Maturation in the Cask
By law, Scotch whisky, including blended whisky, must mature in a cask for at least 3 years and one day. Single malt whisky is usually matured for 10 or more years. You rarely find younger single malts. Excellent single malt whiskies are matured for 12 to 21 years. Only oak casks are used since oak wood is breathable and durable. Softwood contains resin, which agglutinates the pores.
The origin of the casks is crucial for the taste of the whisky. Glenfiddich predominantly matures their whiskies casks made from American white oak that have previously been used to mature Bourbon whiskey. In contrast, distilleries like Macallan or Bowmore use mainly casks that held Spanish sherry before. One malt whisky from Springbank became a legend: it was matured in ex-rum casks from the Caribbean and took on a green colour.
A bit less important for the taste of the whisky is the place of maturation. The old warehouses are dark and have earth floors. The casks are stored on oak beams and are stacked on top of each other in 3 to 6 rows. Modern warehouses have concrete floors so the casks can be moved with forklifts. Until the end of the last century the casks were stored lying on steel racks. Nowadays the casks are mostly stored upright on pallets. Whisky matures differently in the highlands than on the islands, since there are hot summers and cold, snowy winters in the highlands, while the gulf stream provides for a mild climate on the islands and at the coast.
Usually the whisky is filled into the cask with an alcohol content of 63.5%. Over the years some of the cask content evaporates through the cask walls. Alcohol is more volatile than water so it evaporates more quickly. The alcohol content of the whisky decreases by 0.2% to 0.6% annually. The Scots call this evaporated alcohol the Angel’s Share. The fluid level decreases by 2% each year. It is measured with a square wooden ruler that has four scales on each of its four sides corresponding to the various cask sizes. The scales indicate the target level for each year. With this method even smallest leaks can be detected. Experienced controllers tap on the cask ends with a long-handled wooden hammer and deduce the fluid level by the resulting sound.
Due to the evaporation and the absorbing of flavours from the cask wood, the whisky becomes mellower with each year. Samples are taken regularly from each cask to find out when the whisky has reached its prime. The size of the cask is important, too. Larger casks have a smaller surface in proportion to the content, and fewer flavours can be extracted from the wood. Therefore whisky in large casks must be stored longer in order to reach the same level of maturation!
Casks are regularly refurbished in cooperages. Some casks are used several times and over decades. Of course whisky absorbs less sherry flavours in a second-fill sherry cask than in a first-fill cask. Through these various influences each cask produces an individual whisky over time.
In order to achieve a consistent taste, distilleries must blend their single malts accordingly. This has nothing to do with ‘blended malt whisky’ since all whiskies used are from a single distillery, hence the name ‘single’ malt. Most single malt Scotch whiskies are blended from several casks. The age statement on the bottle refers to the youngest whisky used for this bottle.