Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon
These theoretical considerations, applied to a short treatment involving seeding with tartrate crystals, show that great care and strict supervision are required to ensure the effectiveness of artificial cold stabilization. The following factors need to be closely monitored: the wine's initial state of supersaturation, the particle size of the added tartrates, the seeding rate, the effectiveness of agitation at maintaining the crystals in suspension, treatment temperature, and, finally, contact time.
1.5.3 Using Electrical Conductivity to Monitor Tartrate Precipitation
Wurdig and Muller (1980) were the first to make use of the capacity of must and wine to act as electrolytes, i.e. solutions conducting electricity, to monitor tartrate precipitation. Indeed, during precipitation, potassium bitartrate passes from the dissolved, ionized state, when it is an electrical conductor, to a crystalline state, when it precipitates and is no longer involved in electrical conductivity:
The principle of measuring conductivity consists in making the wine into an electrical conductor, defined geometrically by the distance l separating two platinum electrodes with S‐shaped cross sections. The resistance R (in ohms) of the conductor is defined by the relation:
TABLE 1.12 Resistivity and Conductivity of a KCl (0.02 M) Solution According to Temperature (in °C).
| Temperature (°C) | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| Resistivity (Ω/cm) | 446 | 436 | 426 | 417 | 408 | 400 | 392 | 384 | 376 | 369 | 362 |
| Conductivity (μS/cm) | 2,242 | 2,293 | 2,347 | 2,398 | 2,451 | 2,500 | 2,551 | 2,604 | 2,659 | 2,710 | 2,769 |
In this equation, ρ is the resistivity. Its inverse (γ) is the conductivity expressed in siemens per meter (S/m) or microsiemens per centimeter (μS/cm = 10−4 S/m).
The expression of resistivity ρ = RS/l involves the term S/l, known as the cell k constant. This constant is specific to each cell, according to its geometry, and may also vary with use, due to gradual deterioration of the electrodes or the effect of small impacts.
It is therefore necessary to check this constant regularly and to determine it at a conductivity close to that of wine. In practice, a 0.02 M KCl solution is used. The temperature of the KCl (0.02 M) solution must be taken into account when checking the cell constant. The resistivity and conductivity values of this solution according to temperature are specified in Table 1.12.
The conductivity meter cell is subjected to an alternating current. The frequency is set at 1 kHz for the standardized solution (KCl = 0.02 M) and for wine to avoid polarizing the electrodes. A conductivity meter is used for continuous monitoring of tartrate precipitation in wine (see Section 1.6.4, Figure 1.16).
1.6 Tests for Predicting Wine Stability
1.6.1 The Refrigerator Test
This traditional test is somewhat empirical. A sample (approximately 100 ml) of wine, taken before or after artificial cold stabilization, is stored in a refrigerator for four to six days at 0°C and then inspected for crystals. In the case of wines intended for a second fermentation, alcohol may be added to increase the alcohol content by 1.3–1.5% vol. This simulates the effects of the second fermentation and makes it possible to assess the bitartrate stability of the finished sparkling wine.
The advantages of this test are that it is simple and practical and requires no special equipment. On the other hand, it is mainly qualitative and does not provide an accurate indication of the wine's degree of instability. Its major disadvantage is that it takes a long time and is incompatible with short contact stabilization technologies, where rapid results are essential to assess the treatment's effectiveness in real time.
Finally, this test is neither reliable nor easily repeatable, as it is based on the phenomenon of spontaneous, non‐induced crystallization—a slow, undependable process.
1.6.2 The Mini‐Contact Test
A sample of wine with 4 g/l added potassium bitartrate is maintained at a temperature of 0°C for two hours and constantly agitated. The wine sample is cold filtered, and the weight increase of the tartrate collected (exogenous tartrate + wine tartrate) is assessed. It is also possible to dissolve the precipitate in a known volume of hot water and measure the increase in acidity as compared with that of the 4 g/l exogenous potassium bitartrate added to the wine.
The mini‐contact test is based on homogeneous induced nucleation, which is faster than primary nucleation. However, this test does not take into account the particle size of the seed tartrate, although the importance of its effect on the crystallization rate is well known. The operative factor in this test is the surface area of the liquid–solid contact interface. Furthermore, this test defines the stability of the wine at 0°C and in its colloidal state at the time of testing. In other words, it makes no allowance for colloidal reorganization in wine, especially red wine, during aging.
It is normal to find potassium bitartrate crystals, associated with precipitated condensed coloring matter, in wine with several years' aging. When phenols condense, they become bulky, precipitate, and are no longer able to express their protective colloid effect.
It should be noted that mini‐contact test results tend to overestimate a wine's stability and therefore the effectiveness of prior treatment. This statement is based on work by Boulton (1982). After two hours' contact, only 60–70% of the endogenous tartrate has crystallized, and therefore the increase in weight of the crystal precipitate is lower than it could be. These results are interpreted to mean that the treatment was more effective, 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