After fermentation, young wines are usually saturated with potassium bitartrate. When a wine is cooled the solubility limit is exceeded and the 'excess' precipitates out as crystals. The lower the temperature to which it is cooled the more crystals will form, until the saturation equilibrium at that temperature is reached.

Calcium tartrate (CaT) crystals can also be formed in wine, although this is much less common. Formation is less influenced by temperature, but is time dependent and can take months to develop. Use of any calcium-based additives should be avoided.

If bottled wine is subjected to lower temperature than it has experienced previously, then tartrate crystals may appear in the bottle, looking like tiny pieces of broken glass. The crystals are of course harmless but their presence is generally undesirable to consumers.

With all the concern about contamination of foodstuffs, preventing any possibility of tartrate crystals appearing in bottled wine has become an important task for a winemaker. To achieve this, winemakers 'cold stabilise' their wine while it is still in tank, by exposing it to temperatures around freezing to enable the excess tartrates to crystallise out.

Cold stabilisation is an important stage in the process of winemaking. It lowers the total acidity, and raises the final pH, of the wine.
At a pH of about 3.6, depending on alcohol content, the solubility of potassium bitartrate is at it's highest; either side of this pH the solubility decreases. Blending together two already 'cold stable' wines could alter the pH so that the blended wine is not cold stable; hence, it is important to cold stabilise any wine after blending.

Where wineries experience cold winter temperatures cold stabilisation is usually left to occur naturally over the winter months.
In other situations it is induced using refrigeration/chilling.
Usually this is done only to white and rosé wines. This isn't because red wines do not form crystals but they have lower acidity anyway and they are served at a relatively warm temperature; consumers do not normally put bottles of red wine into refrigerators.

The simplest technique for inducing cold stabilisation

Soon after the fermentation is complete, the tank is cooled/chilled to around freezing (between -2 °C and +2 °C) and held at this temperature for 1 to 3 weeks to allow crystals to form. The crystals stick to the sides of the vessel, and the wine is then racked off leaving the crystals in the tank.
Some winemakers argue that a portion of the flavour and uniqueness of the wine is lost with the crystals.

To reduce the time for the stabilisation process a small amount of very finely powdered potassium bitartrate (cream of tartar) can be added to the tank and roused (continually for one hour) into the cold wine. This 'seeds' the wine and provides a very large surface area for the excess tartrates in the wine to precipitate on to.
The minimum amount of cream of tartar to use varies with the wine, but 4 grams/litre is the recommended amount to achieve stability within one hour, in wine at between -2 °C and 0 °C. Lower quantities provide less contact area for nucleation of the crystals so longer time is needed to achieve stability.

The wine then has to be left to clear before racking, or it can be centrifuged.

Following racking, hot water is used to wash out the empty tank. The wet crystals can be collected, sieved, dried and ground very finely for re-use, the excess can be sold as 'cream of tartar'.

With red wines it is necessary to chill for 24 hours and then rack, before treatment, in order to remove cold-unstable pigments and tannins, otherwise the cream of tartar would be contaminated and not re-useable.

Testing for tartrate stability

Testing should can be carried out to check if the wine has reached stability.
Such tests are particularly worthwhile if refrigeration is used, since cold stabilisation requires a large amount of refrigeration power and can cost a considerable amount of money. Winemakers should always strive to use the minimum amount of refrigeration to make the wine cold stable. Not long enough refrigeration and the wine won't be stable, although too much refrigeration will certainly make the wine stable but will waste refrigeration resources unnecessarily.

The test most commonly used, having been found to be the best indicator of a cold stable wine is:-
'Storage of a sample of the wine at –4 °C for 72 hours'.
The only major piece of equipment required is a small 'water-bath' which is capable of holding a temperature of –4 °C.

A sample is filtered, by gravity through a small conical filter, then held at -4 °C for 72 hours and then inspected under bright light for any crystal formation. The sample is examined again after warming to ambient temperature, and if crystals are present then the wine is deemed unstable, and further treatment is required.

This is a relatively easy test to perform. It is important to ensure the water-bath temperature is accurately measured and that the wine is filtered into a clean container.

Although this test takes more than 3 days to complete, I suggest that initially it is carried out on samples taken periodically, in order to gain experience of the time necessary to achieve stability. The same water-bath could be used for all samples.

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