A few miles southwest of the Berkeley campus, in an area commonly referred to by locals as “The Flats”, a chain-link fence gives way to The Rare Barrel, one of the few breweries in America that love it when their beer goes sour.

“We like to joke that sour beer is the oldest style of beer in the world,” said Alex Wallash, cofounder of The Rare Barrel. “Brewers five thousand years ago didn’t know about microbiology, so their beer would automatically go sour if they didn’t drink it fast enough.”

Sour beer is an acquired taste, with a flavor profile that blurs the line between beer, wine, and cider. As Alex describes them, they’re tart like a handful of raspberries, acidic like a glass of wine and crisp like a flute of Champagne.

The style makes up such a small segment of the craft beer pie that it doesn’t even get listed when the Brewer’s Association collects information on beer styles. But as beer becomes increasingly popular, and drinkers demand more interesting and varied offerings, that’s changing. Technical seminars on sour brewing at the Craft Brewer’s Conference in Portland, Oregon have been among the most well attended, and at Boston’s Extreme Beer Festival, which has become something of an incubator for new brewing trends, lines for sours stretched across the room.

It’s also telling that despite the myriad oak barrels stacked to the ceiling of The Rare Barrel’s expansive warehouse and taproom, Alex and his cofounder, Jay Goodwin, still only have enough beer to open their tasting room three days a week. Bottle releases sell out on the same day; when they opened their online store to the Ambassadors of Sour, their opt-in fan club, they received such a flood of interest that their servers crashed. Twice.

Several popular breweries, including Sonoma-based Russian River, makers of the famed Pliny the Elder and Pliny the Younger, have sour programs, though few have gone all sour. That’s what makes The Rare Barrel so unique.

“We get to focus all our resources and time on making better sour beer,” Alex said. “It’s going to be better than if we try to master 40 different styles.”

Brewers five thousand years ago didn’t know about microbiology, so their beer would automatically go sour if they didn’t drink it fast enough.

To make a sour beer, most brewers create a grain tea called wort — essentially cracked grain steeped in water — and set it out to catch floating bacteria and yeast: Lactobacillus, Pediococcus, Brettanomyces. Recognize those names? They’re the same bacterial strains found in your yogurt and miso. This is one way that sour beer making differs from making other types of beer: in IPAs, for instance, you want to keep Lactobacillus, Pediococcus, and Brettanomyces out. You do this by adding an anti-microbial — which is where hops come into play, as hops kill bacteria and yeast. In his beers, Jay only uses about five pounds of hops for every thousand gallons; an equivalent amount of 3 Floyds’ Dreadnaught, a double IPA, requires almost 72.

In the next step of the brewing process, the sour wort gets aged in oak barrels, where it undergoes fermentation and becomes sour beer. This process can take anywhere from a few months to a few years. After that time, the beer is pulled out of each barrel and blended to achieve a certain flavor profile; for the kegs that feed the Rare Barrel’s draft lines, this means mixing anywhere from one to four barrels, while their bottled offerings combine selections from between 10 to 15 barrels.

The result is, in the case of Rare Barrel, award-winning sour beer. Ensorcelled captured a Gold Medal in 2014’s World Beer Cup, and Cosmic Dust got one at the Great American Beer Festival. And although the Rare Barrel only make sours, they achieve stunning variation, with beers that range from pale yellow to dark red, and flavor profiles that hop from fresh berries (Wise Guise, red sour beer aged with raspberries) to coffee (All Systems Go, dark sour beer aged with Sightglass coffee beans). Instead of simply producing a “golden sour beer”, which is strange enough itself to be on the outer edge of the flavor spectrum for the majority of American breweries, Jay will cook up something like Tigerlily, a golden sour beer aged with ginger and hibiscus, or Soliloquy, a golden sour beer aged with rose hips and orange peels.

“15 or 20 years ago a brewery like this couldn’t have been possible,” Jay said. Perhaps it’s a sign of the changing times, but it might also be a testament to Berkeley’s eclectic tastes. As the Brewer’s Association notes in a recent article analyzing craft beer trends, “It is easy to forget that the many markets across the country are still just beginning their craft journey.” So don’t expect to see people quaffing sours at your next Superbowl party. But in savvy markets like San Francisco, Portland, Seattle and Boston, where drinking is synonymous with craft beer, you might get lucky and find the rare bottle of the Rare Barrel. Just don’t pour it down the drain when it tastes sour.

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