Concept card Process
Malolactic fermentation
In one sentence
The conversion, by a lactic acid bacterium, of the wine's malic acid into softer lactic acid and carbon dioxide.
Carried out or avoided?
Almost always done
Reds
Near-universal: suppleness, roundness, and a wine more stable in bottle.
Whites
A choice: carried through to round the wine out, blocked to keep its tension.
Rosés
Generally avoided: it would soften the bright acidity and leave the wine less aromatic.
Usually avoided
Key points
- The work is done by a bacterium, Oenococcus oeni, not a yeast: it makes no alcohol.
- Malic acid has two acid groups, lactic acid only one: the acidity changes in kind and bites less.
- It needs warmth, around 20 to 22 °C; above 13% abv, alcohol hinders the bacteria.
- Its end can be measured: it counts as complete below 0.2 g/L of malic acid left.
- To prevent it, the winemaker uses cold and sulphur.
What it changes in the glass
- Softer acidity: the wine feels rounder, less sharp.
- It can bring notes of butter or cream.
- In a white, it can transform some of the grape's own aromas.
Not to be confused with…
Alcoholic fermentation
There, yeasts turn sugar into alcohol. In malolactic fermentation, a bacterium converts an acid and makes no alcohol.
Inside the intact berry, its own enzymes also break down part of the malic acid, without producing lactic acid.
A red meant for ageing is bottled before its malolactic fermentation has taken place. What risk does it run?
One question to test yourself: it does not count towards your progress.
What happens in the wine
The IFV defines it as "the biological deacidification of wine under the action of bacteria". The species that does the work is Oenococcus oeni. It turns malic acid, the acid of green apples, into lactic acid, the acid of fermented milk, and carbon dioxide.
Malic acid carries two acid groups, lactic acid only one. So malolactic does not remove the acidity: it changes its nature, and the wine bites less. A wine that has been through it is not flat, it is rounder.
A hard bacterium to please
20 to 22 °C
the temperature that suits it
13% abv
above this, alcohol "hinders the bacteria"
0.2 g/L
of malic acid left: below this, it counts as complete
- pH 3.7 to 3.8: this is where the bacterium multiplies best.
- pH 3.0 to 3.2: this is where it converts malic acid best. The winemaker has to strike a balance between the two.
- A big red above 13% abv is a hostile medium at the very moment malolactic is wanted.
And if sugar is left?
The bacterium goes after an acid first, but it does not ignore sugar. If any is left in the wine, lactic acid bacteria can ferment it into acetic and lactic acid, which is a fault. Winemakers head this off by taking the alcoholic fermentation all the way to the end.
Carried out or blocked, wine by wine
- Reds: the IFV credits it with "suppleness, roundness and microbiological stability". In the cellar, it clears the malic acid before bottling.
- Whites: it "can transform certain varietal aromas". What the grape put into the wine, the bacterium can shift, so it is a choice.
- Rosés and some whites, such as Sauvignon Blanc: it is generally avoided, because it changes their taste and aromas "undesirably".
- To block it, the winemaker uses sulphur and cold.
To learn it in depth
Related reading
Sources (4)
- Official sourceFermentation malolactique — fiche pratique— Institut français de la vigne et du vin (IFV)
- Reference bookWSET Level 3 Systematic Approach to Tasting Wine (Issue 2, 2022) and Wine-Lexicon— Wine & Spirit Education Trust
- Official sourceQuelques clés d'élaboration des vins rosés — fiche pratique— Institut français de la vigne et du vin (IFV)
- Academic publicationAvoiding spoilage caused by lactic acid bacteria (fact sheet)— Australian Wine Research Institute (AWRI)