Not really a fermentation
Despite its name, malolactic fermentation (MLF) is not done by yeast and produces no alcohol. It is a conversion carried out by lactic acid bacteria, mainly Oenococcus oeni, usually after the alcoholic fermentation.
The bacteria turn malic acid, the sharp acid you know from green apples, into lactic acid, the softer acid found in milk. The wine tastes less tart and feels rounder.
What MLF does to the taste
Besides softening acidity, MLF can add a buttery note from a compound called diacetyl, and sometimes creamy or nutty character. That is the classic style of many oaked Chardonnays.
Winemakers who want crisp, fruity whites, for example Riesling or Sauvignon Blanc, usually prevent MLF by keeping the wine cool and using sulfites.
Why almost all red wines go through it
Nearly all red wines undergo MLF. Softer acidity suits their tannins, and it makes the wine more stable: if MLF happened later in the bottle, it would cause fizz, cloudiness and off-flavors.
The process depends on temperature, acidity and sulfite levels, so it takes an experienced winemaker to manage it well.
Does a wine need MLF to age?
This is where the debate starts. Some winemakers believe completing MLF is essential for stability and long aging. Others argue that wines with more malic acid stay fresher for longer.
In The Science of Wine, Dr. Jamie Goode travels to Rioja, one of the great regions for age-worthy red wine, to find out whether MLF is really needed to make wines that last.
Watch the episode · 23 min
The Science of Wine: Is Malolactic Fermentation Necessary for the Long Aging of Wines?
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Watch the episodeFrequently asked questions
Why is malolactic fermentation called a fermentation?
Historically it was seen as a second fermentation. Technically it is a bacterial conversion of malic acid into lactic acid, without producing alcohol.
Does malolactic fermentation make wine buttery?
It can. Lactic acid bacteria can produce diacetyl, which smells of butter, especially in some Chardonnays.
Do white wines go through MLF?
Some do, such as many Chardonnays; many aromatic whites like Riesling and Sauvignon Blanc usually do not.