts through fat and can mirror acidity in the dish.

It is also a preservative: low pH inhibits spoilage organisms, which is why high-acid wines are generally more microbiologically robust and often age longer.

And it is directly manipulable in the winery, which is why the same grape in the same place can produce noticeably different wines depending on whether malolactic fermentation was allowed to run.