Malolactic Fermentation (MLF)
Definition - What does Malolactic Fermentation (MLF) mean?
Malolactic Fermentation (MLF) is a winemaking process in which malic acid is converted to lactic acid. Commonly referred to as secondary fermentation, the process can occur naturally in wine, however winemakers tend to force it by adding lactic acid bacteria (LAB) to the wine.
Depending on the conditions of the wine and the time afforded to the winemaker, the process can occur simultaneously with primary fermentation or immediately after; many winemakers suggest waiting, so as to prevent the conflict of the different conversions within the wine. It generally takes between two to three months for the process to complete. Winemakers monitor the pH levels closely; when the bubbles stop forming and the wine is still, MLF is complete. If a winemaker chooses not to force MLF, they have to take measures to prevent it from occurring later.
WineFrog explains Malolactic Fermentation (MLF)
Most red wines undergo MLF; though only a few white wines do, including Chardonnay, Pinot Blanc and Pinot Gris. MLF diminishes fruit characteristics. Light, fruity wines with low acidity and strong aromatic characteristics - Gewurztraminer, Riesling, Ehrenfelser - don’t undergo MLF.
There are several reasons for MLF to be forced in a wine. The first is to prevent it from occurring in the wine after bottling. Technically, MLF is a decarboxylation - which means it releases carbon dioxide as a byproduct. Since MLF can occur naturally, there is a chance that, if it isn’t forced, it will occur in the wine after bottling. This would release carbon dioxide and make the wine taste as if it were still going through fermentation.
The second reason MLF is induced in a wine is to control the end result of the natural process. Some LABs create unsavory characteristics in a wine. Lactobacillus, for example, has been blamed for fetid milk, sauerkraut and sweaty characteristics, as well as mousy taints. Whereas Oenoccus Oeni, the most common bacteria used, produces a smoother taste and a buttery flavor, such as you would find in Chardonnay.
The third reason is to control the end style of a wine. MLF smoothens the high acidity in wine. Malic acid creates tart, Granny Smith apple flavors that, when in abundance, is over powering. MLF converts these to the more subtle Lactic acids. Lactic acids have softer flavors found in milk, butter, cheese and yogurt.