You may have heard malolactic fermentation described as a secondary fermentation. While it never precedes the primary, alcoholic fermentation (sugar+yeast=alcohol+CO2+heat), it’s not a true fermentation. The process is carried out by bacteria (lactic acid bacteria), not yeast. It is sometimes shortened to MLF or to the French la malo, and it is the conversion of malic acid to lactic acid.
Malic comes from malum, Latin for apple and this type of acidity is found in nearly all fruits and berries. It’s used commercially to acidify food and beverage in industrial processes. Lactic acid is named after lactis, Latin for milk and is one of the milder acids found in wine.
Is it a good thing? Well, that depends. If you have a wine with excess acidity, like a red wine from a cool climate, then yes. It can also add additional flavor and complexity to both red and white wines as well as prevent the process from happening later on once these wines are bottled. If, however, you have a wine from a warmer climate or a hotter vintage and you want to preserve its zip, then no.
Some grapes, especially Chardonnay, take better to it than others. Producers of Riesling or Chenin Blanc usually avoid it (through maintaining cooler temperatures and using sulfur dioxide), even though these grapes are naturally high in acidity.
So, what can you expect if a wine’s undergone the process? A rounder, fuller mouthfeel – some would say buttery. The buttery-ness is from diacetyl, which is a by-product of the process. If the diacetyl is not kept in check, it can be overpowering in the wine, even coming across as rancid butter. Some winemakers think that if the malolactic fermentation happens while the wine is in the barrel, the fruit and oak flavors will be better integrated.