Malolactic Conversion, commonly known as Malolactic Fermentation (as ML or MLF for short) is started by bacteria and not yeast. The bacteria that causes ML is conveniently called, lactic acid bacteria or LAB. Therefore, technically ML is not a fermentation at all but a conversion (yet it does give off carbon dioxide!) Yet most people will refer to this process as a ‘secondary fermentation’ as it only happens after the primary alcoholic fermentation has started and then is usually finished by the time the alcoholic fermentation is done. Although, sometimes a winemaker may start ML by adding the bacteria AFTER the alcoholic fermentation is complete.

The carbon dioxide produced in the reaction disappears into the atmosphere unless the wine has already been bottled, in which case it makes a pétillant wine. Originally, this was the source of the sparkle in Vinho Verde’s wines – although now, most producers use the pompe bicyclette way. (from David Bird’s ‘Understanding Wine – Third Edition’)

Almost all red wines go through full MLF but the winemaker chooses whether or not to put a white wine through ML.

Which white wines do we use ML for?

Chardonnay is a grape that is often put through ML. Yet, aromatic varieties such as Riesling or Gewurtraminer are never put through ML. Chenin Blanc producers usually avoid it too.

Strangely, LAB is pH sensitive so that wines with very low pH (therefore super high acidity) can prevent ML from occuring. In this case, the winemaker must carry out a chemical de-acidification to decrease the acidity for the bacteria to grow.

In this process, the harsh ‘malo’ apple acids are converted to softer ‘lactic’ or milk acids.

There are 3 reasons why a winemaker might put a wine through ML:

  1. to decrease the acidity
  2. to stabilize the wine
  3. to produce buttery aromas and flavours

How does a winemaker start ML?

Generally speaking, in established wineries, there is already lactic acid bacteria present and a winemaker just allows it to continue. However, winemakers could add LAB as either Leuconostoc oenos, Lactobacillus, and Pediococcus.

How to encourage ML?

It’s a bacteria that is naturally present in the wine. As bacteria, they like warm temperatures. So, if a winemaker wants ML to occur, they simply warm the must, add bacterial culture and avoid adding any sulphur dioxide.

How to stop or prevent ML?

Conversely, add sulphur dioxide and cool the must to prevent ML.