Malolactic fermentation (MLF) may sound mysterious, but it's a technique every home winemaker should master. It's quite different from "regular" fermentation, in which yeast convert sugar into alcohol. MLF involves bacteria instead of yeast, and it usually begins when primary fermentation is complete, around 0° Brix.
Malolactic fermentation is conducted by Leuconostoc bacteria cultures. These bacteria convert malic acid, which is naturally present in fruits like grapes and apples, to lactic acid. This reduces the acidity of the must and improves the flavor of your wine. After MLF, the wine's flavor profile is more smooth, round and complex. Malolactic fermentation can occur spontaneously, but it's best to conduct it yourself.
MLF is generally used for dry red wines but can also enhance some dry white wines, such as Chardonnay, Sauvignon Blanc and Pinot Gris. MLF is not recommended for sweeter wines, like Riesling, Gewürztraminer and Muscat.
Malolactic bacteria are finicky about their conditions. They aren't tolerant of high alcohol, high sulfur dioxide, low temperatures and low pH. High-acid grapes make it difficult to cultivate malolactic bacteria; in general, it will work in red wines with a pH of 3.3 or higher and in whites with a pH of 3.1 or above.
The most-accepted rule of thumb is to wait until the end of primary fermentation before adding the culture. Malolactic activity can be detected by the presence of tiny carbon-dioxide bubbles. When the bubbles stop, MLF is complete. This should take one to three months.