Everything you've ever wanted to know about barrel-aged beer

It used to be that the only age bar owners were concerned with was 21, but nowadays every serious craft beer bar has at least one brew that's spent time in a wooden barrel before it's old enough to drink.

To better understand how barrel-aging works, we consulted a pair of experts: the trend-setting Scottish brewers Innis & Gunn, who've been sneaking oak flavors into beers since 2003, and Austin, TX's young-gun experimentalists Jester King, who're known for using wood to make their fermentation go wild.

charring barrels
Innis & Gunn

Why barrel age?
Innis & Gunn ages primarily to give their beer the flavor characteristics of wood, whereas Jester King uses the wood to affect the fermentation process. Hence, I&G chars their barrels heavily to allow the flavors to easily seep into the beer, while Jester King wants barrels to be as neutral as possible.

What flavors result from aging?
Oak's most prominent flavor is vanilla, but the barrel's previous contents have a big effect on the sloppy-second flavor characteristics. Bourbon barrels give off a toffee finish, gin has a more complex suite of botanicals, mezcal imparts more smoke and spice, and cedar spirals lend a piney aroma.

beer barrels
Innis & Gunn

What type of wood do they use?
The most typical barrel used for aging is American oak, which imparts flavor much faster than its French oak counterpart. Innis & Gunn has experimented with everything from Irish whiskey-infused oak (for the Irish Whiskey Cask Stout), to Canadian Black Cherrywood.

Jester King primarily uses oak from a range of sources (mezcal barrels for Encendía farmhouse ale, Old Tom gin barrels for Viking Metal), but have also experimented with sherry barrels and cedar spirals. Other breweries, like Russian River, experiment with wine barrels ranging from Chardonnay to Pinot Noir.

jester king barrel foam
Jester King

How long must the beer age?
"You see flavor changes in a matter of hours," says Dougal Sharp, head brewer at Innis & Gunn. "But there are lots of variables that have an impact on the speed in which this chemical reaction takes place." Innis & Gunn uses charred casks to bring out vanilla and piney notes and for their Toasted Oak IPA they've built a special oak chip percolator in which the beer ages for 41 days. Jester King, on the other hand, usually leaves its beers in wood for several months, not for the flavor characteristics, but primarily for each vessel's unique fermentation properties. Every barrel is a funky ecosystem of bacteria fueled by an extra dose of oxygen let in by the porous wood, creating unpredictably delicious results.

What the hell is going on in that photo?
One of Jester King's secret weapons is spontaneous fermentation. They let the wort (the sugary liquid remains of the mash) sit out overnight, and by the morning, the yeast and bacteria in the air have made sweet love to the wort, priming it to be moved to a barrel for fermentation and maturation. Once it's in the barrel, it oozes a ton of krausen, which is basically a nascent form of the same foam you see at the top of a pint.

wooden barrel pyramid
Innis & Gunn

What types of beer respond best to aging?
Lighter-hopped beers, porters, and stouts respond best to aging. Generally, hops and wood don't get along well -- the citrus and vanilla flavors clash worse than plaid and polka dots, so it's a real trick that I&G have managed a Toasted Oak IPA that doesn't taste like a rotten Creamsicle. Jester King, on the other hand, barrels mostly sours, farmhouse ales, and wild yeast: beers whose flavors are largely derived from the unique fermentation variables.

Is the aging process complete once it's bottled/kegged?
Not necessarily. When bottled, the wood-aged effects of I&G's beers subside for about a month before reasserting themselves. Since Jester King's beers aren't filtered or pasteurized, they're still alive in the bottle and will slowly evolve over time. And of course the whole process isn't truly complete until the beer is in your stomach.

Dan Gentile is a staff writer on Thrillist's national food and drink team. He used to be pretty loose with the "wood" jokes, but over time his writing has become much more mature and slightly stiffer. But not that much stiffer. Get it? Follow him to more zingers at @Dannosphere.

Innis & Gunn