|Consumer resistance to the occurrence of potassium bitartrate (tartrate) crystals in wine has compelled cellars to stabilise wine against the possible settling of such crystals.
Different methods may be used, but refrigeration methods approximating the wine’s freezing point are the most commonly used. As this could possibly impact negatively in certain respects, improved methods are continuously being evaluated.
The chilling of wine to a temperature approximating the freezing point of wine is an effective means of cold stabilising wines before bottling, provided the low temperature is maintained for a sufficiently long period of time. It is possible, however, that treatment of this nature may have a detrimental effect on the sensorial character of the wine. This could possibly impact on the mouthfeel and also be a factor in the loss of wine flavour and the so-called bottle shock of wines. It has also been documented that apart from tartrates being removed by the process, phenolic acids, polysaccharides, various proteins and tannins have been identified, inter alia, in the sediment resulting from the cold stabilisation of wines. The removal of such compounds will undoubtedly influence the flavour, mouthfeel and colour of wines.
European researchers have developed a stabilisation method based on electrodialysis. This offers an alternative tartrtate stabilisation method which definitely warrants consideration. The settling of saturated potassium bitartrate (tartrate) and calcium tartrate is prevented by the concentration of the precursors of the settling reaction, namely the potassium ions, calcium ions and tartrate ions to such a level that the settling will no longer take place. This is achieved by pumping the wine between two membranes with a weak electrical charge on either side, at which time the cations and anions will pass through the respective membranes, depending on the charge that applies to the particular wine in question. This is a “soft” treatment seeing that the operation takes place at a pressure of 3 bars or less. The Selective Tartrate Removal System (STARS) also includes an analytical method used to measure the change in conductivity so as to monitor the effective removal of the ions. This method of stabilisation has the following practical benefits:
- The impact on the sensorial qualities of the wine is less than with the refrigeration method.
- Considerable energy is saved as it is a continuous process and no refrigeration is required.
- The product losses are less than with other methods.
- A fair amount of time is saved, as wines may be stabilised on the day of bottling. This also simplifies the logistical planning.
The concept of using electrodialysis for tartaric stabilisation is not new. The well-known company Gallo experimented with the process as early as the late 1960s, but was only partially successful. In the mid 1980s the process was successfully modified in France, but it was too expensive to apply commercially. During the 1990s the French, in collaboration with Japanese membrane producers, succeeded in making the process effective and economical. The result was that the OIV acknowledged it as an acceptable practice and in 1997 the use thereof for French vins ordinaires and estate wines was approved. A request to make it acceptable for French AOC wines was submitted to the OIV in 2003.
Extensive research was done in France and other European countries and the results of research conducted between 1992 and 1998 may be summarised as follows:
- The pH is usually reduced by 0,1 but never by more than 0,15.
- The volatile acidity is reduced by less than 0,12 g/l.
- The alcohol content is reduced by less than 0,1%.
- Various stationary and mobile units are currently being operated in several European countries.
In 2003 the well-known Fetzer Vineyards in California did a comparison between the effect of conventional cold stabilisation and the STARS method on a Sauvignon blanc and a Shiraz rosé wine. In the case of the Sauvignon blanc the STARS method had less impact on the alcohol and titratable acid content and in the case of the rosé it had less impact on the titratable acid content. The colour of the rosé was unchanged by the STARS method, while cold stabilisation did have a considerable influence on it.
Blackburn, D. & C. DiManno. 2004. New choice for tartaric stability. Practical Winemaking January/February 2004: 70-74
Various units for electrodialysis are already being used locally. Concept regulations to legalise the process have already been drawn up.