Casks must be oak and comply with the 'traditional casks' principle, ie. which would have traditionally been used. In the past various wines and spirits were shipped in casks to Scotland and bottled locally, providing a ready supply of empty casks that malt whisky distillers could make use of.
Regulations state: "There is sufficient evidence of traditional use to justify use of the following oak casks for maturation or 'finishing' of Scotch Whisky: Bourbon and other whisky, grape brandy (including Armagnac and Cognac although they are technically wine spirits), rum, fortified wine (including sherry, Madeira, port and Malaga), still wine (of whatever type of origin) and beer/ale."
These cask types can be used for the entire ageing process, or, more typically for secondary maturation. This means transferring malt whisky already matured in Bourbon and sherry casks into a different type of cask for an additional ageing period (ie. secondary maturation). The aim is to further develop the flavour profile produced by the 'primary' maturation in Bourbon and sherry casks.
Bourbon barrels typically contribute vanilla, honey and fruit, with a light, dry sweetness. Sherry casks contribute richer sweetness with vanilla and dried fruit notes including raisins and prunes. These differences are partly down to Bourbon barrels being made of American oak, with sherry casks European oak (though there are examples of sherry casks made from American oak). Various malts are a 'recipe' of both types of casks.
The range and intensity of flavours that secondary maturation in a different type of cask can contribute, depends on various factors, including the type of oak, the character of the previous incumbent (eg. port) and the time-frame, which varies significantly. It may be relatively short, perhaps up to a couple of years (this can also be referred to as a 'finishing' period). Alternatively, it may be several years, or longer.
"It's about using the right cask for the right amount of time. Using port casks for a short period makes a subtle contribution to the Balvenie Port Wood Aged 21 Years. Port casks add fruity, berry sweetness to the Balvenie, which is already rich and fruity, so it's more a case of fine tuning and not a transformative influence," says Brian Kinsman of William Grant & Sons.
How the same type of cask influences a malt when used for different time-scales, and purposes, provides additional insight.
"We used Bordeaux red wine casks for a shorter, 'finishing' period, and also for full maturation of Bowmore. We monitored the casks every three months to see how they were progressing, with red fruit notes and berries coming through from the Bordeaux casks after about ten months to one year. These red fruit flavours continued to get stronger. Bowmore finished in Bordeaux casks retained more of the classic Bowmore character, which includes vanilla, fruit, honey and coconut, with a good hint of peat smoke. Bowmore fully-matured in Bordeaux casks showed stronger red fruit notes, red grapes and ripe, red berries, with a hint of peat smoke at the end of the palate," says David Turner, Distillery Manager, at Bowmore Distillery.
With a broad range of cask types already being used, secondary maturation is an approach that's all set to continue evolving.
"We set out every year to try out new cask types, that's a very active part of our business, whether it's new sources of cask types we've used before, or other cask types to create brand new things. When trialling a type of cask we haven't used before we'll probably use a 10-12 years old malt which has acquired less oak influence than an older malt, as this enables you to see the effect of the secondary cask more clearly," says Brian Kinsman.
Casks must be empty before being used in Scotland, with any liquid remnants of the original incumbent drained. Meanwhile, some residual liquid is present within the staves of the casks, having been absorbed during the cask's previous use. The 'fresher' and less 'dehydrated' the cask is on arrival in Scotland, the greater the amount of residual liquid in the staves, referred to as 'indrink.' Depending on the size of the cask, indrink can amount to several litres.
Indrink should not be considered as residue port or whatever the previous incumbent was. This is technically a 'wood extractive' liquid, which includes various flavour compounds derived from the oak, in addition to characteristics of whatever the previous incumbent was.
As soon as a cask is filled, the indrink begins to leach from the staves into the liquid within the cask, instigating various reactions, with the most significant release of indrink in the first few years.