Malolactic Fermentation - what is it and why they do it

Malolactic fermentation is commonly referred to as "MLF", or (in winemaker's speak as) "malo" (pronounced may-low).

Malolactic fermentation (or sometimes malolactic conversion) is a process of a change used in winemaking where tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid.

Some people refer to this as Secondary Fermentation and the malolactic fermentation requires monitoring just like the primary fermentation. Malolactic fermentation is a process of fermentation where naturally occurring tart malic acid is converted to softer-tasting lactic acid which imparts a rounder fuller mouth feel. This process is under way before, during and after the primary fermentation and is accomplished by naturally occurring lactic acid.

Malic acid tastes slightly of tart apples while lactic acid is richer and more buttery.

MLF usually occurs shortly after the end of the primary fermentation (when the grape sugar is converted to alcohol by yeast). The family of lactic acid bacteria (LAB); Oenococcus oeni, and various species of Lactobacillus and Pediococcus makes it happen - these bacteria convert one of the two major grape acids found in wine called L- malic acid, to another type of acid, L+lactic acid. This conversion is accompanied by the production of carbon dioxide (so hence the term, fermentation).

Malolactic fermentation (also called secondary fermentation) of the wine occurs in barrel and converts the sharper tasting Malic acid to the softer Lactic acid and is normally done on all red wines. When Malolactic fermentation is complete, the wine gets a dose of sulfur dioxide to kill any micro-organisms and also to protect the wine from oxidation.

Wines that typically undergo, and are improved by MLF, are the full-bodied dry whites, such as the California Chardonnays and medium to full bodied dry reds.

NOT ALL WINES BENEFIT FROM MLF - Rieslings are a classic case of a wine that does not benefit.

As a general rule, the quality of lighter-bodied fruit-driven wines that require crisp acidity are reduced by the action of MLF.

There is also a major practical reason why MLF is encouraged during the making of many wines, and in particular reds wines that have previously undergone malo in tank or barrel are far less likely to go through malo when in bottle.

The onset of MLF in the bottle is disastrous as the wine will appear to the consumer to still be fermenting (as a result of CO2 being produced). The wine may also lose its fruit integrity and take on the unpleasant lactic aroma of cured meats.

For more information (and for home winemaker instructions) ... please click here.