Put simply malolactic fermentation or MLF is the conversion of malic acid into lactic acid within a must or wine. It’s a common – and in some styles necessary – step which takes place in the winery and is facilitated by lactic bacteria, commonly Oenococcus oeni.
The process ‘softens’ the acidity of wines through the conversion of harsh-tasting malic acid into softer lactic acid, and it also raises the pH of the wine. The bacteria can be introduced to the wine via inoculation during or after alcoholic fermentation, but in many wineries the bacteria is present in the cellar so spontaneous MLF can occur.
The bacteria like to operate in a warm environment (+16 degrees Celsius) and traditionally this meant winemakers had to wait until the spring following the harvest for the malolactic fermentation to occur.
However, modern cellars and tanks can be heated up to allow the malolactic fermentation to start as soon as desired.
The process, which is technically a bacterial conversion rather than a fermentation because it doesn’t use yeast, also helps to stabilise the wine by preventing it from going through spontaneous MLF later, potentially after bottling.
Does MLF benefit all wines?
No, not at all. MLF is generally undesirable in some styles of wine, particularly aromatic and zingy, high acid whites such as Riesling and Sauvignon Blanc. Winemakers can inhibit malolactic fermentation by adding SO2 to wines post-fermentation or through the use of enzymes, such as lysozyme.
Red wines more commonly benefit from MLF than whites, where high acidity is a key character of the wine. Exceptions include Chardonnay and Viognier, which routinely go through ‘malo’ and can have great appeal with softer, more rounded acidity.
There are no hard and fast rules, though. In Bordeaux in 2014, for example, châteaux used various techniques – including MLF – to control high acidity in white wines made from Semillon and Sauvignon Blanc grapes.<
‘Several white wine producers took the unusual step of performing the secondary malolactic fermentation of their Semillon or Sauvignon Blanc grapes, in an attempt to tame the high acidity that has been a feature of the Bordeaux 2014 vintage,’ wrote Jane Anson for Decanter.com when reviewing the vintage in 2015.
The late professor Denis Dubourdieu, one of Bordeaux’s most respected winemakers and wine researchers, noted in his summary of the 2014 vintage that partial malolactic fermentation ‘rarely practiced on white Bordeaux… was recommended for the most acidic lots. When well done, this made the wines rounder without detracting from their typicity’.
Are there any red wines where MLF is best avoided?
While most reds undergo MLF to help accentuate fruity and berry characters and remove some of the harsh acid notes, reds that have been grown in very warm climates and lack natural acidity do not benefit from the process because it makes them less balanced and unstable.
Many red wines undergo malolactic fermentation in barrels. As well as taming brisk acidity and enhancing fruit notes, this can promote a wine’s smoky and spicy notes, too.
Wines typically aged in oak undergo MLF, so this is one way of deciphering whether a wine has gone through malo or not; other clues include a creamy, oily texture, a rounder acid profile and a higher pH – if you have access to the technical data.