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Dear Dr. Vinny,
What would happen if I left a bottle of wine open for 10 days? I figured a chemical reaction would occur—fermentation? Would it become ethanol?
Let’s start by making sure we’re on the same page as to what fermentation is. That’s when the sugar in wine grapes is converted to alcohol, with the assistance of yeast. That’s how wine is made.
Once that fermentation process is complete, a wine should not spontaneously start fermenting again. But, if there’s leftover bacteria or yeast and sugar in a bottle, there is a chance that could happen, and that is usually considered a flaw. (However, secondary fermentations, as they're known, can be intentional, as is the case for most methods of making sparkling wine.)
But let’s assume we’re talking about a normal bottle of still wine. If you opened up the bottle and left it open for 10 days, the only chemical reactions that would take place are evaporation and oxidation. The wine’s chemical composition will not change—since fermentation is already complete, the alcohol percentage and residual sugar will remain constant. What will happen is much like what happens when you cut open an apple: Oxidation will cause it to turn brown and taste a little nutty. Same thing with wine—it will take on nutty notes and lose its freshness.
Beyond 10 days, perhaps after a month or so, another conversion might take place: The wine’s alcohol (ethanol) can convert to acetic acid, if there’s help from a type of bacteria known as an acetobacter. And that's how you make wine vinegar!
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