When tartaric acid and potassium in wine form potassium bitartrate, which precipitates easily, and calcium tartrate, which is difficult to precipitate, these compounds crystallize and form deposits in the wine. Most wine throws tartrate crystals over time. These crystals are harmless but can lead to consumer complaints. For this reason, they're often removed using several very debatable methods.
In cold stabilization, wine is chilled to -4ºC for table wines and -8ºC for sweet or fortified wines, and stored for eight days. This causes the tartrates to precipitate around minute crystals present in wine. The process is fairly inefficient because the crystals form at the bottom of the tank, so it is necessary to stir the wine to have it all come in contact with the crystals. The equipment for cold stabilization is expensive and the results are unpredictable.
In the contact process, the wine is chilled to 0ºC and crystals of potassium bitartrate are added. The wine is stirred and crystals form. These are removed by filtration, ground down and reused. A continuous version of this system is now being used where cooled wine is pumped through a bed of crystals into a tank and siphoned off at the top of the tank.
Metatartaric acid is an additive that prevents formation of tartrates for 6-18 months. It eventually breaks down to tartaric acid, encouraging more crystal formation. For this reason, it is used for short shelf-life wines.
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