Pascal Ribéreau-Gayon - Handbook of Enology, Volume 2

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 1.5Buffer Capacity (mEq/l) Depending on the Concentration (g/l) of Organic Acid in Water and in Dilute Alcohol Solution (Dartiguenave et al., 2000b)

TABLE 1.6Demonstration of Interactions Between Organic Acids and the Effect of Alcohol on the Buffer Capacity of Binary Combinations (Dartiguenave et al., 2000b)

These findings indicate that the acids interact among themselves and with alcohol, compensating for the decrease in buffer capacity of each individual acid when must (an aqueous solution) is converted into wine (a dilute alcohol solution). From a purely practical standpoint, the use of citric acid to acidify dosage liqueur for bottle‐fermented sparkling wines has the doubly positive effect of enhancing the wine's aging potential while maintaining its freshness on the palate.

Table 1.9shows the changes in buffer capacity in successive pressings of a single batch of Chardonnay grapes from the 1995 to 1996 vintages, at the main stages in the winemaking process.

The demonstration of the effect of alcohol and interactions among organic acids ( Tables 1.6– 1.8) led researchers to investigate the precise contribution of each of the three main acids to a wine's buffer capacity in order to determine whether other compounds were involved. The method consisted in completely deacidifying a wine, containing all of its malic acid, by crystallizing and precipitating the double calcium tartromalate salt. After this deacidification, the Champagne base wine had a residual total acidity of only approximately 0.5 g/l as H 2SO 4, whereas the sample's buffer capacity was still 30% of the original value. This shows that organic acids are not the only compounds involved in buffer capacity, although they represent 90% of total acidity.

TABLE 1.7Demonstration of Interactions Between Organic Acids and the Effect of Alcohol on the Buffer Capacity of Ternary Combinations (Dartiguenave et al., 2000b)

TABLE 1.8Effect of Hydroxyl Groups in the Structure of the Four Carbon Diacids on Buffer Capacity (mEq/l) (Dartiguenave et al., 2000b)

TABLE 1.9Changes in the Buffer Capacity of Must from Different Pressings of Chardonnay Grapes at Various Stages in the Winemaking Process (Buffer Capacity Is Expressed in mEq/l) (Dartiguenave, 1998)

Among the many other compounds in must and wine, amino acids have been singled out for two reasons: (1) in Champagne musts and wines, their total concentration is always over 1 g/l and may even exceed 2 g/l, and (2) their (at least bifunctional) character gives them a double‐buffer effect. They form salts with carboxylic acids via their ammonium group and can become associated with a non‐dissociated acid function of an organic acid via their carboxyl function, which is mostly dissociated at wine pH, thus creating two buffer pairs ( Figure 1.5).

An in‐depth study of the interactions between amino acids and tartaric and malic acids focused on alanine, arginine, and proline, present in the highest concentrations in wine, as well as on amino acids with alcohol functions, i.e. serine and threonine (Dartiguenave et al ., 2000a).

The findings are presented in Figures 1.6and 1.7. Hydrophobic amino acids, such as alanine, were found to have only a minor effect, while amino acids with alcohol functions had a significant impact on the buffer capacity of an aqueous tartaric acid solution (40 mM). An increase of 0.6 mEq/l was obtained by adding 6.7 mM alanine, while addition of as little as 1.9 mM of serine produced an increase of 0.7 mEq/l, and addition of 4.1 mM of threonine resulted in a rise of 2.3 mEq/l.

FIGURE 1.5 Diagram of interactions between amino acids and organic acids that result in the buffer effect.