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Malolactic conversion, sometime called Malolactic fermentation, is a bacterial transformation (not a fermentation) that can take place in the wine making process. And, wine makers must decide if they want this conversion to occur in their wine.
To get a bit technical, this process allows lactobacilli bacteria to consume malic acid present in the wine, and the by-products are lactic acid, carbon dioxide, and diacetyl. It is this diacetyl that can give a wine (i.e., Chardonnay) a buttery flavor and/or a nutty flavor. This natural process also converts sharper-tasting malic acid (found in green apples) into softer, smoother, creamier lactic acid (found in milk). The process results in the total acidity being reduced, so the wines become softer, rounder, and more complex. Additionally, malolactic conversion stabilizes wines by preventing an undesirable fermentation in the bottle, often referred to as a secondary fermentation.
While malolactic conversion is frequently associated with big, rich, buttery Chardonnay, malolactic conversion is not initiated and/or prevented by the wine maker when fresher, crisper styles of wine are desired.
Most red wines undergo malolactic conversion, while only some white wines, notably Chardonnay, undergo malolactic conversion. This process is most often performed shortly after the end of primary fermentation and just prior to the aging process.
So, malolactic conversion is yet another factor that a wine maker must consider as part of the art of wine making. Cheers!