What Is Malolactic Fermentation

Malolactic Fermentation (commonly referred to as “MLF” or “Malo”) is an often welcomed, yet overlooked, part of a wine’s aging process. Although it is typical for MLF to occur naturally, most winemakers will tell you that it is something that needs to be controlled; as a naturally occurring malolactic fermentation, especially after bottling, may be detrimental to the wine. Unlike a typical yeast fermentation where the yeast is used to convert sugar into alcohol, a malolactic fermentation uses certain strains of bacteria to convert harsh malic acid into smooth lactic acid. MLF can take anywhere from a couple of weeks to a couple of months, and like yeast fermentation, its completion can be determined by the like of activity (bubbling) taking place.
Wine should be inoculated with the malolactic bacteria right after the secondary yeast fermentation has completed. If done prior to or during yeast fermentation the MLF will convert the sugar into volatile acids, such as acetic acid, giving the wine an unpleasant sharpness similar to vinegar. Sulfites will also prevent malolactic fermentation from starting, so the bacteria needs to be added to the must after the initial sulfites “burn off” during yeast fermentation and before sulfites are added again for stabilization. Also, Potassium Sorbate will not prevent MLF from occurring, but will cause the wine to have a foul odor, like ripe fish, if the two are combined. Because there are thousands of different strains of malolactic bacteria that can naturally occur in a wine, each producing its own characteristics for better or for worse, most wineries choose to inoculate the wine themselves with a known strain(s) so as it ensure it has the desired effect on the wine.
The Malic acid that is being converted during the malolactic fermentation is the same harsh, tart acid found in Granny Smith apples; while the Lactic acid that it is converted to is the same smooth acid found in milk, butter, cheese, and yogurt. Bold wines, most notably Chardonnay, are often described as being “buttery” on the palate, this is due to the level of lactic acid converted from malic acid. Because of the decrease in sharpness and the increase in smoothness, wines that undergo MLF are less fruity but have deeper, richer, more complex characteristics.
Lactic acid is a mono-acid, therefore, it is a less distinct acid than Malic acid. By converting the Malic acid into Lactic acid, it will inherently reduce the overall acidity of a wine. Also, during the malolactic fermentation only about 2/3 of the Malic acid is converted to Lactic acid, the other 1/3 is converted to CO2 which is what causes the wine to bubble and is released while the fermentation is taking place.
By manually inducing a malolactic fermentation after the yeast fermentation, it ensures that one won’t take place naturally after the wine is bottles. When MLF takes place in a wine that has been bottle, it typically forms a sediment, clouding the wine, and producing sour odor. The CO2 that is produced cannot escape the sealed bottle, therefore carbonating the wine as well.
It’s important to note that not all wines should undergo malolactic fermentation. Wines made from fruit, or grape wines that have a desired fruitiness should not undergo MLF. Most white wines, with the exception of Chardonnay, should be avoided and reds that don’t typically carry bold, rich tones shouldn’t be considered. Malolactic fermentation should be used to enhance wines that already have subtle, yet, rich and complex characteristics.
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