Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 59

Lactic acid is another secondary product of fermentation. It is also derived from pyruvic acid, which is directly reduced by yeast L(+)‐ and D(−)‐lactate dehydrogenases. Under anaerobic conditions (the case in alcoholic fermentation), the yeast synthesizes predominantly D(−)‐lactate dehydrogenase. Yeasts form 200–300 mg of D(−)‐lactic acid per liter and only a few dozen milligrams of L(+)‐lactic acid. The latter is formed essentially at the start of fermentation. By determining the D(−)‐lactic acid concentration in a wine, it can be ascertained whether the origin of acetic acid is yeast or lactic acid bacteria (Section 14.2.3). Wines that have undergone malolactic fermentation can contain several grams per liter of exclusively L(+)‐lactic acid. On the other hand, the lactic acid fermentation of sugars (lactic spoilage) forms D(−)‐lactic acid. When D(−)‐lactic acid concentrations exceed 200–300 mg/l, it is clear that lactic acid bacteria have transformed substrates other than malic acid.

FIGURE 2.17 Acetoin, diacetyl, and 2,3‐butanediol formation by yeasts under anaerobic conditions. TPP, thiamine pyrophosphate; TPP‐C₂, active acetaldehyde.

Yeasts also make use of pyruvic acid to form acetoin, diacetyl, and 2,3‐butanediol (Figure 2.17). This process begins with the condensation of a pyruvate molecule and a molecule of active acetaldehyde bound to TPP, leading to the formation of α‐acetolactic acid. The oxidative decarboxylation of α‐acetolactic acid produces diacetyl. Acetoin is produced by either the non‐oxidative decarboxylation of α‐acetolactic acid or the reduction of diacetyl. The reduction of acetoin leads to the formation of 2,3‐butanediol; this last reaction is reversible.

From the start of alcoholic fermentation, yeasts produce diacetyl, which is rapidly reduced to acetoin and 2,3‐butanediol. This reduction takes place in the days that follow the end of alcoholic fermentation, when wines are conserved on the yeast biomass (de Revel et al., 1996). Acetoin and especially diacetyl are strong‐smelling compounds that evoke a buttery aroma. Above a certain concentration, they have a negative effect on wine aroma. However, in wines that have not undergone malolactic fermentation, their concentration is too low (a few milligrams per liter for diacetyl) to have a sensory impact. On the other hand, lactic acid bacteria can degrade citric acid to produce much higher quantities of these carbonyl compounds than yeasts (Section 5.3.2).

FIGURE 2.18 Citramalic acid and dimethylglyceric acid.

Finally, yeasts condense acetic acid (in the form of acetyl‐CoA) and pyruvic acid to produce citramalic acid (0–300 mg/l) and dimethylglyceric acid (0–600 mg/l) (Figure 2.18). These compounds have little sensory impact.