Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon
small, as the value of the ΔB/ΔpH ratio at a fixed pH corresponds geometrically to the tangent on each point on the titration curve (Figure 1.4). More practically, buffer capacity can be defined as the number of strong base equivalents required to cause an increase in pH of 1 unit per liter of must or wine. It is even more practical to calculate smaller pH variations in much smaller samples (e.g. 30 ml). Figure 1.4 clearly shows the difference in buffer capacity of a model solution between pH 3 and 4, as well as between pH 4 and 5.
This raises the issue of the pH and pKa at which buffer range should be assessed. Champagnol (1986) suggested that pH should be taken as the mean of the pKa of the organic acids in the must or wine, i.e. the mean pKa of tartaric and malic acids in must and tartaric and lactic acids in wine that has completed malolactic fermentation.
This convention is justified by its convenience, provided that (Section 1.4.2) the neutralization curves of the must or wine have no inflection points representative of the pKa of the organic acids present, since their buffer ranges overlap, at least partially. In addition to these somewhat theoretical considerations, there are also some more practical issues. An aqueous solution of sodium hydroxide is used to determine the titration curve of a must or wine in order to measure total acidity and buffer capacity. Sodium, rather than potassium, hydroxide used as the sodium salt of tartaric acid is soluble, while potassium bitartrate would be likely to precipitate out during titration. It is, however, questionable to use the same aqueous sodium hydroxide solution for both must and for a dilute alcohol solution like wine.
Strictly speaking, a sodium hydroxide solution in dilute alcohol should be used for wine to avoid modifying the alcohol content and, consequently, the dielectric constant and, thus, the dissociation of the acids in the solution during the assay. It has been demonstrated (Dartiguenave et al., 2000b) that the buffer capacities of organic acids, singly (Tables 1.4 and 1.5) or in binary (Table 1.6) and tertiary (Table 1.7) combinations, are different in water and in 11% dilute alcohol solution. However, if the solvent containing the organic acids and the sodium hydroxide is the same, there is a close linear correlation between the buffer capacity and the acid concentrations (Table 1.4).
TABLE 1.4 Equations for Calculating Buffer Capacity (mEq/l) Depending on the Concentration (mM) of the Organic Acid in Water or Dilute Alcohol Solution (11% vol.) Between 0 and 40 mM (Dartiguenave et al., 2000b)
| Solvent | Water | Dilute alcohol solution |
|---|---|---|
| Tartaric acid | Y = 0.71x + 0.29; R2 = 1 | Y = 0.60x + 1.33; R2 = 1 |
| Malic acid | Y = 0.56x + 0.43; R = 0.998 | Y = 0.47x + 0.33; R2 = 0.987 |
| Succinic acid | Y = 0.56x − 1.38 × 10−2; R2 = 0.993 | Y = 0.53x + 0.52; R2 = 0.995 |
| Citric acid | Y = 0.57x + 0.73; R2 = 1 | Y = 0.51x + 0.62; R2 = 1 |
Table 1.5 shows the values (mEq/l) calculated from the regression lines of the buffer capacities for acid concentrations varying from 1 to 6 g/l in water and in 11% dilute alcohol solution. The buffer capacity of each acid alone in dilute alcohol solution is lower than in water. Furthermore, the buffer capacity of a four‐carbon organic acid varies more as the number of alcohol functions increases (Table 1.8). Thus, the variation in buffer capacity of malic acid, a diacid with one alcohol function, in a dilute alcohol medium, is 1.4 mEq/l higher than that of succinic acid. When the hydroxy acid has two alcohol functions, the increase is as high as 5.3 mEq/l (17.7%) between tartaric and malic acids, even if the buffer capacities of the three acids are lower than in water.
However, the fact that the buffer capacities of binary (Table 1.6) or ternary (Table 1.7) combinations of acids in a dilute alcohol medium are higher than those measured in water is certainly unexpected. This effect is particularly marked when citric acid is included, and it reaches spectacular proportions in a ternary TMC blend (Table 1.7), where the buffer capacity in dilute alcohol solution is 2.3 times higher than that in water.
TABLE 1.5 Buffer Capacity (mEq/l) Depending on the Concentration (g/l) of Organic Acid in Water and in Dilute Alcohol Solution (Dartiguenave et al., 2000b)
| Acid concentration and type of medium | Tartaric acid | Malic acid | Succinic acid | Citric acid | |
|---|---|---|---|---|---|
| 1 g/l | Water | 5.0 | 4.6 | 4.7 | 3.7 |
| Dilute alcohol | 5.3 | 3.8 | 4.0 | 3.5 | |
| 2 g/l | Water | 9.7 | 8.8 | 9.5 | 6.7 |
| Dilute alcohol | 9.3 | 7.3 | 9.4 | 5.9 | |
| 4 g/l | Water | 16.4 | 17.1 | 19.0 | 12.6 |
| Dilute alcohol | 14.9 | 14.3 | 17.5 | 11.3 | |
| 6 g/l | Water | 28.7 | 25.5 | 28.4 | 18.5 |
| Dilute alcohol | 25.3 | 21.3 |