How does a winemaker stop a fermentation to make an off-dry wine?

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Dear Dr. Vinny,

How does a winemaker stop fermentation when they want to make an off-dry wine instead of a dry wine?

—Laura, Cancun, Mexico

Dear Laura,

During fermentation, yeasts gobble up the sugar in grapes and convert it to alcohol. That’s the simple explanation, at least: There are a lot of approaches to fermentation, including whether or not to add commercial yeast or rely on native yeasts, how hot or cold to ferment, whether or not you do it whole cluster, and what to do when the fermentation gets “stuck” or stops on its own.

As you point out, sometimes it’s desirable to halt a fermentation before it’s complete, to make a wine in an “off-dry,” or slightly sweet style.

One way is to lower the temperature, which can slow or stop the fermentation process. A more complicated method is to remove the yeasts from the wine, which typically also involves some racking and fining. For example, bentonite clay can be added while a wine is still fermenting. The clay acts as a clarifying agent, binding to the yeast cells and other suspended solids in the wine, and settling to the bottom of the tank or barrel. Then you can gently rack, or move, the wine from one container to another, leaving the sediment (and yeast) behind.

There are a couple of other additives that can help. A dose of sulfites can slow fermentations down, and potassium sorbate can stop a yeast colony from regenerating.

—Dr. Vinny

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