What is malolactic fermentation, and why do winemakers use it?

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Dear Dr. Vinny,

What is malolactic fermentation, and why do winemakers use it?

—Bruno, Tucson, Ariz.

Dear Bruno,

We’re really getting into the fine details of winemaking science when we’re talking about Malolactic fermentation, which technically isn’t a fermentation at all. Malolactic fermentation, aka “malo” or just “ML,” occurs after the primary fermentation, when yeast converts sugar into alcohol. ML is a conversion of harsh, tart malic acid to softer, creamier lactic acid and carbon dioxide.

More specifically, a winemaker can inoculate their wine with lactic acid bacteria, primarily Oenococcus oeni (yes, “oeno” as in "oenology"), which then consume malic acid and … “expel” lactic acid, carbon dioxide and an organic compound called diacetyl. Depending on its concentration, diacetyl can impart light nutty or butterscotch notes to overpowering buttery flavors.

ML is a standard practice, used in pretty much all red wines and many whites. In white wines, ML tends to soften acidity and can impart a creamy mouthfeel, so the decision to use it is often made on whether or not the desired finished product is a crisp, bright white (skip the malo) or a fuller-bodied, rich white (full malo ahead).

Intentionally initiating the malolactic conversion also ensures that ML won’t take place later, accidentally, in the bottle, which can turn a wine into a cloudy, smelly, fizzy mess.

—Dr. Vinny

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