39.2
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. They may contain oxaloacetic acid, but the main impurity is pyruvic acid, representing 1–6% by weight of the metatartaric acid product, depending on preparation conditions (Table 1.18). It is therefore important to correct the esterification number to compensate for this impurity. The formation of these two acids results from the intramolecular dehydration of a tartaric acid molecule, followed by decarboxylation (Figure 1.19).
There are many laboratory tests for assessing the effectiveness of a metatartaric acid preparation. Table 1.19 presents an example of a procedure where a saturated potassium bitartrate solution is placed in 10 ml test tubes and increasing quantities of metatartaric acid preparations with different esterification numbers are added. This inhibits the precipitation of potassium bitartrate induced by adding 1 ml of 96% vol. ethanol and leaving the preparation overnight at 0°C. Only 1.6 mg of a preparation with an esterification number of 40.8 is required to inhibit crystallization, while 4.0 mg is necessary if the preparation has an esterification number of 26.6.
FIGURE 1.19 Impurities in metatartaric acid.
Metatartaric acid acts by opposing the growth of the submicroscopic nuclei around which crystals are formed. The large uncrystallized molecules of metatartaric acid get in the way during the tartrate crystal building process, blocking the “feeding” phenomenon, i.e. crystal growth. If the dose is too low, inhibition is only partial, and anomalies and unevenness are observed in the shape of the crystals.
TABLE 1.19 Inhibition of Potassium Bitartrate Precipitation by Various Metatartaric Acids (Peynaud and Guimberteau, 1961)
| Number | Esterification number | Metatartaric acid added to each tube (in mg) | |||||
|---|---|---|---|---|---|---|---|
| 0.4 | 0.8 | 1.6 | 2.4 | 3.2 | 4.0 | ||
| 1 | 40.8 | 12.0 | 15.8 | 17.2 | 17.2 | 17.2 | 17.2 |
| 2 | 38.2 | 12.0 | 15.6 | 17.2 | 17.2 | 17.2 | 17.2 |
| 3 | 37.3 | 12.0 | 15.3 | 17.2 | 17.2 | 17.2 | 17.2 |
| 4 | 33.4 | 9.6 | 12.0 | 16.3 | 17.0 | 17.2 | 17.2 |
| 5 | 31.5 | 8.6 | 11.0 | 15.3 | 15.9 | 16.5 | 17.2 |
| 6 | 26.6 | 7.9 | 10.5 | 12.7 | 15.0 | 16.0 | 17.2 |
| 7 | 22.9 | 6.4 | 7.6 | 11.2 | 13.6 | 15.6 | 16.8 |
The numbers indicate potassium remaining in solution (in mg) in each tube containing 10 ml of a saturated potassium bitartrate solution. The original amount was 17.2 mg.
The fact that metatartaric acid solutions are unstable has a major impact on their use in winemaking. They deteriorate fairly rapidly and are also sensitive to temperature. Hydrolysis of the ester functions occurs, accompanied by an increase in acidity. After 20 days at 18–20°C, there is a considerable decrease in the esterification number (Figure 1.20). Under experimental conditions, total hydrolysis of a 2% metatartaric acid solution takes three months at 23°C and 10 months at 5°C. Consequently, it is necessary to ensure that metatartaric acid solutions are prepared just prior to use in wine.
