How Do Barrels Affect the Taste of Whiskey?

A common refrain in the spirits world is that whiskey gets 70–80% percent of its flavor from the barrel it’s aged in. Though nearly impossible to prove the exact figure, one thing is certain: barrel maturation is an integral part of whiskey production. 

Barrel aging is what gives whiskey its color and the caramel, vanilla, and spice notes that the spirit is often known for. 

Whiskey producers use a variety of barrels, most of which have been previously used to mature other spirits, wines, and sometimes even beer. Every type of barrel imparts different aromas and flavors to the whiskey.

Everything about a barrel will influence the whiskey that’s aged within it. What it was previously filled with, the quality of the oak, and the degree of char all contribute to the whiskey’s flavor. 

Used barrels generally maintain traces of the original liquid that has soaked into its wood. When another spirit is aged in that barrel, it interacts with the wood and extracts some of the flavors and aromas trapped deep within the barrel’s staves. 

“Used casks are fantastic for whiskey makers in several ways. If the aim is to produce a whiskey with muted wood notes then used casks are essential, plus there’s the added benefit of picking up some influence of what was previously in it,” says Miles Munroe, master blender at Portland, Oregon’s Westward Whiskey. 

This applies to all manner of whiskey, from rye and bourbon to more highly regulated styles like Scotch whisky, which use barrel finishes to impart flavors in a way that doesn’t violate strict production codes.

“Barrels which have previously held liquid will add a myriad of different flavors to the spirit,” says Adam Hannett, head distiller at Bruichladdich Distillery, who produce whiskey on Scotland’s isle of Islay. “For example, sherry casks are typically quite strong and dominant in terms of flavor, with the wood imparting rich dried fruit and nutty notes. In contrast, ex-brandy casks tend to have a much more delicate influence, with gentle oak and floral notes.”

Essentially, previously used barrels give whiskey producers the ability to manipulate the flavor of a whiskey while staying within the set rules that each region imposes on producers.

“For me, it’s great to work with a wide variety of casks to really explore the depth of flavors which pull from the oak when combined with the spirit,” says Hannett. “It also challenges and excites us as whisky makers as we never know for absolute certainty how the spirit will take to the wood.”

The types of used barrels in whiskey finishes

Although many kinds of barrels have been used to fully or partially age whiskey, there are a few specific types that are used more often than others. 

Ex-bourbon barrels are the most commonly used barrels in whiskey production. In the U.S., bourbon production is highly regulated. 

“Bourbon is required to be aged in new charred oak. This legislation defines the flavor profile,” says Dan Callaway, vice president of new product development and blender at Bardstown Bourbon Company and Green River Distillery, located in Bardstown and Owensboro, Kentucky, respectively. “There’s a greater concentration of wood sugars and compounds in a first-use barrel. As the liquid is pushed in and out of the wood over time, it absorbs the caramel, vanilla, and baking spice that we love in our spirit.”

Because these barrels can only be used once, they are then shipped to other distilleries where there are no restrictions on reusing barrels. Due to the ease of availability, most whiskey producers in Ireland and Scotland age the majority of their new-make spirits in ex-bourbon barrels for most of the aging process. 

Though the ex-bourbon barrels may have been previously used, the barrels still contribute desirable flavors and aromas. 

“White oak from the U.S. provides rich vanilla and honey notes, along with coconut and baking spice,” says Munroe. “These casks are sent all over the world for whiskey, rum, tequila, and many other styles so these are very common to find as finishing barrels.” 

Sherry casks are also commonly used to age whiskey, especially in Scotland. There are a handful of different styles of sherry, a type of fortified wine made in Spain, and each is defined by differences in the production process. Some scotch distilleries, like The Glendronach and The Macallan, rely heavily on ex-sherry casks, while many others use these barrels to finish the whiskey.

Compared to bourbon barrels, the casks used to age sherry are different in size and shape and are made from a different species of oak wood. 

“Sherry producers use French oak which is by far the most commonly used cask, [imparting] flavors [of] raisin, nuts, citrus, and chocolate hints,” says Munroe. 

French oak tends to impart more savory and spicy flavors, along with a dark chocolate note which many winemakers find to be useful.

Pushing the boundaries

In recent years, creative producers have begun to look to different types of used barrels to create exciting and unique whiskeys.

Within the past year, ex-rum barrels have been used by many producers to impart sweet, tropical fruit notes and in certain cases, a bit of pot-still funkiness as well.

In 2023, Wild Turkey distillery took a 10-year-old bourbon and finished it in casks used by Appleton Estate to age a pot still rum for 14 years. The result was the critically acclaimed and limited Master’s Keep Voyage bottling.

Different types of wine barrels are also being utilized by producers. 

“One of my favorites has been Sauternes casks which match so well with Westward,” says Munroe. “It’s a late harvest wine that adds honeysuckle, pineapple, cedar, and tobacco notes to the whiskey, along with an elegant mouthfeel.” 

Westward has also experimented with locally produced Tempranillo barrels. “Big notes of orchard fruits, herbaceous sweetness, and juicy red wine have made this release an amazing expression of Westward,” says Munroe.

Through their Collaborative Series of whiskeys, Bardstown Bourbon Company crafts a variety of unique barrel-finished whiskeys. 

“[Our] Collaborative Series is one of my favorite expressions in spirits. We partner with another wine, beer, or spirit producer to create a one-of-a-kind profile,” says Callaway. “We are explorers at heart and barrel finishing is an opportunity to find new flavor.”

Bardstown Bourbon Company has experimented with everything from ex-Foursquare rum casks to barrels that once held Amaro Nonino. Their most recent Collaborative Series release is a blended whiskey partially aged in Amrut whisky barrels from Bangalore, India.

“Amrut’s barley is grown at the foot of the Himalayas and imparts beautiful ripe fruit and malt notes,” says Callaway. “We created a custom rye blend to age in the Amrut barrels for 18 months, then surrounded this concentrated flavor with 10–14-year Kentucky bourbon. It’s a three-way balance between malt, rye, and bourbon.”

Both Westward and Bardstown Bourbon Company have also experimented with whiskeys finished in casks previously used for barrel-aged beer. 

“Another knockout for us has been beer casks like stouts, which elevate the roasty, chocolate flavors in the single malt and bring great toffee notes to the forefront,” says Munroe.

Bardstown Bourbon Company collaborated with Chicago brewery Goose Island, who famously age their annual Bourbon County Stouts in ex-bourbon barrels. 

“We really captured the essence of premium stout with this release,” says Callaway. “The authenticity of the malt, honey, and chocolate stout flavor comes from the residual liquid left in the barrel after emptying.”

As whiskey producers continue to experiment with unique used barrels, the potential for collaborations across the spirits, wine, and beer industry grows. 

“With so many casks traveling around the world, they’re part of the community of makers and part of how we communicate with each other and the folks who enjoy our offerings,” says Munroe. 

As this industry-wide barrel collaboration proliferates, more satisfying and unique whiskey is sure to find its way to liquor store shelves.