Читать книгу Handbook of Enology, Volume 2 - Pascal Ribéreau-Gayon - Страница 35

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

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	−1.15	18.2
After cold	6.97	3.04	740	0.77	7.26	9.6	9.77	−9.4	19
Activated charcoal (30 g/hl)	Before cold	7.21	3.1	940	1.59	20.97	17.05	17.2	−2.7	19.75
After cold	6.89	3.1	750	1.01	10.24	9.1	10.33	−11.3	20.4
Gum arabic (3 g/hl)	Before cold	7.31	3.08	940	1.45	18.07	16.8	16.98	−3.8	20.6
After cold	7.04	3.03	730	0.91	8.37	11	11.32	−10.95	21.95
Tannin (6 g/hl) and gelatin (3 g/hl)	Before cold	7.25	3.08	970	1.42	18.26	18	17.97	−4.9	22.9
After cold	7.2	3.08	970	1.32	17.46	16	16.16	−5.5	21.05
Metatartaric acid (5 g/100 bottles)	Before cold	7.19	3.01	975	1.23	20.35	19.25	18.91	<−3.75	>23
After cold	7.26	3.09	975	0.23	16.06	18.65	18.61	−6.09	24.7
Membrane filtered 103 Da	Before cold	6.51	3.08	955	1.25	15.83	16.9	16.54	2.85	14.05
After cold	5.67	3.01	535	0.3	2.24	1.8	0.63	−12.8	14.6
Membrane filtered 0.22 μm	Before cold	7.22	3.08	970	1.54	19.8	17	17.06	−3.65	20.65
After cold	7	3.03	970	0.94	9.08	11.6	11.21	−8.5	20.1

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

^a The 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_CS values 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

where TPI represents the total polyphenol index.

These rules, based on the solubilization of KHT, which is independent of the medium's composition and colloidal rearrangement, are applicable to monitoring cold stabilization treatments. However, they do not check the effectiveness of treatments via addition of crystallization inhibitors, such as metatartaric acid (Section 1.7.6) or mannoproteins (Section 1.7.7). Only crystallization tests, with or without added KHT, are able to show the effect of protective colloids, even though the values obtained are lower than the actual stabilization temperatures.