In Quercus we trust. There are over 600 species of oak trees in the Quercus genus, to which whisky maturation owes all. Without time in oak barrels, whisky would remain white and fiery, devoid of the toasty, caramel, nutty, or vanilla notes that make our mouths water. It’s simple—without oak, there is no whisky as we know it today. Considering oak’s profound effect on the taste of whisky, it’s not surprising that the details count. The specific type of oak, the origin of the tree, and all the finer points of its treatment during production will influence the flavors the barrel imparts, whether vanilla or citrus zest, baking spices or dark red fruits, or a myriad of other possibilities. To understand whisky, you must understand oak.
Using oak for barrels isn’t exactly an emerging trend. “Oak became a barrel of choice as far back as the Roman empire,” says Chris Morris, master distiller at Brown-Forman, producer of Woodford Reserve and Jack Daniel’s whiskeys.
Oak is the ideal choice for several signature traits—strength and durability, its liquid-tightness, and suitability to coopering. “The presence of a large volume of medullary rays in the wood structure contribute to this extra strength,” explains Kevin O’Gorman, Midleton’s master of maturation. “The cells of white oak contain tyloses, which are outgrowths on parenchyma cells of the tree’s xylem. These cells dam up the vascular tissue, and it is these clogged pores that prevent an oak cask from leaking.” In other words, not just any tree species can become a proper whiskey barrel.
Technology may be available for simpler and cheaper storage vessels for whisky, yet it would be a fool’s errand, as there remains no substitute for oak’s flavor—derived not only from the oak itself but also from its treatment. For Morris, the three manipulatives of stave and barrel creation that impact resulting flavors include, “How long do you dry the wood outdoors (seasoning), how long do you toast, and how long do you char?”
The science of this is better understood today than ever before. “When you char a barrel, you break down hemicellulose into its ten constituent sugars, which caramelize in a very tight layer underneath the char in existing, unburned wood,” explains Morris. “And then, as heat radiates and cools, you get the breakdown of lignin into vanillin, and tannic acid begins to turn into the color red. So the heat passes through the barrel and you get different effects.”
Toasting offers a different impact. “If you pre-toast the wood, you can better control your vanillin and your color,” says Morris. “You can only char a barrel so long until it burns up. With toasting, the wood is not at risk. You can toast long, short, whatever you want; deep-set your vanillas, deep-set your color, knowing you’re going to burn some of that away, but you’re going to have plenty as a result of the pre-toast.” The combinations are endless.
The Quercus Family Tree
Quercus alba, also known as white oak or American oak, dominates the whiskey industry thanks to the mandated use of new charred oak barrels for bourbon aging. Contrary to popular belief, there is no legal mandate to use white oak, however its abundance has made it the de facto choice. After one use, these barrels then find a second life maturing scotch, other whiskies, or even rum, brandy, or tequila. The next most common variety is Quercus robur, also known as pedunculate oak or European oak. It’s prominent thanks to its use in Spain’s sherry industry, and therefore the sherry casks which are highly valued for maturing scotch, and sometimes hold other whiskies for a brief but flavorful stay in a process called barrel finishing.
“Quercus robur is very porous as a species and it’s slower growing than its American counterpart Quercus alba,” explains Stuart MacPherson, Macallan’s master of wood. “The spiciness, the dried fruits, are the different flavor characteristics you associate more with Quercus robur.”
Macallan sources slow growing, 100-125 year old Quercus robur from northern Spain. Slow growth tends to produce a tighter or more narrow grain, with more rings per inch. “Take the flip side [with] Quercus alba; it’s generally cut a lot younger, and it’s a much faster growing oak,” says MacPherson. Faster growth leads to a wider grain, with fewer rings per inch, yet increased density, thanks to more cellulose-rich summer wood comparative to spring wood.
For the new Double Cask 12 year old, Macallan uses first-fill sherry casks constructed from both species. “With the influence of the American oak, you start to get lighter colors, more sweetness, the vanilla flavors,” says MacPherson. This serves as a counterpoint to the Spanish oak influence, which can be experienced on its own in the standard Macallan 12 year old, officially part of their Sherry Oak series.
Not all American oak is Quercus alba, either. Enter Quercus garryana, or Oregon oak, a species being deployed with whiskey for the first time by Westland Distillery. “It’s kind of been a forgotten oak from an industry standpoint,” says master distiller Matt Hofmann.
“Since Quercus garryana has much higher tannin levels than Quercus alba it requires a longer seasoning period to ensure the bitter and astringent qualities of tannins don’t over-influence the whiskey,” says Hofmann. The longer the staves are seasoned, the more those tannins break down. Conveniently enough, when Westland came across an unwanted stockpile of garryana from Hardwood Components, a Salem, Oregon mill, the wood had already been left seasoning for between three and seven years.
As for garryana’s flavors, “it’s like everything in American oak, but darker in flavor,” offers Hofmann. “So instead of caramel, it’s molasses. Instead of baking spice, it’s like heavy cloves. There’s blackberry jam, there’s actual smoke in there. It almost has a fruity, Kansas City-style barbecue sauce feel to it.”
Another lesser seen species is Quercus petraea, or sessile oak. French Limousin oak, most commonly used for wine or brandy casks, including Cognac, is Quercus petraea sourced from the Limousin forest. Brenne puts virgin Limousin oak barrels to use, as well as used Cognac barrels, for their French single malt whisky, with Limousin’s wide grain allowing for easier passage of the spirit into the oak and more influence from tannins.
The World’s Most Elusive Oak
Heading to Japan, the type of oak that has been literally and figuratively on the tip of many drinker’s tongues as of late is Quercus mongolica, Mongolian oak, or as whisky lovers know it, mizunara oak.
“Mizunara is unique in that it takes from 200 up to 500 years to grow mizunara trees for cask making,” says Shinji Fukuyo, Suntory’s chief blender. That’s not the only challenge with mizunara, either. “It is so porous that it is said to be not suitable for cask making,” says Fukuyo.
To counter that, Suntory sources straight trees a minimum 70 centimeters (about 27 inches) in diameter, which are ideal for mizunara cask making, according to Fukuyo. Finding trees that match all of these qualities is difficult. “In Japan, mountains are normally steep and there is less flat, wide area, so it is difficult to grow mizunara trees straight,” he says.
Suntory restarted their mizunara cask program in 2000 after a 40-year hiatus, during which they only reused existing casks. Even with new cask production, mizunara still accounts for less than one percent of Suntory’s inventory, and a mere 150 to 200 new mizunara casks are produced each year at their cooperage.
Given its rarity and expense, mizunara oak is very much a signature of Japanese whisky. Fukuyo describes its influence as “distinctively Japanese,” with a long, spicy finish, and flavor notes such as sandalwood, spice, and incense.
Mizunara casks are in such short supply that Suntory doesn’t even have enough to use for their full range of expressions, with the barrels entirely absent from the Hakushu distillery and its single malts. “We wish we could use them here,” says Mike Miyamoto, currently Suntory’s global ambassador and a former distillery manager at Yamazaki. There’s just not enough to go around. “We can’t afford to give mizunara to Hakushu,” he says. “Yet without mizunara character, Hakushu can survive.”
That’s not the case for the pinnacle of Suntory’s portfolio, the blended Hibiki lineup. “Without mizunara, Hibiki wouldn’t be Hibiki,” says Miyamoto. Mizunara is also important for signature Yamazaki releases. “This subtlety works really well with Yamazaki 12,” he says. “The distinctive oriental aroma does a great job and makes a big difference.”
Distinctively Japanese as mizunara may be, Suntory isn’t the only producer working with the oak. Consider Scotland’s Bowmore, which used mizunara casks to create Bowmore Mizunara Cask Finish. “Mizunara oak unlocks Bowmore’s exotic traits, accentuating the complexity of ripe tropical fruit, fresh balsam, silky sweetness, and fragrant smoke,” says Rachel Barrie, Bowmore’s former master blender. (Barrie left Bowmore in February 2017 to become master distiller for GlenDronach, BenRiach, and Glenglassaugh.)
Bowmore began a series of experiments with virgin mizunara casks and sampled progress every three months. First Bowmore’s peat and iodine dissipated, and then flavors such as coconut and tropical fruits emerged. Eventually came oriental incense, sandalwood, and aromatic smoke. “A pinnacle was reached [after three years] when I sensed the scent from an oriental shrine becoming one with Bowmore No. 1 Vault,” says Barrie, referring to what’s touted as the oldest warehouse in Scotland, dating to 1779.
Oak Tree Terroir
Even the same species of oak can showcase divergent characteristics. One factor is region of origin, and Morris identifies three locales from which Brown-Forman sources oak. “We bring wood in from the three major terroirs—the Appalachians, the Ozarks, and the northern forests,” he says, invoking that French word for sense of place familiar to every wine lover. “You see some terroir specificities and differences because of growing conditions.”
Brown-Forman’s three levels of wood control—seasoning, charring, and toasting—exclude what Morris refers to as a fourth, terroir specificity. They don’t manage what wood goes into which barrel, focusing instead on blending the varied characteristics.
“Obviously oak from the tip of the Cumberlands, say in northern Georgia, Alabama, southern Tennessee, is grown in a warmer climate than oak from up in Minnesota, Wisconsin, where it’s a colder climate,” says Morris. “Not as much sunshine over the course of the year, so you see growth rings being tighter or broader in that regard. Then you have different soil conditions and rainfall, which have a pattern of tree development.”
Buffalo Trace examined oak variables in even finer detail, exploring individual tree variances as well as a myriad range of maturation variables—seasoning, char, portion of the tree, and more—in the extensive Single Oak Project. The winning Single Oak bourbon emerged after tallying over 5,000 reviews across 192 expressions, and will become an annual release starting in 2025. The barrel itself was constructed from the bottom half of the tree, with a #4 char level, average grain size, and 12 months of stave seasoning.
Another example of intra-species differences can be seen with Midleton Dair Ghaelach, which was aged in Quercus robur casks sourced from Ireland rather than Spain. “Irish oak shows lower density and higher porosity compared to Spanish and American oak, which leads to a more open structure and allows more compounds to be extracted into the spirit and at a faster rate,” says Midleton’s O’Gorman.
“Irish oak contains higher levels of some lignin derivative compounds, such as vanillin, vanillic acid, and furfural,” says O’Gorman. “These compounds further enhance the whiskey with vanilla, caramel, and chocolate flavors, which are detectable on the nose of Midleton Dair Ghaelach, and perfectly balance the classically rich, spicy single pot still taste profile.”
Oak’s contribution to maturation is commonly reduced to a single number on a label: how many years the whisky spent in the barrel. Yet lost in the mess of age statements or the increasing lack of them is that the species of oak, its region of origin, the nuances of its treatment including seasoning, toasting, and charring, and even the specific tree itself all contribute to the beauty and diversity of whiskies currently being bottled up and poured down.