Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 57

When a molecule of glycerol is formed, a molecule of pyruvate is also formed. The latter cannot be transformed into ethanol following its decarboxylation into acetaldehyde. Under anaerobic conditions, oxaloacetate is the means of entry of pyruvate into the cytosolic citric acid cycle. Although the mitochondria are no longer functional, the enzymes of the citric acid cycle are present in the cytoplasm. Pyruvate carboxylase (PC) catalyzes the carboxylation of pyruvate into oxaloacetate. The prosthetic group of this enzyme is biotin; it serves as a CO₂ transporter. The reaction makes use of an ATP molecule:

Under these anaerobic conditions, the citric acid cycle cannot be completed since the succinate dehydrogenase activity requires the presence of FAD, a strictly respiratory coenzyme. The chain of reactions is therefore interrupted at succinate, which accumulates (Figure 2.7) up to levels of 0.5–1.5 g/l. The NADH generated by this portion of the citric acid cycle (from oxaloacetate to succinate) is reoxidized by the formation of glycerol from dihydroxyacetone.

Under anaerobic conditions, α‐ketoglutarate dehydrogenase has a very low activity; some authors therefore believe that the oxidative reactions of the citric acid cycle are interrupted at α‐ketoglutarate. In their opinion, a reductive pathway of the citric acid cycle forms succinic acid under anaerobic conditions:

Bacteria have a similar mechanism. Camarasa et al. (2003) demonstrated that this is the main pathway found in S. cerevisiae yeast under anaerobic conditions. Furthermore, additional succinate is formed during alcoholic fermentation in a glutamate‐enriched medium. Glutamate is deaminated to form α‐ketoglutarate, which is oxidized into succinate.

Among secondary products, compounds with a ketone function (pyruvic acid and α‐ketoglutaric acid) and acetaldehyde predominantly bind with sulfur dioxide in wines made from healthy grapes. Their excretion is significant during the yeast proliferation phase and decreases toward the end of fermentation. Additional acetaldehyde is released in the presence of excessive quantities of sulfur dioxide in must. A high pH and high fermentation temperature, anaerobic conditions, and a deficiency in thiamine and pantothenic acid increase the production of ketoacids. Adding thiamine to must limits the accumulation of ketone compounds in wine (Figure 2.10).

FIGURE 2.10 Effect of thiamine addition on pyruvic acid production during alcoholic fermentation (Lafon‐Lafourcade, 1983). I, control must; II, thiamine‐supplemented must.

Other secondary products of fermentation are also derived from pyruvic acid: acetic acid, lactic acid, butanediol, diacetyl, and acetoin. Their formation mechanisms are described in the following paragraphs.