The process of decanting is necessary only for red wines which have a lot of natural sediment which has deposited at the bottom of the bottle over time. By tilting the bottle at a 45 degree angle and slowly pouring the wine down the neck of a decanter, preferably with a small candle underneath to see the sediment, one can stop the pouring as the sediments reach the neck of the bottle in order to serve a clear wine.
Most wines however can benefit from a breathing period in a decanter or a jug. This is especially true for tight, young red wines. Not only is serving a wine in a decanter visually appealing, it helps the wine release all its aromas.
As soon as the liquid comes in contact with air, it oxidizes. In other words, it undergoes a speedy aging process, with tannins and acidity softening to create a more harmonious and flavorful wine.
The fact that acidity subsides with oxidation means that white wines too can stand to benefit from breathing, something that is seldom done. As a general rule, one hour is enough for most wines, though you may find big reds which truly only reveal themselves after 4-5 hours.
What about champagne? Champagnes can actually be decanted and some vintage wines will reveal more flavors and complexity this way. Opt for a soft decant and fairly quick serving, away from exposure to sunlight. If you want to keep the bubbles though, it’s best to serve straight from the bottle.
There is one type of wine which you should avoid pouring into a decanter: very old vintages, or wines which should have been opened earlier in their life. It’s best to leave them in the bottle until you’re serving your guests, as you may deteriorate the already softened tannins and put the delicate wine over the top.