By law, appropriately distilled spirit in the UK has to have spent at least three years maturing in a barrel to be called whisky; bourbon in the US takes only two years. For a long time, all the Scotch whisky you could get your hands on was likely to have been matured in oak from the quercus robur – commonly called the English Oak, but in fact native to all of Europe. This was because distillers purchased Spanish casks that had already been used to store sherry, hoping to impart a little of its character to their malts. As the supply of sherry butts began to diminish in the last half-century, some distillers went as far as paying for casks to be constructed, then lending them to Spanish bodegas before storing whisky in them.
There was, though, an alternative to sherry: bourbon casks. Bourbon is traditionally matured in new casks, made from the American white oak, quercus alba. Once used, American distilleries were happy to transfer them to Scottish distilleries – often with the same corporate owners – where they impart a lighter character to the malt than the sherried style. Increasingly, casks that have previously stored other fortified wines and spirits – port, madeira (as in our most recent review) – are being used to produce whiskies with different qualities (though also with heftier, and not always justified, price tags). The folks at Glenmorangie have developed a particular aptitude for lucratively creative whisky maturation.
The casks impart some of their own flavour too. Bourbon barrels are charred on the inside, to help the spirit permeate them and thus magnify their effect. Some whisky producers take advantage of this to achieve particular effects. Johnnie Walker Double Black Label incorporates some whiskies from deeply charred casks to lend a darker appearance; a characteristic it shares with Loch Dhu’s Black Whisky, which enjoys the singular reputation of being, by some distance, the very worst single malt money can buy.
The environment in which a whisky matures plays a role, too. Oban, sitting for fourteen years in a warehouse by the sea, has a distinctly maritime aspect; Islay whiskies possess a peatiness partially acquired during their long wait for maturity by the island’s carboniferous bogs. Individual warehouses can produce casks of whisky that vary greatly: that’s why most single malts are blended together from several casks, but also why ‘single cask’ bottlings are praised for their individuality. There’s so much more to say about whisky maturation – ironically, we could go on about it for years – but we hope that we’ve made the point that there’s some important stuff going on in the barrels: the whisky’s not just lying around!