Despite the intimidating appellation, spontaneous fermentation is nothing new to anyone who’s ever been to a frat house. Remember that cup of orange juice Brother Chad left out in the living room that, weeks later, bubbled into a mimosa? That was because microscopic yeast and bacteria — more casually referred to as “bugs” — had settled on the open liquid, consuming sugar, farting out CO2 and alcohol and creating carbonated booze.

Though alcoholic beverages have been made this way for centuries, “wild brewing” is a relatively new process in the States, due to lack of control and more chance for complications compared with modern techniques. This is also part of its appeal. Vinnie Cilurzo, brewmaster and co-owner of Russian River Brewing Co. in Santa Rosa, California, had been experimenting (unsuccessfully) with the process since 2005. But it wasn’t until the following year, after a life-changing trip to Belgium with fellow brewers Sam Calagione of Dogfish Head Brewery, Rob Tod of Allagash Brewing, Adam Avery of Avery Brewing and Tommy Arthur of the Lost Abbey, that he “really got serious about it,” he says.

During this excursion, Cilurzo and company spent time with Jean-Pierre Van Roy, brewmaster at the legendary Brasserie Cantillon in Brussels. Van Roy and his family have been spontaneously fermenting beer the very same way since 1900, producing some of the world’s most sought-after unblended lambics (naturally fermented sour beers) and gueuzes (blends of one-, two-, and three-year-old lambics). “What you’re seeing in America,” Cilurzo explains, “all the spontaneously fermentation that’s going on — I really think you’ve gotta give Jean Van Roy credit for inspiring and encouraging us to do it. It would have been really easy for him to not say anything, but he has that craft beer industry spirit. When someone calls, he answers.”

Cilurzo wasn’t the only brewer inspired on that voyage. In 2008, Allagash Brewing of Portland, Maine, became the first to utilize a coolship, a large pan used for open-air “wild” fermentation. As Allagash brewmaster Jason Perkins recalls, shortly after the trip, “[owner] Rob Todd came in, and basically said, ‘Fuck it, let’s build a coolship. And let’s do it right.'” The brewery had been emulating old-world Belgian brewing techniques for decades, but had never taken on an experiment like this. Perkins says of Allagash’s renowned witbier, “When Allagash White came out in 1995, no one [in the U.S.] was making those beers at all. It was very innovative. So we were always thinking about the next step, the next thing we could do. We loved lambic-style beers, and that tradition of brewing.” And so the brewery doubled down and began spontaneously fermenting.

It would be years before Allagash started blending any of the lambics for consumption. Once they began selling very limited quantities, in 2011, they still had much to learn about managing the carbonation and alcohol levels of wild fermented beer, and even more to learn about blending, an art form in itself. As Cantillon’s Van Roy says, “Blending is a lot more important than the brew.”

So with a fantastic portfolio of popular beers, why would any successful American brewery go through all the trouble of spontaneous fermentation? For one thing, the process provides a unique flavor profile. “Spontaneously fermented beer is more complex and delicate. It’s a bit like wine or cider,” Van Roy explains. The natural microbial activity lends a special quality that can’t be duplicated by inoculating brew with commercial yeast.

Additionally, the spontaneous process itself — left up to nature every time — is inimitably romantic. “There’s just a traditional component to it that you can’t replace,” says Perkins. “There’s something really magical about that.” Cilurzo agrees: “I just love the idea that the beer is in control. Sometimes it takes a little longer, sometimes it’s a little quicker, but the beer tells you when it’s ready, as opposed to the whole thing being very predictable.”

But perhaps most importantly, the local organisms of any area imprint each spontaneously fermented beer with an identity that’s impossible to duplicate in any other location. Winemaker Matthew Norelli of Preston Vineyards, an organic farm and winery in Dry Creek Valley, California, subscribes to the uncommon practice of spontaneously fermenting his wine for this very reason. “Most winemakers use commercially obtained yeast,” Norelli explains, “which allows the winemaker to slightly tweak aromatics and mouthfeel using different strains developed for those purposes.” However, a spontaneously fermented wine “will be more representative of the place the grapes came from. The yeast in a spontaneous fermentation is from the same area as the grapes and not an isolate from some other region in the world.”

While a brewer’s malt and hops may come from far and wide, he can stamp his beer with an exclusive profile by fermenting with local wild yeast and bacteria. Although spontaneous fermentation can be achieved almost anywhere, the resulting product in different regions will never be identical. Even within the same area, the climate during a specific time of year might not permit a fermentation producing appealing flavors. Thus, breweries such as Jester King in Austin, Texas, Bear Republic in Healdsburg, California, Crooked Stave in Denver, Colorado, and Hill Farmstead in Greensboro Bend, Vermont, have all begun putting unique spins on their own spontaneously fermented beers. At last, the U.S. is producing sour gueuzes and lambics that rival the quality of those produced in Belgium.

Cantillon has recognized the evolution of American quality, too, taking Russian River and Allagash under its wing to collaborate on a special Wild Friendship Blend. Van Roy asked the two American breweries to ship kegs of their own spontaneous beer to Belgium, which were then blended with Cantillon and poured at the brewery’s Quintessence event in the summer of 2014. Shortly afterward, the empty kegs were filled with Cantillon and shipped back to the States so that the American breweries could re-create the blend and release it again the following year. For Van Roy to organize this event says volumes about his respect for his American counterparts, who in turn have the utmost reverence for Van Roy. Cilurzo recalls the collaboration as “one of the coolest things I’ve ever done as a brewer.”

America’s spontaneously fermented beer can be traced back directly to Belgium, and the product’s popularity in the States is, in turn, helping to propel the popularity of Cantillon, which is now in the midst of an expansion that will allow the brewery to produce 70 to 80 percent more beer each year. “It’s nothing in comparison of demand,” says Van Roy. “We could do 100 or 200 times more.” Yet he still wonders about the driving force behind consumer interest: “Is it a fashion because the beer is rare, or is it because people love it?” he asks. “I hope it’s the second option.”