Whoever is asking themselves this question should firstly know that this is not always needed. Its necessity varies depending on different factors, but certainly there are three fundamental reasons why you should decant wine: to separate the dregs or sediment that might be in the bottle, to air it or oxygenate it, and to open it.
In the first case, decanting wine is recommended to separate it from any dregs or sediment that might be inside the bottle. These sediments are found more frequently in wines with prolonged ageing periods. Their presence is normal and originates from the oxygenation or reduction reactions happening during some wines’ fermenting and maturating processes.
When decanting wine, the bottle should be placed in vertical position a day before the time at which it is going to be drunk. This is required when the wine being dealt with has had a long ageing process, usually reserve, and grand reserve wines. This way, thanks to gravity, the sediments precipitate to the bottom of the bottle.
Once opened, moving the bottle too much should be avoided, and the wine must be poured carefully into de decanter, tilting it slightly. It is important to observe the neck of the bottle and stop pouring the wine when the dregs start getting close to it. Afterwards, the wine is left to rest for a few minutes for the dregs that might have slipped through to precipitate to the bottom again.
In second place, it is necessary to decant wine in order to air it or oxygenate it. Sometimes, when a bottle of reasonable ageing is opened, unpleasant smells can be perceived. These scents are generated during the reduction reactions happening in the bottle, and the lack of oxygen makes it smell musty. In this case, when wine comes in contact with the air, it is ventilated, and its authentic aromas come back out.
In these cases, wine must be decanted carefully, as an excessive oxygenation could be counterproductive. Best practice is to taste the wine beforehand by pouring some into a glass and letting it air for about ten minutes. If the smells improve, the rest of the bottle should be decanted, in order to allow the wine to oxygenate quickly.
Finally, in third and last place, it is recommended to decant wine in order to open it. Young wines can sometimes have a mild aromatic intensity, and the decantation process helps opening them. Here, the wine’s contact with air favours the development of the wine’s aromatic potential. This process is the same as when wine is swirled in a glass, where oxygenation is used to strengthen its smells, especially the fruity tones.