What is decanting?
Decanting is the act of transferring wine from a bottle into a decanter, carafe, or jug. This dates back to a time before glass bottles were common when wines would have been drawn straight from casks and served in jugs. Once bottling had become more common, it would usually have been at the wine merchant or importer, rather than at the winery. This means that the filtering and fining* that now occurs would rarely have taken place; leading to wines with deposit after a few years in bottle. Once you opened the wine, the simplest way to avoid deposit in your glass was to decant the wine into another vessel, and stop pouring before the sediment hit the glass.
So why are decanters still used?
Young wines can benefit from decanting, as this aerates the wine. Not only does the act of pouring the wine into a decanter (or jug) increase the contact of the wine with air, once in the decanter a greater surface area is exposed to air. This aeration alters the wine; it softens it and can encourage more complex aromas that would normally have developed with age.
How long do I decant?
Unfortunately there is no exact answer. If it is a relatively young wine (2-4 years old) then a two or three couple of hours should be fine. However, if you have friends coming round for dinner, and you forget don’t worry too much. It’s always an interesting experiment to see how much the wine changes (hopefully improving) as the dinner goes on.
Should I always decant?
Decanting can do wonders to a young wine, but can also help bring out the character of an older wine. Old wines (this term is relative, but in this instance I mean wines that are over 20 years old) should only be decanted if they are throwing sediment, and this should be done just before serving. This is because an old wine is more fragile and will oxidise rapidly, increasing the risk of stripping the aromas from a delicate wine.
*Fining is the adding of a substance to a wine in order to remove organic compounds. This is usually to help improve clarity. This process is used in both brewing and fermentation, traditionally this was done with a variety of substances such as egg whites, blood, milk, and carrageen moss. Nowadays, these substances have been mainly replaced by isinglass and bentonite (a type of clay).