What's the difference between natural and malolactic fermentation?

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Dear Dr. Vinny,

Can you please clarify natural vs. regular vs. malolactic fermentation?

—Stuart, Fairfax, Calif.

Dear Stuart,

Fermentation—or “regular” fermentation—is a process of yeasts converting the sugar in grapes into alcohol. If you just left grapes alone, eventually they would ferment on their own, thanks to native yeasts that are present everywhere. If fermentation is left to happen on its own, its often called “natural,” “wild” or “indigenous” fermentation.

But many winemakers choose instead to inoculate with “cultured” or “lab” yeasts, which are consistent, strong yeasts that can be easier to manage, especially if you’re making larger volumes of wine. Native yeasts can be riskier, slow down or even stop before a fermentation is finished, known as a “stuck” fermentation. In those cases, you can try to add a yeast nutrient to encourage the fermentation to begin again.

Meanwhile, malolactic fermentation (sometimes called ML or "malo" for short) isn’t technically a fermentation, but a conversion of harsh, tart malic acid to a creamier, softer lactic acid. ML isn’t necessary to the winemaking process, but it’s popular among most red wines and many whites, both for added complexity and for stability.

—Dr. Vinny

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