The malolactic fermentation (MLF) is the process in which the sharper malic acid in the wine is converted into the milder lactic acid and CO2 by the lactic acid bacterium Oenococcus oeni. The MLF is therefore not actually a fermentation since it is bacteria and not yeasts that do the work. The MLF can start automatically when the conditions in the wine are favorable and can be prevented by cooling the wine in time, or by using sulfite, lysozyme, sterile filtering or pasteurization. The last four methods kill or remove the Oenococcus oeni from the wine, whereas cooling only makes them inactive. The MLF can be induced by inoculating the wine with Oenococcus oeni (provided that the conditions, such as for example pH, temperature, sulfite levels and lysozyme, etc. are favorable enough for these bacteria). This ensures that the MLF is done by the preferred bacterial strain and not by another type of lactic acid bacterium that is present in the wine.
The MLF results in wine with rich, buttery and milky (due to the lactic acid) aromas and is therefore prevented when a fresher, crisper (due to the malic acid) wine is made.