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Letting wine breathe is another way of saying aerating the wine. The reason we do this is to maximize contact between the surface of the wine and the air. When the wine is exposed to the air, evaporation occurs and this evaporation removes many of the compounds that create a harsh tasting wine. This same process also releases many aromas. The overall effect means that in younger wine particularly, it will become more mellow or softer and the bouquet will improve leading to a better tasting wine.
Typically, it is red wines that will profit the most from aeration, however, certain types of white wines will benefit from exposure to air. Depending on the type wine and how long it has been bottled, the time that it takes for the wine to breathe for maximum flavor to develop, will change. Generally, the younger the wine with higher tannins will need more aeration than older wines, whose tannins have softened naturally. If older wines are left to breathe for too long, the flavor will go downhill because many of the wonderful aromas will be lost to evaporation.
The basic principle to stick to when aerating the wine is that if the wine has more tannins in it, then it should receive longer aeration. Generally, oaky, heavy, full bodied and wines with lots of tannins will benefit the most from a longer period of aeration and the wine will become much softer and smoother. Subtle, low tannin, light and many white wines will not benefit so much from a longer breathing time. Too much aeration will let too many of the aromas out and the wine will start to oxidize.
The old school of thought that it is okay just to simply remove the cork from the bottle to enable the aeration process to proceed, is a flawed belief. This is because only a small surface area of the wine is coming into contact with the air. Wine aficionados know better and will use other ways to ensure more of the wine is aerated.
One way that wine buffs aerate their wine is by emptying it into a decanter, carafe or such a vessel. More wine is exposed to the air as it is poured, and also as it rests in the container, there is more contact with the air because of the wider opening of the vessel.
Another simple way to let the wine breathe is to just pour it into the glass and leave it for a while. The problem with this is that seeing a glass of wine there, not being consumed, may be beyond the fortitude of the drinker. In this case the glass should be placed out of their view. Out of sight, out of mind. But who wants that?
About 30 minutes before consuming your wine, the wine should be decanted into the container of your choice. To check how the flavor of the wine is developing as you decant it, every now and again stop pouring and try a small sample from the vessel. You will notice that the first sample and the ensuing samples will change subtly.