Maturation in casks is one of the main reasons that whisky is so globally popular. Interactions between the organic compounds in the newly distilled spirit and the lignin compounds in wood create a broad and full range of flavours, and aging in different types of oak can achieve incredible variations in taste.
Scotch whisky must be matured in oak casks for a minimum of 3 years. There is no maximum age, and some whiskies are bottled at 50 years or older – however, casks themselves can degrade over time, and in some cases the flavour from the cask can overpower the whisky, so the vast majority of casks will not be aged for so long.
How old a whisky should be is a matter of personal preference, and dependent on a number of factors. For example, Scotch whisky is almost always matured in second-hand casks which impart their flavour less quickly, and the cooler climate means a more measured maturation as well. By comparison, bourbon whiskeys which are aged in new (virgin) oak barrels, and in the warmer American climate, are usually bottled much younger.
The cask that is used also has a large impact on the maturation, and the flavour of the resulting whisky. Most commonly Scotch whisky is aged in hogsheads, barrels or butts that have previously held bourbon whiskey or sherry. Distilleries are increasingly using other types of cask such as pipes or quarter casks, which have held port or red wine, for example. However, these remain a very minor part of the industry as a whole, and for the moment are limited to small-scale experiments.
Part of the distiller’s challenge is to select casks that work well with the spirit that their distillery produces. While time spent in cask is usually a reliable sign of quality, if the casks used are poor then the resulting whisky may be worse than one that has spent less time in a better quality cask.