Most Scotch whisky is matured in bourbon casks, shipped over here after maturing American spirit. But that hasn’t always been the case, finds Richard Goslan

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As our spirits manager Euan Campbell tells us, the overwhelming majority of the casks in the SMWS warehouse – in pretty much every Scotch whisky warehouse, in fact – will have found their way over here from across the Atlantic Ocean, having previously held bourbon (or Tennessee whiskey, if you want to include Jack Daniel’s by its own definition).

But that wasn’t always the case. In fact, it was a landmark piece of legislation in the United States from 1935 that really turned the tide from whisky being matured predominantly in sherry, madeira and port casks.

Ex-bourbon wood makes up the vast majority of casks used to mature Scotch whisky.

In 1933, the US had finally repealed the Volstead Act after 13 years – the end of the era of Prohibition. The US Senate sat two years later to discuss what would become the Federal Alcohol Administration Act, to regulate the alcohol industry post-Prohibition. But it was intense lobbying by the Coopers’ International Union that resulted in a requirement being included that bourbon casks could only be used once. The addition of that all-important word ‘new’ in front of ‘oak containers’ would prove to be the saving of a US cooperage industry that had suffered through Prohibition, as well as a huge boon to a Scotch whisky world hungry for inexpensive casks.

The bourbon cask's future was sealed thanks to US legislation passed after Prohibition.

It’s also a stroke of good fortune that the casks being used by the bourbon industry in the US proved ideal for the lengthy maturation of Scotch. Bourbon barrels are made from American white oak, or Quercus alba, which as well as being watertight allow oxygenation to take place – which contributes to helping create new flavour compounds. American white oak is also low in tannins and higher in oak lactones – responsible for the woody character and coconut and vanilla notes we often find in bourbon-matured whiskies. By law, bourbon barrels must also be charred, which further exposes the wood’s flavour and aroma characteristics of spice and caramel and helps remove sulphury and immature aromas.

Traditionally, used bourbon barrels with a capacity of 200 litres used to be first broken down and shipped to Scotland as staves, in bundles known as ‘shooks’ to save space. When they arrived here they were then usually re-assembled as hogsheads with a capacity of 250 litres. The slightly larger size allows both more capacity to be stored in warehouses and lends itself to a longer period of maturation, due to the lower level of spirit contact with the wood. It’s now more likely that the casks are shipped as ‘standing barrels’ and less are being recoopered into hogsheads.

American white oak, or Quercus alba, is ideal for the lengthy maturation of Scotch whisky.

But there is still a huge amount of variation even within the ex-bourbon cask, which you can see in this Outturn – from a first or second fill bourbon barrel (indicating how many times the barrel has been used to mature whisky and how much influence the wood is likely to have imparted) to refill barrels or hogsheads (which have been used more than twice and will have less wood impact, ideal for long-term maturation) to recharred hoggies, customized levels of toasting or the shaved, toasted and recharred (STR) casks pioneered by whisky expert Dr Jim Swan.

There is still a huge amount of variation within the bourbon cask, from first or second fill (or more) to customised levels of heat treatment.

So whatever bourbon cask-matured whisky you choose, we should all be grateful to our friends across the pond. As Jeff Arnett, master distiller at Jack Daniel’s, once told me: “If there’s a Scotch you like, it’s because there are about three gallons of Jack Daniel’s in it! Our barrels gain about 20lb in weight from the whiskey that soaks in. That’s the penalty we pay as the first user.”

Here’s to the humble bourbon cask!