What exactly does aeration do to a wine?

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Dear Dr. Vinny,

What is the science behind the aeration of wine? What does air do to a wine? Why is it if a wine is closed and then you put air though it (via a funnel of some type) that it can release the aromas and flavors of the wine?

—Heidi Y., Grande Prairie, Alberta

Dear Heidi,

Exposing wine to air does two things: it triggers oxidation and evaporation. Oxidation is what makes an apple turn brown after its skin is broken, and evaporation is the process of liquid turning into vapor. Wine is made up of hundreds of compounds, and with aeration, usually the volatile undesirable compounds will evaporate faster than the desirable, aromatic and flavorful ones.

There are a few particular compounds that are reduced with aeration, such as sulfites, which are added to wine to prevent oxidation and microbial activity but can smell like burnt matchsticks, and sulfides, which are naturally occurring but can remind you of rotten eggs or onionskins. Ethanol is also a highly volatile compound, and a wine that smells too much like rubbing alcohol when you first open it might lose the ethanol note and become more expressive with some aeration.

You mention funnels as a way to aerate wine, but just opening a bottle and pouring a glass will also provide aeration, as will swirling your glass of wine. For more extreme aeration, decanting a wine works well too. After a while, aerated wines begin to oxidize, and the flavors and aromas will flatten out. The more dense and concentrated a wine is, the more it will benefit from aeration and the longer it can go before beginning to fade. On the other hand, you probably don’t want to aerate delicate older wines for long, as you can miss out on their unique aromas, but they’re often decanted to remove sediment.

—Dr. Vinny

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