Decanting a wine is the process of pouring it out of the bottle and into a decanter before pouring it into a wine glass. Often the wine is left in the decanter for an hour, or so before serving. This process helps to speed up aging of a wine and is generally only done with young red wines.
A decanter generally has a long neck and then a large area to hold the wine. They should be able to hold a 750mL (750mL bottle is the most common size sold commercially) bottle of wine. The wine is poured down the neck and this process exposes the wine to a large amount of oxygen very quickly. With a decanter, like the one pictured above, the wine sits with a large surface area exposed to oxygen.
All of this exposure to oxygen is what helps to speed up the aging process of the wine. It helps soften the tannins. Tannins are the molecules that give the wine astringency. Astringency is what causes the drying sensation on your tongue and gums. Your mouth feels a little like sandpaper when you take a sip of a very astringent wine. If you rub your tongue on the roof of your mouth and it is no longer smooth (ignore the normal bumps in the roof of your mouth and just consider the ability to slide you tongue back and forth on the roof of your mouth) then you have an astringent wine. Some people love astringent wine so.. there is no need to decant a wine if you like it that way. If your preference is something a little smoother, a little softer then consider getting a decanter. A decanter does not need to be expensive. The last one I bought was almost 20 years ago and it cost me about $20. Prices have probably gone up, but it isn’t necessary to spend tons of money if you don’t want to.
The interaction with oxygen can help “open up” a young wine. Sometimes a young wine does not have a lot of aroma. That is when a wine is described as closed. When you expose a closed wine to oxygen via decanting you can often bring out more aromas.
So, if you have a young red wine that is astringent and not very aromatic, then if will likely benefit from decanting.
If you are serving an older vintage of wine then I do not recommend decanting it. Older wines have been exposed to small amounts of oxygen through their corks over time. The process of softening tannins, and opening a wine up are done in the bottle over time for you. Some people will decant an older wine because there is likely sediment. Sediment naturally occurs over the life of a wine and although it is a nuisance it is not harmful. If you decant a wine that has been aged in bottle for many years then you will likely expose it to too much oxygen and end up serving a wine that is “over the hill”. This is a description of a wine that has been exposed to too much oxygen. A tell tale sign of a wine that is over the hill is a brown hue. If the wine does not look red, or purple, but instead looks orange, or brown then the wine will probably taste oxidized.
An oxidized wine tends not to have lost the fruity character that we enjoy in a wine. It can taste more like vinegar that wine. It may also have a sherry character, which is considered a fault in all wines except sherries.
Decant young wines and leave the older vintages alone is my advice to you.