Earlier this year, scientists fired molecules of unaged malt into outer space in order to explore how whisky ages in a micro-gravity environment. While we wait for the results of that experiment back on Earth, I thought it would be a fitting moment to sip a dram of “Ardbeg Galileo,” a delightfully fruity new bottling from Islay, and ponder the wonder of wood ageing.
Up to 70 per cent of a whisky’s flavour comes from the cask it’s aged in. You can distil the finest spirit in Christendom, but if you age it in a lousy cask you’ll end up with lousy whisky. This is because the cask performs a variety of quasi-mystical feats, absorbing unpleasant sulphury notes and rounding off rough edges in the young spirit while, in turn, imparting colour, flavour and complexity as the whisky moves gently in and out of the wood’s pores.
Scotch whisky must, by law, be aged in oak (people have tried other woods but they simply don’t work as well). The two most common types are American oak, which is normally imported in the form of cast-off casks from the US bourbon industry (bourbon distillers are only legally allowed to use each cask once), and European oak in the form of ex-sherry casks (typically oloroso sherry, but occasionally fino or PX sherry).
If the whisky you’re drinking has big notes of vanilla, coconut or bananas, then it’s probably been aged at least partially in American oak—Glenmorangie are masters in this department and their Glenmorangie Astar is a massive, creamy bourbon cask bomb. European oak, by contrast, tends to contribute more dried fruit and spicy Christmas cake flavour, beautifully typified by Orkney’s Highland Park 18 year old.
Less common in whisky production, but a mainstay of cognac making, is French oak or Quercus Petraea. Pitched somewhere between US and European oak in terms of flavour, French oak has been skilfully employed by artisan whisky blending company Compass Box in their Spice Tree whisky, a bold blend of Highland malts full of fiery stem ginger in syrup, silky crème brûlée and a touch of soot.
The rise of Japanese whisky has also introduced Japanese oak, or mizunara, into whisky distilling. Expensive, difficult to cooper and prone to leaking, it’s a tricky wood to work with, but in the right hands it produces whiskies with an exotic, incense-filled note. The new limited edition Yamazaki Mizunara, or the slightly more affordable Yamazaki 12 year old are both great examples of this.
A bit more of a thorny issue is the whisky finish, where a whisky is transferred to a second cask which has typically been used for wine, port, madeira or, occasionally, rum, for its last few months or years of ageing. Done well, a finish can add layers of complexity to a whisky, but unfortunately a rash of rather gimmicky whisky finishes a few years back saw the style lose some credibility. But there are still some excellent examples out there—Balvenie Portwood is a dense, dark, richly fruity whisky that’s a stunner after dinner, while Glenmorangie’s Nectar d’Or is lifted by sweet, musky citrus notes gleaned from a final phase in Sauternes casks.
Before being filled, casks are “charred” on the inside, a process that helps to caramelise the wood sugars and release an array of flavour compounds. The strength of the char is therefore another way the distiller can shape the character of a whisky—a dramatic example is Ardbeg Alligator, a complex, smoky vanilla whisky named after a heavy char popular with American whiskey makers.
And size matters too. A smaller cask creates a greater wood-to-whisky ratio, accelerating and amplifying the wood’s effect on the spirit. To see the difference try comparing standard Laphroaig 10 year old with the ultra-concentrated, sweeter opening hit of Laphroaig Quarter Cask.
The number of times the cask has been refilled, and the temperature and humidity of the warehouse will also affect the cask’s behaviour. Factor in the potential effects of the gravitational pull, and you’ve got a process that’s one hell of a fine balancing act.