During malolactic fermentation in wine by Oenococcus oeni, the degradation of citric acid was delayed compared to the degradation of malic acid. The maximum concentration of diacetyl, an intermediary compound in the citric acid 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 concentration 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 rather strongly with SO2 (Kf = 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 concentration of SO2 decreases during storage of the wine, the diacetyl concentration increases again.