Also called cask-conditioned ale, cask ale is an old-fashioned type of beer hailing from England that has a lot in common with yogurt. It contains live yeast cultures that are not filtered out in the brewing process, like they are in most beers, but are rather left in the keg (called a cask in this case, or sometimes referred to as a firkin). These yeasts create natural carbonation. Most American beers, even micros, are charged with CO₂ to carbonate them. To the uninitiated, cask ale tastes flat, because its bubbles are more delicate than those in beer that’s been pumped full of CO₂. And because, like red wine, it’s supposed to be served at cellar temperature (50 to 55 degrees Fahrenheit) to allow its rich flavors and aroma to fully bloom, it can strike some as too warm. But to its small, rabid fan base, it’s the real McCoy. Hence its nickname, real ale.

“There’s a small amount of vociferous people who won’t drink anything but cask-conditioned beer, and look down their noses at forced-carbonated beer,” says Griz Miller, “head cook and bottle washer” at brewing supply store San Francisco Brewcraft. “And for good reason! If you go to a bar and judge beer by its quality, cask will always be better.”

Steve Dresler, Sierra Nevada’s brewmaster, counts himself a fan. “[Cask ale has] very soft CO₂ with not a lot of bite, so you’re really enjoying the aromas and flavors of the beer. The ester fermentation aroma, the hops come out real nice. Some of my favorite beers I’ve ever had in my life were cask beers.”

Until recently, in this country you could only find naturally carbonated beer in bottles (otherwise known as bottle-conditioned ale) at better liquor stores. If you wanted to try the fresher, more nuanced cask ale, you had to go to a microbrewery that made it and served it at an adjoining pub. That’s because it doesn’t transport well. The live yeast in the casks must settle before the beer can be poured (often breweries add isinglass or gelatin to help the particles clump together and sink to the bottom).

It also requires special know-how and equipment to serve. A typical beer keg is pressurized, so the bartender just pulls the handle of the tap and the beer shoots out. But casks are unpressurized and require a special hand-pumping device to suck the beer out. The process gives servers an upper-body workout. All that, and the fragile beer goes bad after just three days.

“It’s a fussy product,” says Jonathan Tuttle, the U.S. representative in the British consumer rights’ group Campaign for Real Ale (CAMRA). “Over the course of two to three days, [cask ale] will oxidize enough so that it’s not right anymore. But somewhere in that process it will reach a peak of perfection that all drinkers look for, which is why it’s an adventure.”

The last five years, however, have seen more and more Americans seeking out just such an adventure. Attendance has grown at the handful of real ale festivals around the country (see sidebar), more brewpubs are hosting cask ale nights à la Sierra Nevada’s, and slowly bar owners are investing in hand pumps and buying cask ale (like Lanesplitter, a Berkeley pizza parlor and pub, pictured). More than 400 bars, restaurants, and brewpubs now serve cask ale, with over 15 bars in New York City alone carrying it. Ray Deter, co-owner of popular East Village bar d.b.a. (he owns a second d.b.a. in New Orleans), says that even customer complaints about it being “warm” and “flat” are on the wane. “As long as the ale is in good shape, almost everyone loves it,” he says.

Up to now, cask ale has been the labor of love of microbreweries. But the entry of Sierra Nevada into the market indicates that real ale’s day has finally come. Although the Chico craft-beer giant has made cask ale for the last three years, the brew has only been available to the company’s employees and diners at its pub, and only on Thursdays. As any kind of ale, from porter to hops-heavy pale ale, can be made without forced carbonation, each week Sierra features a different beer made in the style. (Brewmaster Dresler had to cut a Thursday afternoon interview with CHOW short in order to tap one.)

Last year, the brewery experimented with sending a cask to a festival in San Diego. It was a risk, particularly for a big outfit like Sierra whose customers expect consistency and a high level of craft.

“We’re new to this,” says Dresler. “We’ve been playing around with this for the last couple of years, but once in a while we’ll go to tap and we’re like, ‘Bummer.’”

But the beer settled well and delighted many. That led to its quiet debut in a bar: Last month Sierra sent a cask to the Blind Tiger Ale House in New York City. It, too, was a hit.

“You’ve heard of Slow Food?” asks Brenden Dobel, master brewer at San Francisco brewpub ThirstyBear Brewing Company. “Cask ale’s like slow beer: It’s all natural, no artificial ingredients, usually local.” Cask ale may have finally found its audience.