Whisky is made in the cask. To put it simply, the fiery young spirit that goes into the cask emerges transformed into the complex liquid gold we know as whisky. In Scotland whisky must by law spend three years maturing in cask to be legally labelled as Scotch, while in Japan any whisky classified as Japanese Whisky must be cask aged for a minimum of two years. So, what exactly goes on inside the cask and how does it affect the flavour of the whisky?
Once inside the cask the young spirit gradually begins to change in character thanks to evaporation. Oak is used to make whisky casks since it is porous and allows a small quantity of liquid to evaporate. Alcohol is more volatile than water, meaning that a greater proportion of alcohol is lost compared to water and other components of the whisky. The result is that over several years the whisky becomes smoother and rounder, and also absorbs wonderfully complex spiced flavours from the oak itself.
Another key factor to consider is the size of the cask. Smaller casks mature more quickly since there is greater contact between the whisky and the side of the barrel which facilitates a faster rate of evaporation and thus maturation. In larger casks there is also less extraction of flavours from the oak due to the reduced surface contact per litre of whisky. This is why Scotch must be made in a cask that is no larger than 700 litres and made of oak to ensure consistency.
Scotch must be matured for at least three years in the cask, but many highly-prized whiskies are aged for decades in the cask before being bottled and released. Single malts are usually aged for at least 8-12 years. It is important to bear in mind that the whisky only continues to evolve and develop inside the cask. When it is bottled the flavour profile will not continue to improve.
Another way to tweak the flavour profile of whisky is by ageing it in a barrel which has previously been used to mature sherry or bourbon. Casks which have already been used to mature spirits will mature the whisky more slowly and also add unique flavour characteristics from the previous occupant. A great example of a distillery that frequently works with sherry casks is Macallan, while distilleries like Springbank have also successfully experimented with Carribean rum casks.