Distillers around the world are pushing the envelope when it comes to the types of wood they’re aging their spirits in.

If you like the taste of bourbon, you like the taste of white oak.

Bourbon makers say that somewhere between 50- and 80-percent of the flavor in bourbon is derived from interactions with the white oak staves used to make a barrel. The oak not only has its own natural flavors in it—after all, it’s a plant like rutabaga or corn—but treating it with a flame to char the interior of the cask releases myriad other compounds, including a sort of butterscotch top-note that arises when the wood’s natural sugars are caramelized. The interaction of oak and the molecules naturally found in spirits also create new and inviting compounds.

One of the people who likes the complex flavors derived from oak is Melkon Khosrovian, who in 2004 founded Greenbar Distillery in Los Angeles with his wife, Litty Mathew. About seven years ago, Khosrovian started thinking that, yes, oak is tasty and all, but what flavors might be able to coaxed of out of other types of wood?

“The question was, ‘what are we missing?’” he said. “Why doesn’t anyone use these others woods? Distillers are twisting themselves into knots trying to make whiskey more interesting by aging them—at least secondarily—in sherry casks, vermouth casks, wine casks, beer casks, you name it. If we’re going to go that way, what about the rest of the forest?”

Khosrovian had a few ideas about where to start. He’d been an amateur furniture builder for a time, which afforded him an inadvertent education in the smell and tastes of various woods. So he selected 30 different wood samples (sourcing many from suppliers of barbecue enthusiasts), and began a series of experiments. “We stuck the wood inside jars with whiskey to see what kind of flavor, color and aroma we could produce,” he said. He spent about a year sampling and tasting before selecting several for blending. “In the musical vernacular, we took a symphonic approach to wood, versus than the soloist approach that virtually everyone takes with white oak.”

Greenbar’s experimenting was innovative but not wholly unique. Other distillers both large and small have been dabbling with nontraditional wood aging, often involving varied forms of oak. Buffalo Trace in Kentucky released its Old Charter Oak Mongolian Oak Bourbon, aged from ten years in barrels sourced from Mongolia. Louisville-based craft distiller Copper & Kings has been aging brandy in “zebra barrels”—made with alternating staves of used bourbon barrels and new American oak, with cask heads from used chardonnay barrels. Makers Mark 46 inserts a rack of toasted French oak staves into its finished bourbon. (Bourbon by law must be aged in new oak barrels, but the law doesn’t restrict the type of oak or where it’s grown.)

Other producers have been journeying outside the oak forest, if a bit tentatively. Copper Fox in Virginia has experimented with a sort of tea-bag consisting of oak and applewood chips that steep inside a bourbon barrel. Woodford Reserve included maple casks as part of its Master’s Collection Four Wood Bourbon released several years ago. (The other woods were white oak, along with used oak casks that once held port and sherry.)

But other producers are embracing alternative woods in full. Michael Myers, founder of Distillery 291 in Colorado Springs, wanted to add something to his whiskey that was uniquely Coloradan. He started experimenting with the toasted wood from one of the state’s iconic trees, the aspen. “I know there are traditions and stuff,” Myers said, “but I don’t pay attention to them very much.”

He initially began to filter his spirit through aspen charcoal, similar to the filtering of Tennessee whiskey through maple charcoal. That worked nicely, but when he started looking deeper at aging, he wondered if longer exposure to aspen staves would further improve his whiskey. He couldn’t make barrels out of aspen—the cell structure would mean rapidly emptying containers—but he could insert aspen staves inside traditional casks.

Myers ages his whiskey for about a year in small oak barrels (ten to 30 gallons), then finishes each batch for two to four weeks with three toasted aspen staves, each about 10 inches long. (A friend cuts the wood for him; he toasts and chars them on a Weber grill.) For a small-barrel whiskey, it tastes more mature than might be expected. Myers stays that’s in part due to aggressive cuts in distillation, which can temper the young whiskey flavor. But he suspects the wood also plays a part. “The aspen may clean up some of that graininess,” he said.

In Brazil, two producers of barrel-aged cachaça are taking terroir to the next level by producing spirits aged in indigenous woods. Larger vats there have been traditionally made of the tight-grained local woods, but using casks to add flavor hasn’t been much seen until recently. Avuá Cachaça rolled out its Amburana Cachaça, aged up to two years in amburana casks (also known as Brazilian teak), which gives it a rich, an almost savory edge, offsetting the natural sweetness of the cachaça.

Another distiller, Novo Fogo, has released three aged expressions of cachaça using Brazilian woods to finish its spirit, which is first aged for a year in used bourbon barrels. They’ve also released an amburana expression (finished for an additional year in 250-liter amburana casks), as well as a Brazilian nut wood spirit (aged two years in 250-liter casks), and a flavorful zebra wood variation, aged in a 350-liter cask. The latter is a dense, distinctively striped wood full of resin and flavor, and the spirit is finished for three months.

The Brazilian woods add a layer of flavor that is at once familiar and unfamiliar. The dense zebra wood adds curious bitterness to the cachaça. The nut wood brings creamy, baking spice notes and a touch of something that hints of menthol. And the amburana adds deeper vanilla notes than typically achieved with ex-bourbon barrels.

Production of these three expressions has been limited—Novo Fogo currently has a total of just seven finishing barrels—owing in part to the scarcity of the wood. Novo Fogo doesn’t want to further stress endangered hardwoods; while the teak and nut wood are available in limited quantities from sustainable sources, the zebra wood remains all but impossible to source ethically. The one cask they had made was of reclaimed flooring from a building remodeling. (They’re scouting for another rebuilding project for wood to make a second barrel.) “At this point, we don’t have a sustainable solutions,” admitted Luke McKinley, Novo Fogo’s marketing director.

Back in Los Angeles, the woods used in Greenbar’s Slow Hand Six Wood Whiskey are widely available. In addition to American white oak, Khosrovian uses hickory, maple, mulberry, red oak and grape (from vines) to infuse flavor. Because most of these woods are poorly suited to making casks, he employs planks—up to two feet long, and about two or three inches thick, which are toasted before they’re used. (He experimented with wood chips, but found the flavor “duller.”)

The planks are loaded to a height of several feet in large stainless steel vats—Greenbar has one holding 1,000 gallons and another of 2,000 gallons—which is topped off with his all-barley malt whiskey. The wood floats for about nine months, he said, and then grows spirit-logged and sinks. “That’s when we get access to the full flavor,” Khosrovian noted. Total aging is just under three years.

“It’s been an experiment, and the reaction has been all over the place,” Khosrovian admitted. Those new to whiskey are curious about the new flavors his approach yields, “but among the whiskey aficionados, it’s a harder sell. Some people think it’s heresy.”

He’s hopeful that even traditionalists will come around, lured by Slow Hand’s unique flavor profile—its tastes like bourbon on entry, but finishes like Scotch, making it a sort of intriguing Pegasus of the spirit world.

“The flavors aren’t weird—they come from trees,” he said. “But they’re flavors that no one has ever tasted.”

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