Malolactic Conversion
Winemaking · Intermediate · 3 min read
Malolactic conversion, sometimes abbreviated to ML or called malolactic fermentation, is a secondary fermentation carried out by lactic acid bacteria, primarily Oenococcus oeni. The bacteria convert sharp malic acid, the same acid found in green apples, into softer lactic acid, the acid found in milk. The result is a wine with a rounder, creamier mouthfeel and a different aromatic profile.
What the conversion does
Two things, chemically. First, it de-acidifies the wine, dropping total acidity by 1 to 3 grams per litre. The effect is to soften the wine and to make it feel less sharp on the palate. Second, it produces diacetyl, a buttery compound that gives a distinct creamy, butterscotch character. In Chardonnay, diacetyl is usually welcome. In delicate aromatics such as Bacchus and Riesling, it is usually avoided.
Beyond flavour, malolactic conversion also de-stabilises the wine microbiologically. The bacteria consume the residual malic acid, leaving the wine less hospitable to spoilage organisms. The conversion is almost always carried out for red wines, for full-bodied Chardonnays, and for any wine that needs a softer, rounder mouthfeel.
How it is triggered
The conversion happens naturally if the wine is left warm after alcoholic fermentation. Most UK producers trigger it deliberately by warming the cellar or the tank to 18 to 22 degrees Celsius, then inoculating with a cultured strain of Oenococcus oeni. The conversion takes two to four weeks, after which the wine is racked off the lees and sulphured to prevent further bacterial activity.
In sparkling wine, malolactic conversion is usually done on the base wine before the second fermentation, to give a softer, more rounded mouthfeel to the finished bottle. The English PDO rules allow malolactic conversion for both still and sparkling wines, although some producers choose to block it for their top aromatic whites.
Malolactic in the UK
Almost all English red wines, particularly Pinot Noir, go through full malolactic conversion. The cool UK climate produces grapes with high malic acid, and the conversion is essential to bring the wine into balance. Without it, English reds would feel aggressively sharp.
For whites, the choice is more nuanced. Chardonnay and Pinot Gris usually go through malolactic, sometimes in barrel where the micro-oxygenation helps integrate the buttery character. Bacchus, Ortega, Müller-Thurgau and Riesling are typically kept on the lees without malolactic, to preserve their fresh, aromatic character. Seyval Blanc and Schönburger usually go through malolactic, depending on the winemaker's preference.
Risks and challenges
Malolactic conversion is a biological process, and it can fail. The bacteria are sensitive to low pH, high alcohol and residual sulphur, all of which are common in UK base wines. A stuck conversion leaves the wine high in malic acid and microbiologically unstable, which is a serious problem. UK producers monitor pH, alcohol and temperature closely during the conversion to make sure the bacteria are happy.
There is also the risk of spoilage. Lactic acid bacteria other than Oenococcus oeni can produce off-aromas, including mousiness (a smell like a dirty mouse cage) and buttery diacetyl in excessive amounts. The winemaker's job is to give the right bacteria the right conditions and to keep the wrong ones out.