The grape’s principle acid, tartaric, is very soluble in wine. However, its mono-salt, potassium-bitartrate, is less soluble and can precipitate out during or after wine processing – forming small, clear glass-like crystals. These, whilst harmless, may offend the consumer.In fact, newly-fermented wine is normally saturated with potassium bitartrate, and, as it cools, bitartrate crystals will form. Therefore it is important that wine is cooled to the minimum temperature to which it is ultimately likely to be subjected to prior to bottling so that these crystals do not form in the bottle.

To test a wine for tartrate stability, it should be seeded with a small weighed tartrate crystal, and then chilled to around -4ºC for 24 hours. If the weight of the crystal increases, then the wine is unstable.

In order to stabilise the wine, it should be seeded with 4 g/l of cream of tartar and held at -4ºC for a few hours. The cream of tartar is separated from the wine by racking and can be re-used.
Note that wine freezes at roughly half its alcoholic content multiplied by -1, so there is no need to go much beyond -4ºC.
Alternatively, Mannoproteins such as Mannostab™ or gum Arabic (acacia gum) acacia may be used. These are protective colloids that prevents the tartrate crystals from growing large enough to be visible. The effect of gum Arabic is short-lived (less than a year), but mannostab is more reliable. Adding metatartaric acid is also permitted. This disrupts tartrate crystal formation and thus stops the crystals becoming large enough to be visible, but only lasts for a short period, months for warm temperatures or up to year when stored cold.