[INTRO].

Any good sommelier will tell you that it’s important to let your Cabernet "breathe." After you pour your glass, you should take a moment to gently swirl it around. Look at the pattern of "legs" as the wine drips down the sides.

Maybe take a good sniff and marvel at the wonderful notes of chocolate, cherry, and blackberry. Or, at least, just wait a second or two before you down the whole thing. And they’re not actually making up the whole "breathing" thing.

Exposing wines, at least red wines, briefly to air does make them taste better by subtly changing their chemistry. There are two key processes happening when a wine is “breathing”: evaporation and oxidation. You won’t see bits of wine vaporizing when you pour your first glass, but the most volatile components—those that evaporate easily—have already started leaping into the air.

That includes some of the wine’s ethanol, the alcohol that gets you buzzed. And while you wouldn’t want all of it to evaporate, ethanol has a pretty strong smell, so it’s not the first thing you want heading up your nose when you tip your glass towards your face. At the same time, sulfites, which are added to wine as preservatives, also take their leave.

These sulfur-containing molecules can give the first whiff of wine a rotten eggy note. Ugh. But once these scents are lessened, you can actually smell the goodies in the wine, and how something smells has a huge impact on how it tastes.

Aerating also exposes the wine to oxygen, and oxygen causes oxidation: a rearrangement of electrons that can shift a chemical’s properties and help break down molecules. In wines, among the first to react with oxygen are phenolics: the stuff mostly responsible for the color of your favorite red or rosé. You may have heard of tannins, for example—those are phenolics that don’t have strong flavors themselves, but cause that drying feeling in your mouth after you drink a bold red.

Oxidation breaks them down, mellowing that astringent texture and allowing other flavors and aromas to shine through. Phenolics come from the grape’s skin and seeds, and since the red wine-making process involves more time with the whole grapes, more phenolics end up in the final product. That’s why rich reds like Cabernet Sauvignons, Malbecs, and Sangioveses benefit from "breathing," and why red wine glasses are rounder and larger, to increase amount of the wine that’s in contact with air.

Fruity, fresh red wines have fewer tannins, so they don’t need to breathe as much, if at all. And too much aeration can ruin a wine. If oxidation is allowed to go too far, it will change and break down the tasty bits of the wine, too, so the flavors and aromas flatten out.

Red wines tend to be okay with a stopper for a few days, while whites become oxidized more quickly. And you can even see it happen: Whites become brown-tinged, and reds turn more orangey. Eventually—like, if you forget about your bottle of wine for months—your wine will become really gross.

That's because bacteria get into it that convert ethanol to acetic acid, the molecular flavor of vinegar. So, if you’re drinking a robust red on date night, letting it sit in the glass for a few minutes or decanting the bottle might enhance the experience. Cheers to our Patreon patron Nick for asking, and to all our other patrons who voted for this question in our poll.

If you’d like to suggest questions or vote to help decide what questions we answer, head over Patreon.com/SciShow and become our patron. [OUTRO].