A Basic Guide to Malolactic Fermentation in Wine

Malolactic fermentation (MLF) is the process by which bacteria convert malic acid into lactic acid and carbon dioxide. These lactic acid-producing bacteria can include Oenococcus oeni and other species of Pediococcus and Lactobacillus. Bacteria may be naturally present in the winemaking equipment (such as used oak barrels), or the winemaker may inoculate the wine with a specific malolactic culture, such as O. oeni. Malolactic conversion happens after or during yeast fermentation (primary fermentation), which is why it's sometimes called secondary fermentation.

There are three main reasons why winemakers facilitate malolactic fermentation:

1. 1. Acid reduction: Malolactic fermentation decreases acidity, since malic acid is more acidic than softer lactic acid. The reduction of total acidity can lead to spoilage, so winemakers sometimes have to re-acidify wines by adding tartaric acid.
2. 2. Flavor: MLF can add a buttery, creamy complexity to wine by mellowing out tart fruity flavors. It also can make for softer wines with a full, smooth mouthfeel.
3. 3. Stability: Allowing wines to undergo MLF before bottling increases stability by preventing malolactic fermentation from occurring after bottling. If wine undergoes malolactic fermentation during bottling, the wine can look cloudy (due to the presence of malolactic bacteria) and become slightly sparkling.

After alcoholic fermentation, most red wines—like pinot noir—undergo the purposeful conversion of malic acid into lactic acid, and about one-fifth of white wines do as well. Certain white wine grapes, such as chardonnay and cabernet sauvignon, lend themselves better to MLF than others like riesling and gewürztraminer, which tend to be more sugary. The region and climate also impact the use of MLF. Malolactic fermenting is more likely to occur in colder regions, such as Burgundy and Champagne, where low temperatures can cause grapes to become more acidic.

Malolactic fermentation can add flavor and a rounder, creamier mouthfeel to some wines, while reducing aroma in others. There are three primary reasons for this:

1. 1. Diacetyl: Diacetyl is a byproduct of malolactic conversion that has a nutty, toasted flavor at low concentrations and an overwhelming buttery flavor at higher concentrations. Diacetyl is responsible for the buttery flavor of certain Chardonnays. The amount of diacetyl present in a wine depends on levels of citric acid, sulfur dioxide, temperature, oxygen, and pH during malolactic fermentation.
2. 2. Malic acid: Malolactic fermentation reduces malic acid, which has a tart, green apple flavor. Depending on the wine style, winemakers may choose to avoid MLF or have only a portion of the wine undergo MLF to preserve the tart flavor of malic acid.
3. 3. Acetic acid: Acetic acid may be another byproduct of malolactic fermentation. Too much acetic acid can make a wine taste vinegary.

Some winemakers prevent malolactic fermentation to preserve acidity, usually in warmer climates where the wine is less naturally acidic. (Exceptions include the acidic white wines made in cold climates from riesling, gewürztraminer, and chenin blanc grapes.) Malolactic fermentation can only occur at temperatures higher than 68 degrees Fahrenheit, so keeping wine cold is one way of preventing malolactic fermentation. Another method is early racking; malolactic fermentation requires a specific pH and won't work with wines that have a very low pH (below 3.1). Other techniques include the addition of sulfur dioxide, which kills the lactic acid bacteria. To prevent spontaneous malolactic fermentation after bottling, winemakers can filter finished wine.

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