Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 55

Pasteur was the first to compare yeast growth under aerobic and anaerobic conditions and to observe an inhibition of fermentation by respiration. At low concentrations of glucose on culture media, yeasts utilize sugars through either respiration or fermentation. Aeration induces an increase in biomass formed (total and per unit of sugar degraded) and a decrease in alcohol production and sugar consumption. Pasteur therefore deduced that respiration inhibits fermentation.

The “Pasteur effect” has been interpreted in several ways. Two enzymes compete to catalyze either the respiration or fermentation of pyruvate. This competition explains the respiratory inhibition of fermentation. PDC is involved in the fermentative pathway. It has a much lower affinity toward pyruvate than does pyruvate dehydrogenase. Furthermore, oxidative phosphorylation consumes a lot of ADP and inorganic phosphate, which migrate to the mitochondria. A lack of ADP and inorganic phosphate in the cytoplasm ensues. This deficit can limit the phosphorylation and thus slow the glycolytic flux. The inhibition of glycolysis enzymes by ATP explains the Pasteur effect for the most part. The ATP from oxidative phosphorylation inhibits phosphofructokinase in particular. Phosphorylated hexoses accumulate as a result. The transmembrane transport of sugars and thus glycolysis is slowed down.

TABLE 2.1 Energy Balance of Oxidation of Glucose in Respiration

Stage	Reduction coenzyme	Number of molecules ofATP formed
Glycolysis	2NADH	4 or 6
Net gain of ATP from glycolysis		2
Pyruvate → acetyl‐CoA	NADH	6
Isocitrate → α‐ketoglutarate	NADH	6
α‐Ketoglutarate → succinyl‐CoA	NADH	6
Succinyl‐CoA → succinate		2
Succinate → fumarate	FADH2	4
Malate → oxaloacetate	NADH	6
Net yield from glucose		36–38

For high glucose concentrations—for example, in grape must—S. cerevisiae only metabolizes sugars by the fermentative pathway. Even in the presence of oxygen, respiration is impossible. Discovered by Crabtree (1929) on tumor cells, this phenomenon is known by several names: catabolite repression of respiration by glucose, the inverted Pasteur effect, and the Crabtree effect. Yeasts manifest the following signs during this effect: a degeneration of the mitochondria, a decrease in the proportion of cellular sterols and fatty acids, and a repression of the synthesis of Krebs cycle mitochondrial enzymes and constituents of the respiratory chain. With S. cerevisiae, there must be at least 2 g of glucose per liter for the Crabtree effect to occur. The catabolite repression exerted by glucose on wine yeasts is very strong. In grape must, at any level of aeration, yeasts are forced to ferment because of the high glucose and fructose concentrations. From a technological viewpoint, yeasts consume sugars via the respiratory pathway during the industrial production of dry yeast, but not in winemaking. If must aeration helps the alcoholic fermentation process (Section 3.7.2), the fatty acids and sterols synthesized by yeasts, which proliferate in the presence of oxygen, are responsible, not respiration.

Saccharomyces cerevisiae can metabolize ethanol via the respiratory pathway in the presence of small quantities of glucose. After alcoholic fermentation, oxidative yeasts develop in a similar manner on the surface of wine (Sections 14.5.2 and 14.5.3) as part of the process of making certain specialty wines (Sherry and vin jaune from Jura in France).