DENVER

A CREATURE is lurking here in Chad Yakobson’s warehouse, inside the oak barrels where he ages most of his beers. Its name is Brettanomyces, and it’s a cousin of the domesticated yeasts that humans have brewed with for thousands of years. Often called wild yeast — a reference to its natural habitat (fruit skins) and to its volatile temperament — “Brett,” as it is widely known, can lead to unpredictable fermentations and gushing beer bottles, aromas politely described as funky, and fear. Most brewers work hard to keep it out of their tanks by sterilizing every piece of equipment.

But Mr. Yakobson, 28, who studied winemaking before founding his brewery, Crooked Stave Artisan Beer Project, in 2010, treats Brett like an old friend, pointing out that it was common in beer before the advent of modern sanitation in the late 19th century. He prizes the hints of tropical fruit, earthiness, spiciness and, yes, funkiness that it lends to his beers, which include an herbal India pale ale, Belgian-inspired saisons and sour ales reminiscent of Belgian lambics.

“It’s an entire new category of beers,” often called wild or Brett beers, Mr. Yakobson said one afternoon as he stood among old wine and spirits casks and elephant-size barrels called foudres.

Bitter or mild, light or dark, acidic or barely tart, and frequently barrel-aged, these ales all share winelike nuances that most other craft beers lack. Although they comprise only a sliver of the beer market and challenge many drinkers’ ideas of what beer should taste like, they have nonetheless captured the imaginations of a growing number of brewers and aficionados. They also show off the sense of artisanship and depth of flavor that increasingly define American craft beer.