As harvest begins to wind down and wines move out of primary fermentation, one question comes up frequently: When is a wine actually considered done fermenting?

Primary, or alcoholic, fermentation is the conversion of sugars into alcohol and carbon dioxide. While every winemaker has their own target for glucose/fructose, a wine technically needs to be below 2 g/L Glu/Fru before it is considered unable to spontaneously re-ferment. Most winemakers typically target below 1 g/L Glu/Fru.

Then there’s malolactic fermentation (MLF).

MLF involves lactic acid bacteria converting malic acid into lactic acid and is generally used for red wines, while being more of a stylistic choice for whites. For wines going through MLF, the general completion threshold is under 0.2 g/L malic acid.

For winemakers targeting less than 0.10 g/L malic acid, an enzymatic confirmation is necessary to accurately measure those very small concentrations.

One other important reminder during harvest: a wine does not need to have completed MLF to be analyzed as wine. Once alcohol is above 10%—usually when the wine is below approximately 5 °Brix—it can be run as a wine sample.

Not sure which analysis to request? Mark the sample as Fermenting Must and the lab can help determine the appropriate profile.

As we move from harvest into the next stage of winemaking, knowing exactly where your wine is can help guide the decisions that come next.

What targets do you use to determine when fermentation and MLF are complete?

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