Malolactic fermentation is a secondary fermentation that most red wines and many white wines go through during the barrel aging process. The whole process is brought about by a benign bacterium, lactobacillus, which converts the tart malic acid in the wine into a softer lactic acid, essentially making the wine taste softer as well. This process can occur at a number of different times during the winemaking process depending on the winemaker’s preference and even the ambient cellar temperature.
Basically, the bacteria needs a high temperature to facilitate this transition, something established commercial winemakers can achieve easily but is a little more difficult for smaller home winemakers. In fact, when my family’s home winery was making one of our first chardonnays we used an aquarium heater to bring the wine up to temperature and start the fermentation.
But not all wines will go through this secondary process, for red wines crisp acidity is not usually a goal so it routinely goes through malo, while some white wines benefit from preserving that tart, bright acidity. Sauvignon Blancs, Rieslings, and Pinot Grigios rarely go through malolactic fermentation because that acidity is a key part of the wine. Chardonnay on the other hand typically does. During the malolactic fermentation a compound called diacetyl can be produced, this compound is very buttery in flavor and is what gives the wine that buttery, rich quality that many Chardonnays are known for. Interestingly, diacetyl is also added to margarine to make it taste more like real butter. However, there is a new trend towards Chardonnays that do not go through malo or only partially through it. These Chardonnays are not the rich, buttered popcorn wines that many are used to. Instead they are routinely still aged in oak, giving them some richness but with out the butterines, and retaining their crisp fruit. They are personally some of my favorites, I find them to be much more balanced than the Chardonnays that seems to have been soaked in butter or their counterparts, stainless steel Chardonnays that taste more like a bland Sauvignon Blanc than a Chard.
How do winemakers prevent a wine from going through malolactic fermentation? There are two different ways, they can either fine or filter the remaining yeast cells, proteins, or bacteria out of the wine. Or they can chill the wine down quickly to stun the bacteria. Basically they just want to make sure to remove all the bacteria so that the wine will not spontaneously go through malo once it has been bottled. This process is much easier to accomplish in a commercial winery than it is in a home winemaking operation, hence the aquarium heater.