Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon

, respectively. The difference, , defines the width of the DS of the wine in which i added KHT has been dissolved, expressed in degrees Celsius. The width of the domain of supersaturation is independent of the addition value i, as exponential curves (A) and (B) are roughly parallel. Thus, in the example described (Figure 1.11), the width of the domain of supersaturation is close to 21°C, whether 1.1 g/l or 1.8 g/l of KHT is added. If 21°C is subtracted from the true saturation temperature of the wine , i.e. no added KHT (i = 0), it may be deduced that spontaneous crystallization is likely to occur in this wine at temperature .

The experimental method for finding the width of the domain of supersaturation has just been described, and the relationship between the saturation temperature and the temperature below which there is a risk of crystallization has been deduced. The width of the domain of supersaturation, corresponding to the delay in crystallization, must be linked, at least partially, to the phenomenon of supercooling (the effect of alcohol), as well as the presence of macromolecules in the wine that inhibit the growth of the nuclei. These macromolecules include carbohydrate, protein, and phenol colloids. It seems interesting, from a theoretical standpoint, to define the contribution of these protective colloids to the width of the domain of supersaturation. It also has a practical significance and should be taken into account in preparing wines for tartrate stabilization. For this purpose, aliquots of the same white wine at 11% vol. alcohol were subjected to various treatments and fining (Table 1.16). At the same time, a model dilute alcohol solution was prepared: 11% vol. buffered at pH 3, containing 4 g/l of KHT, with a saturation temperature of 22.35°C. The spontaneous crystallization temperature of the same solution was also determined after 1.4 g/l of KHT had been dissolved in it, . It was thus possible to find the width of the domain of supersaturation, i.e. 15°C.

      The spontaneous crystallization temperature of each sample of treated wine (Table 1.16) was also determined using the same procedure. Examination of the results shows that a wine filtered on a 10³ Da Millipore membrane, i.e. a wine from which all the colloids have been removed, has the lowest value for the domain of supersaturation , closest to that of the model dilute alcohol solution. Therefore, the difference between the results for this sample and the higher values of the domains of supersaturation of fined samples defines the effect of the protective colloids. It is interesting to note that the sample treated with metatartaric acid had the widest supersaturation field and cold stabilization was completely ineffective in this case. This clearly demonstrates the inhibiting effect this polymer has on crystallization and, therefore, its stabilizing effect on wine (Section 1.7.6). Stabilization by this method, however, is not permanent.

      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 1.16 show 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 10³ Da 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.

      1.6.5 Applying the Relationship Between Saturation Temperature (T_Sat) and Stabilization Temperature (T_CS) to Wine in Full‐Scale Production

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_Sat for white wines and for red wines, respectively. The relationship between saturation temperature T_Sat and true stability temperature (T_CS) in various types of wine is yet to be established.

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

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Samples	Total acidity (g/l H2SO4)	pH	Potassium (mg/l)	Tartaric acid (g/l H2SO4)	CPK × 105	T Sat measured (°C)	T Sat calculated (Wurdig) (°C)	T CS calculated (°C)	T Sat − TCS measured (°C) a
Control	Before cold	7.03	3.13	970	1.46	19.67	18.19	17.85	−2.60	20.8
	After cold	7	3.05	730	0.98	9.21	9.55	11.06	−12.7	22.25
Bentonite (30 g/hl)	Before cold	7.29	3.09	985	1.59	20.97	17.05	17.14