the basis of research into potassium bitartrate (Figure 1.11), Vallée (1995) used measurements of electrical conductivity to define the width of the domain of supersaturation expressed in degrees Celsius, as well as the calcium tartrate saturation temperature of various types of wines. The low solubility of calcium tartrate indicates that saturation temperatures are likely to be much higher than those of potassium bitartrate.
To avoid the risk of calcium tartrate precipitation, the saturation temperature of white and rosé wines and vins doux naturels must be lower than 26°C to ensure that calcium tartrate deposits will not be formed if the wine is kept at 2°C for one month. The calcium tartrate saturation temperature for red wines must be below 35°C.
According to Postel (1983), the addition of 100 mg/l of metatartaric acid is capable of stabilizing a wine stored at 4°C for several months, so it does not suffer from crystalline deposits of CaT. Furthermore, the use of racemic acid (D‐L‐tartaric acid) or calcium L‐tartrate has been suggested for eliminating excess calcium (Ribéreau‐Gayon et al., 1977). In both cases, the precipitation of calcium racemate, a highly insoluble salt, totally eliminates the cation. The treatment's effectiveness depends on the colloid content of the wine, as colloids hinder precipitation of the salt. These treatments are used to varying degrees in different wine regions depending on the types of wines produced.
Finally, ion exchange (Section 12.4.3) and electrodialysis (Section 12.5) are also processes for preventing calcium tartrate deposits.
1.7.6 The Use of Metatartaric Acid
In the processes described above, tartrate precipitations are prevented by eliminating the corresponding salts. It is also possible to envisage the addition of crystallization inhibitors.
The first positive results were obtained with hexametaphosphate, which certainly proved to be effective (Ribéreau‐Gayon et al., 1977). However, very high doses were necessary in certain wines, and, above all, the increase in phosphate content led to the formation of an iron (III) complex that caused instability on contact with air (ferric phosphate casse, Section 4.6.2).
Metatartaric acid is currently the product most widely used for this purpose. Its properties and conditions of use are described in this paragraph. It is perfectly effective; however, its stability over time is insufficient.
CMC (Section 1.7.8) and mannoproteins extracted from yeast (Section 1.7.7) have also been suggested as stabilizers.
Metatartaric acid is a polyester resulting from the intermolecular esterification of tartaric acid at a legally imposed minimum rate of 40%. It may be used at doses up to a maximum of 10 g/hl to prevent tartrate precipitation (potassium bitartrate and calcium tartrate) (Ribéreau‐Gayon et al., 1977).
When tartaric acid is heated, possibly at low pressure, a loss of acidity occurs, and water is released. A polymerized substance is formed by an esterification reaction between an acid function of one molecule and a secondary alcohol function of another molecule. Tartaric acid may be formed again if a polymerized substance of the lactide family is subjected to hydrolysis. In reality, however, not all of the acid functions react (Figure 1.18).
FIGURE 1.18 Polyesterification reaction involved in the formation of metatartaric acid.
Metatartaric acid is not a single compound, but rather a dispersed polymer, i.e. a mixture of polymers with different molecular weights. There are many metatartaric acid preparations with various anti‐crystallizing properties, depending on the average esterification rate of their acid functions. It is possible to obtain an esterification rate higher than the theoretical equilibrium rate (33% for a secondary alcohol) by heating tartaric acid to 160°C in a partial vacuum. Under these conditions, the thermodynamic esterification equilibrium is shifted by eliminating water.
TABLE 1.18 Detailed Analysis of Various Metatartaric Acid Preparations (Peynaud and Guimberteau, 1961)
| Preparation method | For 1 g of chemical | Esterification number (%) | Pyruvic acid (%) | Corrected esterification number (%) | ||
|---|---|---|---|---|---|---|
| Acidity (mEq) | Esters (mEq) | Acidity + esters (mEq) | ||||
| Reduced pressure, 160°C | ||||||
| 15 min | 10.67 | 3.13 | 13.80 | 22.6 | 0.9 | 22.8 |
| 40 min | 8.77 | 5.14 | 13.91 | 36.9 | 4.2 | 37.5 |
| 45 min | 8.63 | 5.57 | 14.20 | 39.2 | 4.4 | 40.6 |
| 50 min | 8.48 | 5.70 | 14.18 | 40.2 | 4.1 | 41.5 |
| 55 min | 8.32 | 5.74 | 14.06 | 40.8 | 5.6 | 42.7 |
| Reduced pressure, 175°C | ||||||
| 20 min | 9.81 | 3.65 | 13.46 | 27.1 | 5.2 | 28.3 |
| 90 min | 9.56 | 3.76 | 13.32 | 28.2 | 2.3 | 28.7 |
| 105 min | 9.11 | 4.58 | 13.69 | 33.4 | 5.4 | 35.0 |
The esterification number of different metatartaric acid preparations may be determined by acidimetric assay, before and after saponification. Table 1.18 shows the importance of the preparation conditions in determining this value.
Metatartaric acid is by no means a pure product: solutions are slightly colored and oxidizable.