Malolactic fermentation

Malolactic fermentation is the second fermentation of the must during vinification. This transformation of malic acid into lactic acid by lactic bacteria reduces the acidity of a wine, while giving it greater stability.

Fermentation has many advantages for wines, but is not mandatory

Malolactic fermentation consists of the transformation of a strong acid naturally present in the must, malic acid, into a weaker acid: lactic acid. This is a natural reaction, also known as MLF or malo. It is then responsible for the biological deacidification of a wine. This drop in acidity is caused by the action of a lactic acid bacterium, Œnococcus œni. It takes place following alcoholic fermentation, hence the term "second fermentation".

Discovered in the 1960s, malolactic fermentation is not a systematic part of the winemaking process. The winemaker carries out malolactic fermentation according to the grape variety and the desired quality properties of the finished product. It will certainly influence the aromas and other organoleptic characteristics of a wine, but also its stability. For example, it limits spoilage caused by certain undesirable yeasts, such as Brettanomyces or Bretts.

  • Malolactic fermentation has become essential for red wines, reducing their ageing potential but also melting the tannins and reducing astringency. Spicy, floral and buttery notes are on the menu, as well as more roundness and suppleness, appreciated when tasting the reds.
  • For white and rosé wines (still or sparkling, such as champagne), the liveliness provided by malic acid is more often sought after. MLF remains occasional, but more frequent in northern areas. The use of this process also depends on its highly variable impact on the varietal aromas of white grape varieties.

Malolactic fermentation, a delicate stage in the winemaking process

The initiation and progress of MLF by the action of indigenous bacteria alone remains random and depends on several parameters of the must:

  • its acidity level, malolactic activity being optimal in the presence of an acid pH of between 3.0 and 3.2,
  • its alcohol content, which is harmful to bacteria above 13%,
  • its temperature, ideally between 20 and 22°C, linked to the ambient temperature in the winery and the heat produced by alcoholic fermentation.
Modulating and controlling malolactic fermentation in order to guarantee its success is therefore no easy task and requires specific techniques.
  • Wine inoculation, or sequential inoculation, is a powerful and reliable first option, carried out just after alcoholic fermentation. The winemaker uses direct inoculation or simplified acclimatisation strains.
  • Co-inoculation, the second option, involves co-inoculating the must with yeast and lactic acid bacteria. This method saves a considerable amount of time by speeding up MLF, which is an advantage for primeur wines, for example.
Le suivi régulier et pointu de la fermentation malolactique se révèle essentiel en œnologie. It helps to avoid the appearance of compounds that are harmful not only to the wine, but also to its health. It is carried out once or three times a week, initially by chromatography and then by enzymatic assay at the end of the process. Once the malic acid content is below 0.2 g/l, malolactic fermentation is considered complete. This is when the wine is racked, i.e. separated from its lees and any residual lactic acid bacteria that could spoil the cuvée.