The Secret to the Whisky You Love? Wood.

When it comes to whisky, Scottish law is stricter than strict. To legally earn the moniker “Scotch whisky,” it can contain only a mere three ingredients: malted barley, water, and yeast. And yet, even the most casual Scotch whisky drinker knows that the range of flavors and aromas possible in a single glass is vast. From smooth, clean whiskies with notes of vanilla and toffee to heavy, smoke-scented whiskies that linger on the palate, the category is hugely varied and exciting. How is such diversity of flavor possible? Wood, when wielded properly, behaves more like an ingredient than a tool. When fashioned into sturdy barrels for aging, wood can impart nuanced and complex flavors.

“We have casks built specifically for us, to our specifications. We control the wine that we put into those casks to season them. And then we ship them across and fill our whisky into them,” says Gordon Motion, the master whisky maker for Highland Park, explaining the importance of barrel-aging for Highland Park’s award-winning single malt Scotch whisky.

Søren Solkær

Søren Solkær

Not all types of wood, or barrels, are created equal

To be legally defined as a Scotch, the spirit must have been aged in oak casks for a minimum of three years. But the type of casks is up to interpretation, and it’s a serious decision on the part of any distiller. It’s not an understatement to say that a Scotch’s flavor is dependent on the casks it’s aged in. At least 80 percent of Highland Park’s flavor and 100 percent of its color is said to have come from the casks it’s aged in.

But there’s a wide range of oak casks used in Scotch. Most casks come from either America, used for aging bourbon, or Europe, used for fortified wine. One particular spirit plays a huge part in the flavors you ascertain in your Scotch: bourbon. By law, barrels can only be used once to age bourbon, so Scotland has experienced an influx of used bourbon (and Tennessee whiskey) American oak casks. “With the growth of bourbon and the fact that bourbon distillers could only use their casks once and then have to get rid of them, there became a glut of ex-bourbon casks available to the industry,” says Gordon Motion. “The Scots were saying, ‘Okay, that’s great. We’ll take them.’”

It’s been said that 9 out of 10 casks used to mature Scotch whisky were first used for bourbon or Tennessee whiskey. But the bourbon casks deliver a completely different flavor, Motion says. You’ll find stronger notes of butter, vanilla, and coconut in a Scotch aged in American oak casks. There’s also one downside to using bourbon casks: The first spirit, the bourbon, extracts more of the flavors from the wood before it’s filled with whisky the second time around. “Before these casks are seasoned with a wine or bourbon, you’ve basically dried the wood out,” Motion explains. “It’s like an empty sponge and it’ll soak some of that spirit or wine into the cask.” That bourbon before it extracts much of what a Scotch would, especially because of its higher proof, compared to a wine with a lower strength.

Søren Solkær

Søren Solkær

Highland Park goes the traditional route for its barrels: sherry casks. “Traditionally, Scotch whisky [distillers] would have used ex-wine casks, ex-sherry casks,” Motion says. “They were shipped into Britain many years ago to be bottled over here, and when the casks were emptied, the Scotch whisky [distillers] would grab hold of them.”

Highland Park ships staves, made of European and American oak, to Jerez in southern Spain where they’re made into casks. These casks are filled with oloroso sherry and left to mature for about two years before they’re emptied and shipped to the Orkney Islands, where they’re filled with the Highland Park new-make spirit. Those two types of oak used make a huge difference in the final Scotch, Motion says. “We have sherry casks made from American-grown oak, which gives a real rich, oily vanilla character to the whisky. And then we have sherry casks, which are made from European oak. European oak is a lot more tannined and so you get a darker color and a more spicy dry fruit.” It’s why so many of Highland Park’s Scotches have that deep rich flavor that is characterized by notes of dried fruits, zesty orange, and cloves.

The art of barrel-aging

The more time the whisky spends in the barrels, the more it matures — but if you mature it for too long, you risk over-aging your whisky and letting it go bad. After the first stage of maturation (called subtractive maturation), the wood from the barrels eventually passes on or adds its flavors to the spirit inside of it (called additive maturation). And the longer it ages in barrels — the last and slowest phase of maturation — is “where compounds in the whisky and compounds in the wood interact,” says Motion. “And they can create something new that wasn’t there in either of them.”

The art of making whisky is the diversity in the wooden vessels: No two casks are alike. “The variation in each of those casks, even if they’ve been filled on the same day and lain in the warehouse next to each other for 17 years, is remarkable. They could be completely different characters and recent Highland Park special releases — The Light and The Dark are perfect examples of this,” says Motion. “That’s also where the blending side of it comes in, where you have to pick and choose the right style of cask to give you the character you’re looking for.”

Søren Solkær

Søren Solkær

The decision to use sherry casks doesn’t come without its challenges — namely cost and having the expertise in wood management. But Highland Park has devoted itself to every detail of their casks, right down to the wine that goes inside to season them. Motion says the distillery works closely with a number of sawmills, cooperages, and bodegas in Spain to have the casks built to its specifications. “We have strict conditions on how our casks should be built and toasted, depending on where the tree grew, and whether it was American or European oak. And we also control the wine that goes into the cask to season it,” Motion says. “And the fact that we’ve got a lot of color coming from these sherry casks means that Highland Park can be an entirely natural color. We don’t add any artificial colors to it.”

Though the Scots may not have birthed the practice of barrel-aging — the Greeks began transporting their wine and beer in wooden barrels around 500 B.C., modeled after the Gauls they’d seen do it first — they surely have gone a long way to perfect the art of it. Motion believes there’s a chance this discovery of barrel-aging, and the effects on whisky, was a happy coincidence.

“It was probably accidental the first time,” he says. “The ancient distillers would have been making more than they could drink and would have put it into a cask and then left it and then gone, ‘You know, actually that’s better than it was when we made it.’” To that, we toast those who first discovered just exactly how wood could unlock a whole new world of flavor in our whisky.