What actually happens to wine once you decant it?

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Dear Dr. Vinny,

What is the chemical process taking place as tannins soften in a wine that is in a decanter for 2 or 3 hours?

—Helen C., California

Dear Helen,

When you open a bottle of wine, two things start to happen: oxidation and evaporation. Neither of these actually cause tannins themselves to soften (at least, not in that period of time). But other things can happen to make the wine seem more harmonious and its tannins more integrated.

The first thing that happens is evaporation. After a wine has been poured into a decanter, highly volatile components will move to the new headspace and then dissolve into the surface of the wine. The more you swirl or pour a wine and change up the surface, the more this can happen. Typically, the volatile compounds in a wine tend to be the less desirable notes, so evaporation will help notes of burnt matchsticks or rubbing alcohol, for example, to “blow off,” making the wine seem smoother and more appealing.

When it comes to wine, the effects of oxidation happen over hours and days, not seconds or minutes. Sure, oxidation might quickly turn a sliced apple brown, but you’re not likely to pick up any actual changes to the tannins in a wine after just a couple of hours in a decanter. I checked with U.C. Davis professor Roger Boulton about this, and he says, “There is no evidence that any of the tannin components are altered by such an exposure to dissolved oxygen. The smaller phenolics that react much faster, and might be consumed, show an insignificant change in concentration. Neither of these can explain the changes that are attributed to air such as ‘tannin softening.’”

That’s not to say tannins don’t change their structure over time. Certainly, during a wine’s life in the bottle, tannins will bond with oxygen, form tannin polymers and fall out of the liquid, creating sediment and causing the color of a wine to fade or change. But this takes years to play out.

I think most people will agree that 2 or 3 hours in a decanter, as you’re describing, do result in a wine showing differently, and usually for the better. Oxidation and evaporation both seem to smooth out rough edges, and they can make a wine seem more expressive, more aromatic and better integrated. We just can’t say the tannins are softer.

—Dr. Vinny

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