Barrel-aging 101: How Colorado brewers use wood to make unique beers

Matt Malloy will never forget the first time he had to throw out a batch of beer. The recipe, a barleywine aged in Barbados Rum barrels, picked up musty notes of decomposing wood that didn’t compliment the base beer, he said. While dumping beers is often a necessity for brewers who dabble in barrel-aging, it hardly ever gets easier.

“It definitely is hard watching all that time, effort, and cost, just go out the window,” said Malloy, head brewer at Denver’s River North Brewery. “It breaks me, but it’s something that has to be done to make sure we’re known for high quality.”

As far back as the Middle Ages, societies used barrels to ship beer and other “wet” goods such as wine and olive oil. In those days, the barrel was simply a transportation vessel and often produced in a way so that the wood did not affect the flavor. Starting in at least the 1800s, brewers even actively tried to avoid wood flavor in their recipes.

In the modern craft beer era, however, brewers reclaimed the barrel as an ingredient, leveraging the container for its unique flavor characteristics and relying on it to develop complexity through aging, much like winemakers. Today, drinkers can find an array of specialty beers aged in barrels previously home to whiskey, rum, rye, tequila and wine.

The one thing they all have in common: time.

Loveland’s Verboten Brewing and Barrel Project is so named for its lack of adherence to Reinheitsgebot, Germany’s 500-year-old purity law that dictates beer be made exclusively with water, barley and hops, said co-owner and head brewer Josh Grenz. Verboten means “forbidden” in German, and Grenz considers wood among the list of prohibited additives.

Grenz will age beers, primarily stouts and barleywines, anywhere from nine to 18 months, depending on the style and specific recipe.

Matt Hess, founder of River North Brewery, said he won’t try a stout until at least eight months after it’s been in a barrel. The brewery just released its oldest beer ever: An imperial stout called XXO Avarice, which was aged for three years in Colorado bourbon barrels.

Because wood is a porous material, barrels allow small amounts of oxygen to seep in and continually feed the yeast — a process known as micro-oxidation — which enables the beer to evolve over time. The longer the beer sits in a barrel, the more it also absorbs the flavors of the wood and whatever else previously resided inside.

Bourbon barrels are among the most popular beer-aging vessels since distillers require brand-new oak, using a barrel once before tossing it. Additionally, as the spirit’s warm, pungent characters meld well with bold styles like porters, stouts and barleywines.

“The marriage between beer and spirit in the barrel and the oak flavors, that you can’t replicate,” Grenz said. “Any alcohol leftover in the barrel – meaning it’s still wet from whatever spirit – is protecting barrels from bacteria. If there’s an ounce or two left in the bottom, that’s a good sign the barrel is fresh and protected.”

Brewers say freshness is key when they are looking to showcase woody flavors in their beers. Oak, specifically, is known for lending spicy, caramelized and vanilla notes depending on its level of char. And the plethora of craft distilleries in Colorado means it’s easy to find fresh barrels in here, Malloy said.

Verboten Brewing, for example, sources the vast majority of its barrels locally from places like The Family Jones in Denver, Grenz said.

Not all brewers seek out fresh barrels, however. Troy Casey, owner of Casey Brewing and Blending in Glenwood Springs, covets vintage French oak wine barrels from California that have been used to the point they have little, if any, residual flavor.

Since its founding in 2014, Casey has specialized in barrel-aged sours. And while he goes through the same process as the aforementioned breweries, his goal is decidedly different. Casey wants micro-oxidation to coax out esters and acidity from various types of yeast, such as Brettanomyces, which produces funky, earthy and sometimes “barnyard”-like flavors.

The same recipe even varies barrel to barrel depending on the thickness of the wood and the ambient cellar temperature during fermentation, Casey said.

“It’s like magic. You can have the same base beer in two different barrels, but get completely different results,” he said.

Despite their different intentions, all brewers dabbling in barrel aging must account for certain factors, like the local climate, and the perceived risks. Colorado’s dryness means more water evaporates as beer ages, a phenomenon known as the angel’s share. Not only does that reduce volume, but it also increases alcohol content, potentially affecting the taste.

There’s also good reason why brewers typically barrel-age stronger styles instead of lighter ones. “Intensely flavored beers hold up to intense characters you get out of whiskey barrels,” said Hess.

To prep a barrel, brewers purge it of excess oxygen, but they don’t often deeply sanitize to retain the integrity of the wood. With that comes the risk of unwanted bacteria that can contaminate the barrel and spoil the beer. Too much oxygen can also ruin a beer, giving it a cardboard-like taste. But brewers won’t know if something’s gone awry until post-fermentation, often many months later.