On a quiet June afternoon at Philadelphia’s Monk’s Cafe, William Reed, a former Boston Beer Company brewer, popped the top on part of an experimental batch that he’d brewed in 1996. Originally made at the request of Tom Peters, the happy pasha of Monk’s, it was a Flanders red ale called Brewhouse Tart, fermented with a mixture of conventional and wild yeasts, which can wreak sensory havoc. That year, Reed, still green, lucked out, presenting a taste to the late, influential British beer writer Michael Jackson—a frequent guest lecturer at the University of Pennsylvania’s Museum of Archeology and Anthropology—who loved it, immortalizing it in one of his sixteen books. “He was blown away,” Peters said. Reed set aside a single keg, which remained, more or less forgotten, in a cellar for seventeen years, until he opened it last month. The beer, improbably, was in terrific shape, with a brick-like color and tannic, woody, cherry- and port-like flavors. But what was notable was its acidity. Brewhouse Tart tasted like a liquid Sour Patch Kid.

Before the advent of refrigeration and advances in the science of fermentation in the mid-nineteenth century, almost all beer was, to varying degrees, sour. The culprits were pre-modern sanitation and poorly understood, often naturally occurring bacteria including Lactobacillus and Pediococcus, as well as Brettanomyces yeasts, which can contribute a hint of tartness and characteristic “funky” flavors and aromas, sometimes compared to leather, smoke, and “horse blanket.” In a development that would make Pasteur, the father of biogenesis (as well as his method for halting it, pasteurization) roll in his grave, brewers, especially in the United States, have embraced the time-honored Belgian art of deliberately infecting beer with the same “wild” bugs that generations of their predecessors so painstakingly eradicated. The result: pleasingly sour, food-friendly beer, mysteriously complex and engaging.

At brewing conventions these days, the best-attended lectures aren’t about hops. They’re about inoculating wood barrels with wild yeast, with slide shows of oozing bungs and anti-oxidative pellicles. Brewers have put the lessons to use, releasing hundreds of commercial examples of American sour ales. These sometimes have lofty, even sacerdotal names: there’s Allagash’s Resurgam (“I shall rise again”) and Russian River Brewing’s Consecration, for starters. With pH akin to good Pinot Noir, the best make it onto serious menus. The worst taste of nail-polish remover, rotten apple, coconut, or the dreaded “baby diaper.” A consistent product is notoriously tricky to pull off; brewers might be said to guide, rather than master, the beers, hoping for serendipity. Amid all the trial and error, ancient brewing history is repeating, spreading around the nation one foamy, infected-on-purpose barrel at a time: to Billings, Montana; Athens, Ohio; Tampa; L.A.; Brooklyn (of course). Where next?

Commercially available Belgian sour beers first came to the United States in the nineteen-seventies, laying the groundwork for ever-tarter domestic beers. The Cantillon brewery, founded in 1900, in Brussels’s Anderlecht neighborhood, still brews the most uncompromising examples—specializing in lambic, spontaneously fermented sour ale, and gueuze, made of blended, aged lambics. (Other sour Belgian styles, brewed elsewhere, include the red and brown ales of Flanders, such as those made by Rodenbach.) Widely misunderstood at first, Cantillon’s austere, tart, musty character has sometimes led drinkers to declare the beer infected—and to return bottles by the case. In early 1997, when I first visited the brewery, the beers were not available in the United States beyond a few semi-smuggled shipments. “Cantillon seemed crazy at the time,” says Dan Shelton, the beer’s quixotic importer. “It took almost ten years for people to realize that’s what traditional lambic and gueuze is supposed to taste like.”

This beer could scarcely come from a more atmospheric place. Virtually unchanged since the First World War, the brewery is a marvel of weathered beams, steam-powered flywheels, hammered-copper kettles, and a koelschip (often anglicized to “coolship”), an enormous, copper-lined pan in the brewery’s attic, used for chilling wort—unfermented beer. Today, amid the barrels, Jean Van Roy, a forty-five-year-old, square-jawed great-grandson of the Cantillon line, runs the the show. Jean-Pierre, his affable seventy-one-year-old father, credited with saving lambic from death by added sweetener in the nineteen-seventies, works by his side, looking a bit like Max von Sydow.

During the cool winter months in Brussels—brewing takes place only seasonally—each new batch is pumped, steaming, into the koelschip. Then the younger Van Roy opens louvered vents and cranks on a fan, inviting wild yeasts and bacteria to inoculate the beer overnight (there’s plenty stirring in the rafters, too). The next morning, the wort flows downstairs into empty Burgundy barrels, where it will slowly transform for up to four years, and later be blended and re-fermented, sometimes with fruit, to make Cantillon Kriek (cherry), Rosé de Gambrinus (raspberry), and Fou’ Foune (apricot), among other variations. In recent years, Van Roy has also experimented with fermenting beers in terra-cotta amphorae and aging them on rhubarb, elderberries, Cabernet Franc and Merlot grapes from Saint-Émilion and Pomerol, and biodynamically grown Pineau d’Aunis, also known as Chenin Noir, from the Loire Valley.

In the meantime, inspired by Cantillon and other Belgian traditionalists, a motley crew of American rebels is pursuing the art of sour beer. There’s Ron Jeffries, formerly known as “Captain Spooky,” of Michigan’s Jolly Pumpkin, credited with brewing some of the first good, all-wood-fermented American sours in the early two-thousands. California’s Lost Abbey, of San Marcos, and Russian River, of Santa Rosa, have ascended to cult status. Chad Yakobson, founder of the surging Crooked Stave brand in Denver, began his career with a masters on Brettanomyces. Even MillerCoors, through its AC Golden unit, is overseeing a wood-aging program under the watchful eye of an ambitious young brewer, Troy Casey. And in the Oregon wilderness, outside Bend, the brewer Paul Arney, formerly the R. & D. chief of the fifth-largest brewery in the country, Deschutes, has gone off the grid, too, fermenting some of his sour Ale Apothecary line in hollowed-out spruce trees resembling dugout canoes.

Who will lead the movement? Ask a beer geek, especially in the Northeast, and he or she might genuflect to the eighth-generation Vermonter Shaun Hill, who brews his Hill Farmstead beers—including some excellent sours, though they represent a small fraction of his production—on his family’s rambling hundred-acre property, near Greensboro. Hill has attracted an almost crazed fan base; in just two years, the former philosophy major’s operation has rocketed to the No. 1 worldwide ranking on RateBeer.com, although the brewery itself is, at present, a one-barn affair, so small that Hill’s creations are almost impossible to find outside Vermont. With some frequency, visitors get their cars stuck on his property, so he has to fire up the tractor… again. It’s a scenario that the thirty-three-year-old brewer—a self-taught perfectionist, with six years of brewing experience in Denmark, an abiding love of Dylan, and crowds of hundreds, even thousands of enthusiasts on weekends—regards with an air of disbelieving, proud misery. How will he ever keep up?

The demand is incessant, but more or less welcome. The endless attention, however, is less comforting. “I just want to be wanted,” Hill told me on a snowy April night, as we sipped his latest (terrific) Flanders red, Prolegomena, a collaboration with the Boston-area brewer Will Meyers, of Cambridge Brewing Company. He was talking as much of his beers—usually named for his ancestors (i.e., Ephraim) or works of philosophy (Phenomenology of Spirit)—as the wounds of love. What motivates his devotees, in part, might be genetics: some biologists believe that humans evolved to enjoy low-level bacterial sourness to encourage probiotic health. High-proof pucker, on the other hand, can indicate spoilage. According to a study described in Nature, PKD2L1, the sour protein receptor, also resides along the entire length of the spinal cord, possibly monitoring cerebrospinal health. Sour beer lovers sometimes speak of being ruined on conventional beer styles—forever. It must be love. Or is it lightning, bottled? The ions of acidic foods, it turns out, can penetrate the cell walls of our tastebuds, triggering an electrical response, exchanging free radicals, like our skin in the open ocean.

Christian DeBenedetti, the author of The Great American Ale Trail and editor of Weekly Pint, is starting a sour-beer brewery on his family’s hazelnut farm, outside Newberg, Oregon.

Photograph by George Marks/Retrofile/Getty.

View “Mapping the Rise of Craft Beer,” a New Yorker interactive infographic.