Malolactic Fermentation and WineAs wineries increase the amount of information on their labels and websites, often winedrinkers see that a wine has undergone malolactic fermentation to achieve its current flavor. What is this kind of fermentation, and what does it do to a wine?
First, you should understand about how wine is made. In essence, grapes are crushed, the juice is fermented and aged, and bottled. Fermentation is when yeast eats the sugar in the juice and turns it into carbon dioxide and alcohol. Between the fermentation and aging steps, sometimes another step is introduced. This is the malolactic fermentation, sometimes abbreviated to MLF.
What happens during this fermentation is that malic acid, which has a sharp flavor, is acted on by special bacteria. This usually takes place in a barrel set aside for this purpose. The result is that carbon dioxide is given off, and the malic acid is converted into lactic acid. This smooths the flavor of the wine. This was a key development in the past century, and by 1990 most red wines and many sparkling wines underwent malolactic fermentation.
Once a barrel has been used for malolactic fermentation, there are always bacteria in the barrel and from that point forward it must always be used for wines intended for this kind of extra fermentation. Often in a winery you will see barrels labelled "ML" or "MLF", and this is why.
Usually a wine that has undergone malolactic fermentation will be more complex and less acidic. It also can take on buttery and creamy overtones, as lactic acid is the type of acid found in milk.
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