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Dear Dr. Vinny,

How does temperature impact fermentation? Does a cooler or warmer fermentation result in sweeter wine? More acidic wine? Lower alcohol content?

—A.J., Los Angeles, Calif.

Dear A.J.,

During fermentation—that magical process when yeast converts the sugar in grapes into alcohol—there is both the ambient temperature of the winery to consider and the residual heat that is a byproduct of fermentation. Winemakers have to find that Goldilocks sweet spot for both, where it’s not too hot nor too cold for their ideal fermentation.

A hot fermentation is a fast fermentation. When it’s too hot, the yeast can start to die, fruit flavors can start to taste cooked or stewed, and aromatics can be lost. Warm ferments also make it easier for unwanted microorganisms to thrive.

If it sounds like a cold fermentation is preferred, many winemakers would agree. Cooler ferments are believed to preserve colors, aromas and fruit flavors. But too cold and the yeast can go dormant, or get "stuck."

Some winemakers like to start off with warmer ferments and then cool them off, or vice versa, using their experience with their grapes to coax more color and texture, to give the yeasts what they want so they don’t struggle before they are done doing their important job. Winemakers can use tanks with built-in cooling systems, or even just use ice to regulate temperatures.

As far as alcohol content, remember that it’s directly related to how ripe the grapes are (and how much sugar they have) and how much of that sugar is converted into alcohol. While a halted fermentation means that there is probably some residual sugar left that can still convert to alcohol, fermentation temperature is not the primary determining factor in the alcohol equation, rather it is the ripeness of the grapes.

—Dr. Vinny

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