The saturation temperature of a wine was determined by measuring electrical conductivity (Figure 1.14) in a two‐stage experiment.
In the first experiment, the wine is brought to a temperature of approximately 0°C in a temperature‐controlled bath equipped with sources of heat and cold. The temperature is then raised to 20°C in 0.5°C increments, and the wine's conductivity measured after each temperature change. In this way, it can be observed that the variation in conductivity according to the temperature of a wine containing no KHT crystals is represented by a roughly straight line.
In the second experiment, a volume (100 ml) of the same wine is brought to a temperature close to 0°C, 4 g/l of KHT crystals is added, and the temperature is once again raised to 20°C in 0.5°C increments. The wine is agitated constantly, and its conductivity measured after each temperature change. Two patterns are observed:
1 Subsequent to the addition of 4 g/l of KHT, the wine (Figure 1.14a) shows a linear variation in conductivity at low temperatures that could almost be superimposed on that of the wine without crystals, until a temperature TSat, where the conductivity leaves the straight line and follows the exponential solubility curve.FIGURE 1.14 Experimental determination of the saturation temperature of a wine by the temperature gradient method (Wurdig et al., 1982). (a) Example of a wine that is not highly supersaturated, in which no induced crystallization occurs after the addition of potassium bitartrate crystals at low temperature. (b) Example of a highly supersaturated wine, in which induced crystallization occurs immediately after the addition of potassium bitartrate crystals.
2 Following the addition of 4 g/l of KHT, the wine's conductivity (Figure 1.14b) at temperatures around 0°C is below that of the wine alone. This means that low‐temperature induced crystallization has occurred, revealing a state of supersaturation with high endogenous KHT levels in the wine. Its conductivity then increases in a linear manner until temperature TA; then the KHT starts to dissolve, and the conductivity follows the exponential solubility curve. At temperature TB, the exponential solubility curve crosses the straight line showing the conductivity of the wine alone. This intersection corresponds to the wine's true saturation temperature. The temperature TA corresponds to that of the same wine after “contact,” leading to desaturation caused by induced crystallization. It is therefore normal that, following desaturation, the wine should solubilize more KHT, at a temperature lower than its true saturation temperature, TB.
On a production scale, where rapid stabilization technologies are used, experimental determination of the saturation temperature by the temperature gradient method is incompatible with the rapid response required to monitor the effectiveness of ongoing treatment.
On the basis of statistical studies of several hundred wines, Wurdig et al. (1982) established a linear correlation defined by
FIGURE 1.15 Determining the saturation temperature of a wine according to the variation (ΔL) in conductivity at 20°C before and after the addition of potassium bitartrate (KHT) (Wurdig et al., 1982).
This straight‐line correlation (Figure 1.15) between the variation in conductivity of a wine at 20°C before and after the addition of 4 g/l of cream of tartar (ΔL) and the saturation temperature has only been verified for wines where the solubilization temperature of KHT is between 7 and 20°C. The practical advantage of using this equation is that the saturation temperature of a wine may be determined in just a few minutes, using only two measurements.
In some wines, crystallization may be induced by adding cream of tartar at 20°C. This means that they have a lower conductivity after the addition of tartar, i.e. a saturation temperature above 20°C. This is most common in rosé and red wines. To determine their precise saturation temperature, the samples are heated to 30°C. Cream of tartar is added, and the increase in conductivity at this temperature is measured. The saturation temperature is deduced from (Maujean et al., 1985)
Calculating the saturation temperature of a wine prior to cold stabilization provides information on the optimum seeding rate for that wine. Indeed, it is not necessary to seed at 400 g/hl, as often recommended, if 40 g/hl is sufficient.
1.6.4 Relationship Between Saturation Temperature and Stabilization Temperature in Wine
The temperature at which a wine becomes capable of dissolving bitartrate is a useful indication of its state of supersaturation. However, in practice, enologists prefer to know the temperature below which there is a risk of tartrate instability. Maujean et al. (1985, 1986) tried to determine the relationship between saturation temperature and stability temperature.
The equations for the solubility (A) and supersolubility (B) curves (Section 1.5.1, Figure 1.11) were established for this purpose by measuring electrical conductivity. They follow an exponential law of the following type: C = aebt, where C is the conductivity, t is the temperature, and a and b are constants.
The experiment to obtain the exponential supersolubility curve (B) consists of completely dissolving added cream of tartar in a wine at 35°C and then recording the conductivity as the temperature dropped. This produces an array of straight‐line segments (Figure 1.11) whose intersections with the exponential solubility curve (A) correspond to the saturation temperatures
Of course, if only 1.1 g/l of KHT is dissolved in the same wine, crystallization occurs at a lower temperature, as the wine is less supersaturated
