Malolactic fermentation (MLF, or Malo for short) is a distinct and separate fermentation from alcoholic fermentation.

In alcoholic fermentation (primary fermentation), yeast metabolize grape sugars (mainly glucose and fructose) producing ethanol (main alcohol in wine) and carbon dioxide.

Malolactic fermentation occurs after alcoholic fermentation using bacteria, commonly Leuconostoc Oenos and its various strains. Other bacterium can be used, but all fall under the banner of Lactic Acid Bacteria (LAB).

The major role of this LAB is to convert malic acid into lactic acid. Small amounts of other compounds and some carbon dioxide gas are produced, but no alcohol.

So, why bother? Well, winemakers use MLF for a variety of reasons.

The first and most important reason is to achieve enhanced bacterial stability in red wines. In other words, if MLF does not take place in a barrel, it will occur in the bottle, resulting in a cloudy, smelly fizzy red wine.

Lactic Acid Bacteria are present in vineyards, around wineries and in barrels, so if winemakers cannot prevent the occurrence of MLF at least they can manage it and ensure it happens in barrel rather than in the bottle.

Many winemakers allow MLF to occur naturally, using the indigenous LAB to conduct the fermentation, which is fine if you have a strain of bacteria that is reliable. However, a strain that is unreliable or producing off-flavours is possible, so in these instances winemakers add a known, reliable LAB culture to start the MLF in red wine.

White wines are another story. The use of MLF in white wines is to add complexity of flavour or to decrease acidity (more so in Europe than Australia ), rather than impart bacterial stability. Whites are far less susceptible to MLF in bottle than reds; in fact, winemakers usually have to add a LAB culture to start the MLF.

LF is becoming increasingly prevalent in Semillon and Chardonnay as winemakers recognise the flavour benefits. The malolactic fermentation lends complexity to these wines, giving them butterscotch aromas and a creaminess on the palate.