What Is Malolactic Fermentation?

Submitted May 11th, 2009

“Penicillin cures, but wine makes people happy”
~Sir Alexander Fleming, English bacteriologist

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.

In winemaking, malolactic conversion is generally encouraged in many red wines and some white wines, particularly those that are aged in oak. A prominent example of this is the prevalence of malolactic fermentation in California chardonnays.

Malolactic fermentation tends to create a rounder, fuller mouthfeel. It has been said that malic acid tastes of green apples – indeed, malic comes from the Latin word for apple, m?lum, and is present in apple juice – and this can be tasted in the wine. By contrast, lactic acid is richer, even unctuous, and more buttery tasting – corresponding to its presence in milk, as reflected in the word lactic being derived from the Latin word for milk, lac, and it is present in sour milk.

White wines, such as German wines, generally do not undergo malolactic conversion.

Malolactic conversion is accomplished by lactic acid bacteria (such as Oenococcus oeni), which consume malic acid to liberate energy. This can occur naturally. However, in commercial wine making, malolactic conversion typically is initiated by an inoculation of desirable bacteria. This prevents undesirable bacterial strains from producing off-flavors. Conversely, commercial winemakers actively prevent malolactic conversion when it is not desired, to prevent accidental initiation and maintain a tarter, more acidic profile in the finished wine.

Chemically, malolactic fermentation is a decarboxylation, which means that carbon dioxide is liberated in the process.

Sometimes malolactic conversion can occur unintentionally after the wine is bottled. This is almost always a fault, and the result is a slightly carbonated wine that typically tastes bad. The carbonation from this type of change should not be confused with benign carbonation, known as spritz.

Because it consumes malic acid, which is present at the time the grapes are crushed, malolactic conversion can take place at any time during or after alcoholic fermentation. A wine undergoing malolactic conversion will be cloudy due to the presence of bacteria, and may have the curious smell of buttered popcorn, due to the production of diacetyl.

“Malolactic Fermentation.” Wikipedia: The Free Encyclopedia. 01 May 2009.

Editor’s Note: I checked many resources for malolactic fermentation and found answers to be pretty much the same, some more brief, some very technical.

Let’s be sure to ask Doug Brazil about malolactic fermentation at this month’s tasting!

Michelle Jeffers