When winemakers want to induce a soft mouthfeel and reduce acidity in wine, they often turn to malolactic (pronounced may-low-lactic) fermentation. This process converts one type of acid bacteria to another, resulting in more complex wines and a rounder mouthfeel. Winemakers don't use malolactic fermentation (MLF) with every wine, however. Instead, they induce the process after some wines have undergone primary fermentation.
Malolactic Fermentation: The Purpose
Grapes contain malic acid, and it remains in the wines after primary fermentation. This tart-tasting acid imparts a sour green-apple note to unfinished wines. In some cases, malic acid complements the flavor of the wines, particularly in crisp, acidic whites such as Riesling, which benefit from the green apple flavors the acid imparts. In other wines, however, the sour flavors can flatten them and create sharp off tastes and aromas. This occurs primarily in certain red wines, as well as some whites like Chardonnay that benefit from more toasty, buttery flavors. Winemakers convert the malic acid to lactic acid, because lactic acid is a softer, more palatable acid that improves the character of many wines.
Winemakers may also use MLF to prevent the process from occurring naturally after the wine has been bottled. When in-bottle MLF occurs, carbon dioxide becomes trapped in the wine, resulting in an unpleasant fizziness. A consumer opening a wine that has accidentally undergone in-bottle MLF may think the wine is continuing to ferment as the bottle releases carbon dioxide. At the same time, uncontrolled, in-bottle MLF may impart off flavors and aromas to wines, such as cured meats or fetid milk. In cases where MLF has occurred in bottle, the wrong type of lactic acid may dominate, imparting discordant "sweaty" flavors, dampening fruits, or creating a very unpleasant "mousy" taint.
The Process of Malolactic Fermentation
During primary fermentation, winemakers convert sugars to alcohol using yeast. During malolactic fermentation, winemakers convert malic acid to lactic acid. While MLF can occur naturally, winemakers typically prefer to control the process in order to assure that the right kind of lactic acid is introduced into the wine and to make sure the proper balance of acids exists. Just the right amount of lactic acid, for instance, can render a wine buttery and smooth on the palate, while too much may overwhelm the wines with burned caramel and rancid butter flavors.
To induce MLF, winemakers inncolate the fermented wines with Leuconostoc bacteria cultures. This bacteria consumes the malic acid, converting it to lactic acid and softening the wines. Winemakers control the process every step of the way, assuring that the right kind of lactic acid remains in the wine, and that not too much MLF occurs.
In some cases, winemakers may actively try to prevent MLF in wines. This is especially true in crisp, acidic fruit-driven whites, like Riesling, Gewürztraminer, and Moscato. These wines need the acid present in malic acid to balance the fruit and sweetness in the wines. Winemakers actively prevent MLF in these wines by controlling both temperature and sulfur dioxide during wine production and bottling.
Wines That Benefit From MLF
Medium to full-bodied dry reds and dry white wines may benefit from MLF, as do wines made from grapes grown in cooler climates like Oregon. These wine grapes often contain higher levels of malic acid, while grapes grown in warmer climates like California or South America may have less naturally occurring malic acid. Some of the wines that can benefit from MLF include:
Results of Malolactic Fermentation
When properly done, MLF enhances complex, dry wines, providing a roundness and softness in the palate. The process reduces grassy or vegetal notes in wines, and often increases berry and dark fruit flavors. They may also impart earthy or nutty armoas to the wine. Skilled winemakers carefully induce and control MLF to create complex, focused wines of great distinction. The result is better tasting wines with a smoother, more rounded mouthfeel.