Читать книгу Handbook of Enology, Volume 2 - Pascal Ribéreau-Gayon - Страница 18

The main acids produced during fermentation are described in Table 1.2. The first to be described is pyruvic acid, due to its function as a “crossroad compound” in cell metabolism, although concentrations in wine are low, or even nonexistent. Following reduction by a hydride, or H⁻, ion (from aluminum or sodium borohydride) or by a coenzyme (NADH) from L- and D‐lactate dehydrogenases, pyruvic acid produces two enantiomers of lactic acid, L and D. The first dextrorotatory form is mainly of bacterial origin, and the second levorotatory form mainly originates from yeasts.

The activated, enol form of the same acid, phosphoenolpyruvate (Figure 1.2) adds a nucleophile to carbon dioxide, producing oxaloacetic acid, a precursor of aspartic acid via transamination.

The enzymatic decarboxylation of pyruvic acid, assisted by thiamine (vitamin B1) pyrophosphate (TPP), produces acetaldehyde (ethanal), which is reduced to form ethanol during alcoholic fermentation. Its enzymatic, microbial or even chemical oxidation produces acetic acid.

Another acid that develops during fermentation due to the action of yeast is succinic, or 1‐4‐butanedioic, acid. Concentrations in wine average 1 g/l. This acid is produced by all living organisms and is involved in the lipid metabolism and the Krebs cycle, in conjunction with fumaric acid. It is a diacid with a high pK_a (Table 1.3). Succinic acid has an intensely bitter, salty taste that causes salivation and accentuates a wine's flavor and vinous character (Peynaud and Blouin, 1996).

TABLE 1.2 The Main Acids Produced During Fermentation

FIGURE 1.2 Biosynthesis of oxaloacetic acid from phosphophenolpyruvic acid.

Like succinic acid, citramalic acid, or α‐methylmalonic acid, confused with citric acid in chromatography for many years, is of yeast origin.

In conclusion, it is apparent from this description that, independently of their origins, most of the main organic acids in must and wine consist of polyfunctional molecules, and many are hydroxy acids. These two radicals give these acids polar and hydrophilic characteristics. As a result, they are soluble in water and even in dilute alcohol solutions, such as wine. Their polyfunctional character is also responsible for the chemical reactivity that enables them to develop over time as wine ages. In this connection, results obtained by monitoring ethyl lactate levels in Champagne for two years after malolactic fermentation are highly convincing. Indeed, after two years of aging on the lees, concentrations reach 2 g/l and then decrease. The degree of acidity, indicated by the pK_a values of these acids, controls the extent to which they are present in partial salt form in wine (Table 1.3).

A final property of the majority of organic acids in wine is that they have one or more asymmetrical carbons. This is a characteristic of biologically significant molecules.

TABLE 1.3 State of Salification of the Main Inorganic and Organic Acids (Ribéreau‐Gayon et al., 1977)

Category	Name	pKa	Form in wine
Strong inorganic acids	Hydrochloric	Less than 1	Completely dissociated salts
Sulfuric 1	Approx. 1
Sulfuric 2	1.6
Sulfurous 1	1.77	Acidic bisulfite
Phosphoric 1	1.96	Acidic phosphate
Strongest organic acids	Salicylic	2.97	Acid functions partly neutralized and partly free (not highly dissociated)
Tartaric 1	3.01
Citric 1	3.09
Malic 1	3.46
Formic	3.69
Lactic	3.81
Tartaric 2	4.05
Weakest organic acids	Benzoic	4.16	Free acid functions (very little dissociated)
Succinic 1	4.18
Citric 2	4.39
Acetic	4.73
Butyric	4.82
Propionic	4.85
Malic 2	5.05
Succinic 2	5.23
Citric 3	5.74
Weak inorganic acids	Phosphoric 2	6.70	Free acid functions (almost entirely non‐dissociated)
Carbonic 1	6.52
Sulfurous 2	7.00
Hydrogen sulfide 1	7.24
Carbonic 2	10.22
Phosphoric 3	12.44
Phenols	Polyphenols (tannin and coloring)	7–10	Free (non‐dissociated)