or the wine more stable, than was actually the case. To make the mini‐contact test faster, more reliable, and compatible with the dynamic contact process, the Martin Vialatte company proposed the following variant in 1984: seeding a wine sample with 10 g/l of cream of tartar and measuring the drop in conductivity at 0°C.
The rules governing stability under the extreme supersaturation conditions prevailing in wine are as follows:
1 If, in the 5–10 minutes after seeding, the drop in conductivity is no more than 5% of the wine's initial conductivity (measured before adding potassium bitartrate), the wine may be considered to be properly treated and stabilized.
2 If the drop in conductivity is over 5%, the wine is considered unstable.
As this test is based on measuring the wine's electrical conductivity, it has the tremendous advantage that there is no need to collect the precipitate by filtration and determine the increase in weight. This new mini‐contact test, measuring conductivity, is much faster (5–10 minutes instead of two hours). Furthermore, by comparison with the first variant of the mini‐contact test, as the contact surface (A) and, consequently, the state of supersaturation of the wine are multiplied by 2.5 (adding 10 g/l of KHT instead of 4 g/l), it gives a more accurate assessment of a wine's stability.
In spite of these improvements, this test remains open to criticism, and its reliability is limited. Indeed, as is the case with the preceding test, it does not always take into consideration the effect of particle size and is based on excessively small variations in conductivity and too short a contact time. The results in Tables 1.13–1.15 corroborate this point of view. In Table 1.13, results indicate that a variation of over five units in the concentration product CPK (see samples A and D) only caused a 1% decrease from the wine's initial conductivity. In this instance, a white wine with a CPK close to 13 was considered unstable, but this assessment was not confirmed by the percentage drop in conductivity.
The unreliability of this result is confirmed by the experiment described in Table 1.14, involving a wine with an initial CPK of 9.17 × 105, maintained at 30°C, in which increasing concentrations of commercial cream of tartar were dissolved. It was observed that when the CPK of a wine was doubled (e.g. between wine +0.2 g/l of dissolved KHT and wine +1 g/l of dissolved KHT) the percentage drop in conductivity was the same, although there was obviously a difference in stability.
TABLE 1.13 Values of the Concentration Products of Wines and the Corresponding Percentage Drop in Conductivity Produced by the Mini‐Contact Test
| Samples | CPK × 105 | Drop in conductivity at 0°C (%) |
|---|---|---|
| A | 7.28 | 0.5 |
| B | 11.62 | 1.0 |
| C | 11.84 | 0.0 |
| D | 12.96 | 1.5 |
Table 1.15 shows that the effects of variations in cream of tartar particle size and contact time in the same wine are capable of causing a 5% difference in the drop in initial conductivity, which is the benchmark for deciding whether a wine is stable or not.
In practice, a rapid response test is required for monitoring the effectiveness of artificial cold stabilization. The preceding results show quite clearly that the tests based on induced crystallization are relatively unreliable for predicting the stability of a wine at 0°C.
1.6.3 The Wurdig Test and the Concept of Saturation Temperature in Wine
Wurdig et al. (1982) started with the idea that the more KHT a wine is capable of dissolving at low temperatures, the less supersaturated it is with this salt and, therefore, the more stable it should be in terms of bitartrate precipitation. The authors defined the concept of saturation temperature (TSat) in a wine on the basis of this approach.
The saturation temperature of a wine is the lowest temperature at which it is capable of dissolving potassium bitartrate. In this test, temperature is used as a means of estimating the bitartrate stability of a wine, on the basis of the solubilization of a salt.
In comparison with the previously described tests, based on crystallization, this feature seems very convincing. Indeed, the solubilization of a salt is a spontaneous, fast, repeatable phenomenon, much less dependent on the particle size of the added tartrate crystals. The solubilization of KHT is also much less affected by the colloidal state of the wine at the time of testing. It has been observed that protective colloids act as crystallization inhibitors, but do not affect the solubilization of salts. Consequently, estimating the bitartrate stability of a wine by testing the solubilization of KHT, i.e. saturation temperature, is a more reliable measurement in the long term as it is independent of any colloidal reorganization during storage and aging.
TABLE 1.14 Limitations of the Reliability of the Mini‐Contact Test in Assessing the Stability of a Wine by Adding Increasing Quantities of Potassium Bitartrate and Measuring the Percentage Drop in Conductivity
| Samples | pH | K+ (mg/l) | CPK × 105 | Drop in initial conductivity (%) |
|---|---|---|---|---|
| Control | 3 | 390 | 9.17 | 1.5 |
| Wine + 0.2 g/l KHT | 3 | 420 | 10.85 | 11.5 |
| Wine + 0.5 g/l KHT | 3.03 | 469 | 13.33 | 7.5 |
| Wine + 0.7 g/l KHT | 3.05 | 513 | 15.26 | 12.5 |
| Wine + 1 g/l KHT | 3.06 | 637 | 21.16 | 11.5 |
TABLE 1.15 Influence of Tartrate Particle Size and Mini‐Contact Test Time on the Percentage Drop in Conductivity of the Wine
| Drop in conductivity (%) | Commercial KHT | KHT: particle size greater than 100 μm | KHT: particle size smaller than 63 μm |
|---|---|---|---|
|
After 10 min
|
