The Malolactic Fermentation (MLF)

The malolactic fermentation is the conversion of malic acid in the wine to lactic acid through the action of naturally-occurring or added bacteria. It reduces and ‘softens’ the acidity of the wine and so is sometimes used in white winemaking, but is almost always encouraged in red winemaking. The reduction in acidity is due to the fact that malic acid has two acid radicals (-COOH), whilst lactic acid only has one:

HOOC – CH2 – CHOH – COOH → CH3 – CHOH – COOH + CO2 ↑

Malic acid also has a sharper (unripe apples) acidity, whilst the acidity of lactic acid is softer (more milk-like). There are considerable losses of primary (fruit) aromas during the MLF, but many by-products are formed, such as diacetyl, which adds a buttery richness to the wine.

To encourage the MLF To inhibit the MLF
  • Maintain the wine on its yeast lees after fermentation
  • Use low levels of sulphur dioxide pre-fermentation and do not add any sulphur dioxide post-fermentation until the MLF is complete.
  • Maintain the wine temperature around 17 - 20ºC.
  • Add freeze-dried MLF bacteria (Oenococcus oeni) or lees from a tank undergoing the MLF before, during, or after the alcoholic fermentation.
  • Increase the pH to above 3.1
  • Early racking clarification, which reduces nutrient and bacterial levels
  • Sulphur dioxide addition post-fermentation: free levels greater than 25 mg/l will inhibit bacterial growth
  • Low storage temperatures (below 12ºC)
  • Maintaining a low pH (below 3.1)
  • Sterile filtration at bottling

The MLF should never be allowed to occur in the bottle. For instance, base white wines for the production of sparkling wines are usually encouraged to undergo the MLF before the second (in bottle) fermentation.

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winemaking