Malolactic fermentation

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Malolactic conversion is a process of a change in wine where tart malic acid is converted to softer-tasting lactic acid.

It is accomplished by lactic acid bacteria (such as Oenococcus oeni) , which consume malic acid to produce energy. Depending on the bacteria present, it can occur naturally, but 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 accidential initiation and maintain a tarter, more acidic profile in the finished wine.

Because it consumes the malic acid, all of 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 an uncanny smell of buttered popcorn, due to the production of diacetyl.

In winemaking malolactic conversion is generally encouraged in many red wines and some white wines, particularly those that are aged in oak. Unoaked white wines, such as German wines generally do not undergo malolactic conversion. Malolactic conversion tends to create a rounder, fuller mouthfeel in subject wines by converting malic acid into lactic acid. Malic acid tastes slightly of apples and this can be tasted in the wine, while lactic acid is richer and more buttery.

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.