Malolactic fermentation, Secondary fermentation, MLF, ML or “Malo” for short, is the process in which malic acid in wine is converted to lactic acid. Nearly all red wines and various white wines go through ML after the initial fermentation is complete. Contrary to regular or primary fermentation where yeast converts sugar into alcohol, ML is caused by a bacteria Oenococcus oeni. ML occurs naturally, however wines are often inoculated with the bacteria culture to kickstart the process. Malolactic fermentation softens the taste and texture of the wine, adds complexity and character, and stabilizes wines prior to bottling.
Malic acid is the tart acid in grapes also found in green apples, while lactic acid is the more creamy acid found in milk, cheese, and yogurt. Chardonnay is the classic example where the fuller mouth-feel and creamy texture is the direct result of malolactic fermentation and barrel aging. The buttery flavor in many chardonnays comes from a compound called diacetyl which is derived from the ML process. This compound is also replicated and used on microwave popcorn. Makes ya thirsty, eh?
For crisp whites that do not benefit from ML such as Riesling, Gewürztraminer, or Sauvignon Blanc, ML is prevented through chilling, filtering, and/or adding of fining agents or enzymes. Even some chardonnay producers prefer a crisp style, skipping the barrels and fermenting entirely in temperature controlled tanks to keep the wine light and fruity. Occasionally, the two styles are combined and tank wine is blended with wine that has gone through ML in barrel. This results in a wine with both crispness and nuance of butter and toasty oak.
Red wines prior to malolactic fermentation tend to be harsh and astringent on the palate. After the malic acid is converted to lactic, the wine becomes noticeably softer and more approachable. Wines are perceptibly heavier and rounder on the palate and will continue to gain weight through the aging process.
The status of the MLF in a wine must be monitored quite attentively. To check if ML is working, we literally put our ear to an open barrel and listen for CO2 bubbles being released. The crackling sound lets you know that the wine is alive and well. Once the bubbles have ceased, wines are checked in the lab for acid levels. When ML is finished and all of the malic acid has been consumed, S02 (sulfur) can now be added to help preserve the wine and minimize microbial activity.
Microbial stability is one of the most important reasons to have wines go through ML. Without it, unfiltered wine may go through the ML process on its own, AFTER it has been bottled. The result is an effervescent wine that shouldn't be sparkling, accompanied by odd, unintended flavors. Wine is filled with all kinds of nutrients, and if the malolactic bacteria doesn't eat them, something else will. By adding a specific strain of bacteria, you can control what eats the nutrients, therefore controlling how the wine will taste.
To gain a more in depth understanding of malolactic fermentation I recommend buying a bottle of barrel fermented chardonnay and a non-oaked chardonnay. Most likely the non-oaked will have gone through minimal ML if any. Otherwise come visit the winery and we’ll do some barrel tasting with this year's new vintage :) thanks for reading.