Control of flavor development in wine during and after malolactic fermentation by Oenococcus oeni

During malolactic fermentation in wine by Oenococcus oeni, the degradation of citric acid was delayed compared to the degradation of malic acid. The m aximum concentration of diacetyl, an intermediary compound in the citric ac id metabolism with a buttery or nutty flavor, coincided with the exhaustion of malic acid in the wine. The maximum concentration of diacetyl obtained during malolactic fermentation was strongly dependent on the oxygen concent ration and the redox potential of the wine and, to a lesser extent, on the initial citric acid concentration. The final diacetyl concentration in the wine was also dependent on the concentration of SO2. Diacetyl combines rath er strongly with SO2 (K-f = 7.2 x 10(3) M-1 in 0.1 M malate buffer [pH 3.5] at 30 degrees C), The reaction is exothermic and reversible. If the concen tration of SO2 decreases during storage of the wine, the diacetyl concentra tion increases again.