Pascal Ribéreau-Gayon - Handbook of Enology, Volume 2

On the basis of these results evaluating the protective effects of colloids and saturation temperatures before and after cold stabilization, it is possible to determine the most efficient way to prepare a white wine for bitartrate stabilization. It would appear that tannin–gelatin fining should not be used on white wines, while bentonite treatment is the most advisable. The effect of tannin–gelatin fining bears out the findings of Lubbers et al . (1993), highlighting the inhibiting effect of yeast cell wall mannoproteins on tartrate precipitation.

There are quite tangible differences in the performance of slow stabilization when wines have no protective colloids (wine filtered on a membrane retaining any molecule with a molecular weight above 1,000 Da). These effects ought to be even more spectacular in the case of rapid stabilization technologies. Indeed, the results presented in Figure 1.16show the impact of prior preparation on the effectiveness of the contact process.

It was observed that the crystallization rate during the first hour of contact, measured by the slope of the lines representing the drop in conductivity of the wine in microsiemens per centimeter per unit time, was highest for the wine sample filtered on a 10 3Da membrane, i.e. a wine containing no protective colloid macromolecules. In contrast, the addition of metatartaric acid (7 g/hl) completely inhibited the crystallization of potassium bitartrate, even after four hours. In production, bentonite and activated charcoal are the best additives for preparing wine for tartrate stabilization using the contact process.

In practice, the saturation temperature is obtained simply by two electrical conductivity measurements, at 20°C for white wines and 30°C for red wines. The first is measured on the wine alone, the other after the addition of 4 g/l of KHT crystals. Equations (1.10)and (1.11)are used to calculate T Satfor white wines and for red wines, respectively. The relationship between saturation temperature T Satand true stability temperature ( T CS) in various types of wine is yet to be established.

TABLE 1.16Influence of Pre‐treatment on the Physicochemical Parameters of a Cold Stabilized White Wine

Wines treated with slow cold stabilization (10 days at −4°C). Assessment of protective effects (Maujean et al ., 1985).

aThe differences, T Sat− T CS, were determined by dissolving 1 and 2 g/l of KHT to the wine. Conductivity was then recorded at decreasing temperatures until crystallization occurred; the T CSvalues were deduced.

FIGURE 1.16 Crystallization kinetics of potassium bitartrate analyzed by measuring the drop in conductivity of a wine according to the type of treatment or fining. Samples were stored at 2°C, seeded with 5 g/l of KHT, and subjected to the static contact process for four hours (Maujean et al ., 1986).

To define a rule that would be reliable over time, i.e. independent of the colloidal reorganizations in white wine during aging, Maujean et al . (1985, 1986) proposed the following equation:

Note that this equation totally ignores protective colloids and is valid for a wine with an alcohol content of 11% by volume. For white wines with an alcohol content of 12.5% vol., or those destined for a second fermentation that will increase alcohol content by 1.5% vol., the equation becomes

Thus, if stability is required at −4°C, the saturation temperature should not exceed 8°C. The stability normally required in Champagne corresponds to the temperature of −4°C used in the slow artificial cold stabilization process. It is questionable whether such a low temperature is necessary to minimize the probability of tartrate crystallization.

In the case of a rosé Champagne base wine, the equation is as follows:

This equation shows that, if stability is required at −4°C, the saturation temperature must be 11°C or lower.

In the case of red wines, it is possible to be less demanding, due to the presence of phenols. To simplify matters, Gaillard and Ratsimba (1990) related the tartrate stability of wines solely to saturation temperature. They estimated that stability is achieved if