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Dear Dr. Vinny,

If you leave a wine for one or two days out in the open and it starts to taste bad, does the alcohol content increase or decrease? And by how much?

—Georgia, Ohio

Dear Georgia,

Even though a wine will probably taste different if it’s been open for a couple days—including possibly the alcohol sticking out a bit more—that doesn’t mean the percent of alcohol by volume will change. Same thing with changing a wine’s temperature or even aging a wine—alcohol percentages don’t change. Perception is one thing, but the chemistry of wine is another.

A wine’s alcohol content is determined during fermentation, when the sugar in the grapes is converted to alcohol. Sure, if you opened a bottle of wine, poured the entire thing into a shallow pan pointed a fan at it and let the surface of the wine evaporate for a couple of days, you might be able to calculate a difference—the alcohol would start to evaporate more quickly than the wine’s water content. But I’m guessing that once you open a bottle of wine, you keep the wine in its bottle, where there is much less surface area, and much less evaporation occurring, and any changes zero to negligible.

You will notice your perception of wine will probably change as a wine is open for minutes, hours and even days if you want to experiment.

A much more fun science experiment would be to put red wine in a pan and start to heat it up, perhaps even setting it aflame to increase the evaporation, reduce it a little further, and then you have a terrific red-wine reduction sauce to braise short ribs in.

—Dr. Vinny

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