Why is oxidation bad for wine?

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Dear Dr. Vinny,

Why is oxidation bad for wine?

—Mikayla, Victoria, Australia

Dear Mikayla,

Whenever I write about oxidation (and aeration), I like to bring up apples. You know how if you bite into an apple and set it aside, the apple will start to turn brown and then take on a nutty flavor? It’s because once you’ve broken the skin of the apple and exposed its flesh to oxygen, it starts to react with the oxygen in the air.

Same with wine. A wine can become oxidized if it’s exposed to too much oxygen while it’s being made, or if too much oxygen gets into the bottle because of a faulty closure. You might have firsthand experience with oxidation if you leave wine in your wineglass overnight and then take a sip, or revisit a bottle that’s been open for a week. The wine’s flavors and aromas will flatten, and those nutty, Sherry-like notes replace the fresh flavors the wine had. Oxidation will also change a wine’s color to brown and brick tones. What is chemically happening is that the wine’s phenols are susceptible to combining with oxygen, and the wine’s ethanol starts to break down and shift into other compounds, like acetaldehyde.

The ill effects of oxidation take hours if not days to show themselves, however. Sweet wines, wines with higher acidity, and more tannic wines tend to have more resistance to oxidation, but after a while, all wines will suffer from too much oxygen.

—Dr. Vinny

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