Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 56

Wines contain about 8 g of glycerol per 100 g of ethanol. During grape must fermentation, about 8% of the sugar molecules undergo glyceropyruvic fermentation and 92% undergo alcoholic fermentation. The fermentation of the first 100 g of sugar forms the majority of the glycerol, after which glycerol production slows but is never nil. Glyceropyruvic fermentation is therefore more than just an inductive fermentation that regenerates NAD⁺ when acetaldehyde, normally reduced into ethanol, is not yet present. Alcoholic fermentation and glyceropyruvic fermentation overlap slightly throughout the fermentation process.

Pyruvic acid is derived from glycolysis. When this molecule is not used by alcoholic fermentation, it participates in the formation of secondary products. In this case, a molecule of glycerol is formed by the reduction of dihydroxyacetone.

Glycerol production therefore equilibrates the yeast endocellular oxidation–reduction potential or NAD⁺/NADH balance. This “relief valve” eliminates surplus NADH, which appears at the end of the synthesis of amino acids, proteins, and the oxidations that generate secondary products.

Some winemakers place too much importance on the sensory role of glycerol. This compound has a sugary taste similar to glucose. In the presence of other constituents of wine, however, the sweetness of glycerol is practically imperceptible. For the majority of tasters, even well trained, the addition of 3–6 g of glycerol per liter to a red wine is not discernible. Therefore, the pursuit of winemaking conditions that are more conducive to glyceropyruvic fermentation has no enological interest. On the contrary, the winemaker should favor a pure alcoholic fermentation and should strive to minimize glyceropyruvic fermentation. The production of glycerol is accompanied by the formation of other secondary products, derived from pyruvic acid, whose increased presence (such as carbonyl function compounds and acetic acid) decreases wine quality.