Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 60

Saccharomyces cerevisiae partially degrades malic acid (10–25%) in the must during alcoholic fermentation. Different strains degrade varying amounts of this acid and degradation is more significant when the pH is low. Alcoholic fermentation is the main pathway degrading malic acid. The pyruvic acid resulting from this transformation is decarboxylated into acetaldehyde, which is then reduced to ethanol. Malic enzyme is responsible for the transformation of malic acid into pyruvic acid (Figure 2.19). This oxidative decarboxylation requires NAD⁺ (Fuck and Radler, 1972). This malo‐alcoholic fermentation lowers wine acidity significantly more than malolactic fermentation does.

Schizosaccharomyces differs from wine yeasts. The alcoholic fermentation of malic acid is complete in yeasts of this genus, which possess an active malate transport system. In S. cerevisiae, malic acid penetrates the cell by simple diffusion. Yet at present no attempts to use Schizosaccharomyces in winemaking to break down the malic acid in musts have been successful (Peynaud et al., 1964; Carre et al., 1983). First of all, the implantation of these yeasts in the presence of S. cerevisiae is difficult in a non‐sterilized must. Secondly, their optimum growth temperature (30°C), higher than for S. cerevisiae, imposes warmer fermentation conditions. Sometimes, the higher temperature adversely affects the sensory quality of wine. Finally, some grape varieties fermented by Schizosaccharomyces do not express their varietal aromas. The acidic Gros Manseng variety produces a very fruity wine when correctly vinified with S. cerevisiae, but has no varietal aroma when fermented by Schizosaccharomyces. To resolve these problems, some researchers have used non‐proliferating populations of Schizosaccharomyces enclosed in alginate balls. These populations degrade the malic acid in wines having already completed their alcoholic fermentation (Magyar and Panyik, 1989; Taillandier and Strehaiano, 1990). Although no sensory defect is found in these wines, the techniques have not yet been developed for practical use.

Today, molecular biology permits another strategy for making use of the ability of Schizosaccharomyces to ferment malic acid. It consists of integrating Schizosaccharomyces malate permease genes and the malic enzyme (Mae 1 and Mae 2) in the S. cerevisiae genome (Van Vuuren et al., 1996). The technological interest of a wine yeast genetically modified in this manner is not yet clear, nor are the risks of its proliferation in wineries and nature.

FIGURE 2.19 Decomposition of malic acid by yeasts during alcoholic fermentation.