Wines meant for exportation must be stabilized in order to prevent tartrate deposits from forming.
THE COLD OF WINTER CAUSES TARTRATE CRYSTALS to precipitate in bottles of wine in the cellar. These crystals do not harm the quality of a product that holds both symbolic and economic importance for France, but they do hamper its exportation to demanding or poorly informed clients. How can reductions in market value—or, worse still, returns—be avoided? Jean-Louis Escudier, Jean-Louis Baelle, and Bernard Saint-Pierre at the Institut National de la Recherche Agronomique (INRA) station in Pech Rouge and Michel Moutounet at the Institut Supérieur de la Vigne et du Vin in Montpellier have developed a method for balancing wines by electrodialysis.
Tartaric acid is a characteristic constituent of grapes, but the salts associated with it are not very soluble. For this reason potassium bitartrate and calcium tartrate naturally tend to precipitate in wines, forming deposits of what are commonly called tartrates. In the past, producers who wanted to avoid tartrate accumulations kept their bottles at a low temperature for ten days or so, adding to the wine potassium tartrate, which served as a crystal nucleus. The cold triggers crystallization because the limited solubility of tartaric salts diminishes further with the fall in temperature, while the action of polyphenols (a class of molecule that includes many of the coloring agents in red wines) ensures that the wine remains supersaturated with these salts—hence the ineffectiveness of traditional procedures in the case of red wine. Moreover, because the tartrate content of the wine varies, lasting stability is not guaranteed.
The Hunt for Tartrates
Because the cause of the problem is the excessive concentration of tartrate, potassium, and calcium ions in wines, the INRA researchers sought a way to eliminate them. Electrodialysis suggested itself as a promising candidate. Wine was made to flow between two polymer membranes by applying an electrical field perpendicular to the direction of flow: The negative ions were attracted on one side and the positive ions on the other. The researchers reasoned that if membranes were used that selectively let through potassium, calcium ions, and tartrate ions, these ions would be specifically extracted.
The wine was circulated through a stack of parallel anionic and cationic membranes that deionized it while enriching the lateral compartments (through which a solution of fixed composition circulated simultaneously) in tartrate, potassium, and calcium ions. The membranes were made of grafted polysulfones, 20 centimeters (a bit less than 8 inches) on a side and spaced 0.6 millimeter apart, and the electric field applied was 1 volt per cell. The researchers tested a total transfer surface of 4 square meters consisting of sixty stacked cells. In their modeling of the problem they tried to adapt the intensity of the electrical current to the degree of instability peculiar to each wine. As a result, oenologists now have at their disposal a tool suited to the majority of relevant cases.
Why does the new method do a better job of stabilizing the wines than the traditional technique? Because it uses the electrical conductivity of the environment as a measure of the amount of tartrate in the wine. This conductivity depends to a great extent on the concentration of potassium, calcium, and tartrate ions. By regulating the electrical field and the length of time the wine circulates as a function of its conductivity, it becomes possible to extract just the quantity of ions necessary to balance the wine, at a rate of about 1 hectoliter (a little more than 26 gallons) per hour using the pilot device tested. The speed of processing depends on the wine’s adhesive properties: Once a day a cleaning agent must be circulated through the system in order to remove the accreted polyphenols and tannins.
Does the new filtering process alter the quality of the wines? This vital question, one that is naturally of great interest to all gourmets, was studied in the course of a long and careful series of trials by oenologists in the Beaujolais, Champagne, and Bordeaux regions. They reported no qualitative difference in flavor between treated and untreated wines. Baccard S.A., in association with Eurodia S.A., which produces the membranes, plans to manufacture and market detartarizing equipment now that the necessary approvals have recently been obtained from Brussels; the first experiments were conducted under private auspices, but preliminary authorization for the expenditure of public funds in support of this research was received in 1996.
In the meantime the researchers continue to study possible uses for the extracted ions and to explore the processing of sweet fortified wines (vins doux naturels) and apéritifs made from wine, which are difficult to treat using traditional methods, with the aid of cold.