6.1.2 Stabilization

Tartaric stabilization

Grapes, must and wine all contain tartaric acid in varying amounts. They also contain potassium and calcium. Tartaric acid, in its TH- form, can bind either to potassium to form primarily potassium hydrogen tartrate (KHT), or to calcium to form the CaT complex. Both salts are soluble in wine but can precipitate and form tartar deposits at the bottom of the bottle, which is not always acceptable to the consumer.
Tartaric acid and potassium concentrations can affect the wine's tartaric stability as well as the pH, temperature and ABV. For example, the lower the temperature and the higher the KHT salt concentration, the greater the risk of precipitation.

Thermal treatments at a very low temperature over a few days (4°C) help to accelerate the formation of tartar crystals and their sedimentation.

Protein stabilization

The proteins responsible for instability in white wines come from the grapes although the nature of these proteins varies with grape variety.
It is possible to set up a test to evaluate the risk of protein breakdown prior to bottling. Heat treatment in a water bath (80°C for 30 minutes) is the most commonly used, and causes precipitation to occur after cooling.
The heat denatures the proteins and makes them insoluble after cooling. When the resulting cloudiness is greater than 2 NTU, the wine is considered unstable. It is then possible to implement a treatment to stabilize the entire batch before bottling.

Bentonite treatments are the most widely used method to remove excess proteins. Several types of bentonites exist: acid, calcium and sodium. Sodium bentonites are the most commonly used for clarification treatments as they allow water and ion retention and greater swelling, and thus a greater absorption capacity of the proteins involved in protein precipitation.
Bentonite does not denature the wine and preserves organoleptic properties at moderate doses. This treatment is used both on white wines, which are more susceptible to protein precipitation, and on red wines that are rich in colloidal colouring matter.

Other treatments, particularly those using heat, can prevent protein breakdown by denaturing the proteins. Cold treatments can also be used for white wines.