Winemaker's Archives -- George Troquato

Wine Crystals

Occasionally visible in wine bottles after opening, wine crystals are harmless insoluble salts of tartaric acid, a natural grape acid. Most crystals are clear like diamonds (potassium tartrate) while white crystals are salts of calcium tartrate. Most often, they are found at the bottom of the bottle or on the end of the cork.

Wine crystals are insoluble in alcohol but soluble in water. They are as much a part of wine as the pigment or the alcohol. While alcohol content increases during fermentation, tartaric acids decrease by way of crystallization. This process, also called "tartrate precipitation," is very slow and continues long after fermentation ceases.

Some winemakers will attempt to cold stabilize wine before bottling. Since tartrate crystallization is temperature influenced, stability is accomplished by chilling the wine to just above its freezing point and maintaining that temperature until insolubles have crystallized and the majority precipitated. The wine is then racked (moved to another tank) while still cold, leaving the crystals on the sides and bottom of the tank. The entire process usually takes 5-15 days at 28F. Despite these efforts, crystals may still form in bottles, particularly when they are properly stored in a cool cellar.