Decanting a wine may be done for several reasons. However, the most common are to remove sediment (filtration), aerate the wine and for presentation.
Aeration: A young tannic red tastes better once it has been given time to ‘breathe’ – the exposure to air softens the tannic bite allowing the wine’s complexity to show through. Although this can be done simply by removing the cork, the exposed surface area of wine is minimal so it may take several hours. The preferred method to improve the wine’s taste is to decant the wine into a wide decanter. The decanter’s shape exposes more of the wine to air, which reduces the need for aeratation (recommended times vary from 10 minutes to an hour).
Filtration: Aged wines, primarily reds, may have some sediment which should be removed before serving. The actual process here is the same, although some preparation is needed. First, the wine bottle itself should have been stored on its side, and not rotated or agitated for some time: this causes the sediment to collect along one side of the bottle. Then 24 hours before serving the bottle should be stood upright allowing the sediment to collect around the punt at the base of the bottle. Once opened the wine should be decanted (carefully) in the same method as above, stopping, however, the wine before any sediment drains into the decanter. It’s often useful to remove the entire capsule from the bottle before decanting and to do so under good light. This allows you to decant as much of the wine as possible by pouring until the sediment just reaches into the neck of the bottle. One caveat is that the wines that most need decanting (fine aged wines) are often also the most delicate and thus susceptible to rapid oxidation. In this case you should decant to remove the sediment, but you don’t want to let it stand (decant and serve immediately). An extremely delicate old wine may oxidize only 10 minutes after decanting.
Presentation: Put simply, wine looks more elegant when poured from a fine decanter!
Most whites do not need decanting as they don’t have the tannin of a young red nor do they undergo much bottle aging. However, certain whites may have sediment or wine making faults and could benefit from the aeration provided by decanting.