Pascal Ribéreau-Gayon - Handbook of Enology, Volume 2

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 1.18Detailed Analysis of Various Metatartaric Acid Preparations (Peynaud and Guimberteau, 1961)

The esterification number of different metatartaric acid preparations may be determined by acidimetric assay, before and after saponification. Table 1.18shows 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 1.19presents 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 1.19Inhibition of Potassium Bitartrate Precipitation by Various Metatartaric Acids (Peynaud and Guimberteau, 1961)

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.

FIGURE 1.20 Hydrolysis rate of two grades of metatartaric acid in 2% solution ( T = 18–20°C), followed by a decrease in the esterification number (Ribéreau‐Gayon et al ., 1977).