A koelschip—or coolship, as it’s been Americanized—isn’t much to look at. Unlike the tall, gleaming fermenters you have likely seen on a brewery tour, the device consists of a simple metal pan, usually longer than it is wide and only a foot or two deep. No one knows exactly where the coolship originated (there is evidence of shallow, uncovered brewing vessels dating back to at least medieval times), but its practical application is clear to anyone who didn’t sleep through physics in high school.

“I would imagine someone was boiling that liquid a millennium ago, adding spices or hops, and they said, ‘Well, it’s not going to ferment until it cools down,'” says Dan Carey, co-owner and brewmaster of Wisconsin’s coolship-rocking New Glarus Brewing. “And then somebody had the bright idea, ‘Let’s increase the surface area by putting it in a pan.'”

As the years went by and technology advanced, coolships as old-fashioned heat exchangers were no longer necessary. Most modern breweries—adopting the cleanliness-is-next-to-godliness mantra you see in most commercial spaces today—eschewed these shallow pans in favor of more modern refrigeration techniques.

But not everyone.

Belgian tradition and the influence of Cantillon

The Belgians realized that, in addition to speeding up the cooling process, these open vessels also exposed wort (the sugary liquid that ferments into beer) to naturally occurring yeasts and bacteria in the atmosphere. While these microorganisms can easily spoil a brew through contamination, they are also capable of producing stunningly complex flavors under the right circumstances. The catch is that you can only control the process so much; mostly, you just have to sit back and let nature take its course.

Traditional breweries in Brussels and the Pajottenland region of Belgium (such as Brasserie-Brouwerij Cantillon, Brouwerij Boon, Brouwerij Lindemans, and 3 Fonteinen) would let their beer sit out overnight in a coolship, where it would become inoculated with wild yeast from the night air in a process called spontaneous fermentation. When these yeast strains mingled with natural bacteria in wooden barrels after fermentation, the results were often satisfyingly tart. The process also imbued each brew with a distinct sense of place, which explains why traditional lambics—the base beer that’s often aged and blended to create gueuzes and krieks—can only legitimately be made with the yeasts found in a specific part of Belgium.

Cantillon wasn’t first brewery to make spontaneously fermented sour ales, but it may be the best.

It’s hard to pinpoint which brewery first started making spontaneously fermented sour ales, but Cantillon in Brussels is the most commonly cited reference point—both because it’s been around for a while (since 1900), and also because it feels like every U.S. craft brewer interested in wild ales worships at the feet of its tippling, chair-tipping label mascot. Cantillon wasn’t first, but it may be the best.

“It really doesn’t matter who were the first two people that pitched a ball and who hit it,” says EmbraceTheFunk.com proprietor Brandon Jones. “The standard is defined by people like Babe Ruth or Willie Mays. That’s what people strive for—to be the Jean van Roy of lambic, or the Armand Debelder of blending.”

Jean-Pierre van Roy, Cantillon’s fourth-generation brewer, has owned the Brussels institution since 2011. He says very little has changed in Cantillon’s 114 year history aside from the bottling line, and that time and patience are simply part of the process when it comes to making these carefully crafted sour brews. “We don’t control our fermentation, so some beers could take less than one year to be ready for a blend, and some even two years,” van Roy says. “Patience [is important for brewers considering a coolship] because a brewery has to build its own natural environment, and it takes a lot of time. Love and passion are very important for me, as [they are for] a lot of craft brewers.”

The coolship revolution in the U.S.

Inspired by forefathers like Cantillon, U.S. breweries have caught coolship fever in recent years. Allagash Brewing Company in Portland, ME is widely cited as the first to set one up for spontaneously fermented sour beers, back in 2008. (Anchor Brewing, it should be noted, has used one for a while, but not to make sours.) Others, like Russian River, Anchorage, New Glarus, and Bluejacket have followed suit in the years since. Their motives vary, from lambic admiration, to a thirst for innovation, to a desire to make beer with a sense of terroir.

Oftentimes, it’s a mix of these and other factors. Denver’s Crooked Stave Artisan Ales was born from a dissertation that owner/brewer Chad Yakobson wrote on a wild yeast called Brettanomyces. (Crooked Stave has a coolship, but hasn’t put it to use yet.) Jester King in Austin, TX incorporates local yeasts from the surrounding Texas Hill Country into its coolship batches. Allagash is experimenting with different blends and aging with cherries and raspberries—a traditional technique used to balance out sourness with natural sweetness from fruit.

U.S. breweries have caught coolship fever in recent years.

These beers, collected under the nebulous umbrella of “American wild ales” by the microbrewing community, are still very much a small niche in an exploding craft scene that for years has been dominated by the arms race to make bigger, hoppier IPAs. But breweries like Boulevard, Surly, The Lost Abbey, and Wicked Weed are taking home Great American Beer Festival medals in various sour-leaning/wild beer categories, and there are more and more events cropping up to celebrate this funky corner of the brew world, like Chicago’s annual Festival of Wood and Barrel Aged Beer and the roving, collaborative bottle-sharing meet-up, Where the Wild Beers Are. All signs point to a continued rise in the production of complex, tart beers in years to come.

In the meantime, the proliferation of coolships around the country is a testament to the mouth-puckering, refreshing appeal of lambics; the maturing craft-beer market, which is looking for a break from hop bombs and imperial stouts; and the enduring influence of old-world breweries like Cantillon. Asked if it’s fulfilling to see this movement happening in the states, van Roy seems pleased, if modest. “Yes, for sure, but I don’t consider it a success for Cantillon itself,” he says. “More as a success for traditional lambic.”

To get a sense of how the wild-ale landscape is evolving stateside, we caught up with six of the most exciting U.S craft brewers using coolships right to find out how they’re paying homage to—and riffing on—tradition.

Allagash Brewing Company

From: Portland, ME
Founded: 1995
Coolshipping since: Summer 2008

What prompted Allagash to get a coolship?
Brewmaster Jason Perkins: 
“Ever since Allagash opened its doors, we have been focused on exclusively Belgian-style beer. We joked for many years that [coolshipping] would be a fun thing to try. Rob [Tod, Allagash founder/owner] walked in one day in the late summer and said, ‘Let’s put in a coolship,’ and [we had it in place two months later]. We have always functioned with a big degree of innovation here over the years, and this project is just an extension of this mentality. We have always loved the lambic-style beers and loved the idea of seeing if spontaneously fermented beer could be produced outside of the Senne Valley in Brussels.”

What have you learned?
“Patience. Making these beers take a lot of time, and it is very different experience than making our other beers as a result. We have also learned that the temperature and weather has a significant effect of the flavor of these beers, and we need to be cautious about when to brew.”

Coolship beers so far: All of Allagash’s coolships beers arrive unannounced and can be purchased exclusively at the brewery. “Over the last six years, we have brewed 24 batches of our Coolship beer, which serves as the base for Coolship Resurgam, Coolship Red, and Coolship Cerise,” Perkins says. “We have also brewed three batches of Ghoulschip, a beer brewed with local pumpkins and pumpkin seeds on Halloween night.”

Coolship beers coming up next: “The next Coolship beer we will sell will likely be Coolship Red. Utilizing two-year-old Coolship beer, we add two pounds/gallon of fresh local raspberries to the barrel. Resurgam—our blend of one-, two-, and three-year-old, not-fruited beer—will be available shortly after that.”

Russian River Brewing Co.


From: Santa Rosa, CA
Founded: 1997
Coolshipping since: January 2012

What prompted Russian River to get a coolship?
Co-owner/brewmaster Vinnie Cilurzo: “We have been making spontaneous fermented beer for years using a hybrid method we came up with another brewer friend of ours, so graduating to a coolship/horny tank was an obvious move. We only ferment spontaneous-fermented beer in this tank and it is something I have always wanted to do.”

Wait, what in the world is a “horny tank”?
Horny tank is another term for an open fermenter in which the wort is left exposed to wild yeast and bacteria.”

What have you learned?
“We’ve learned that there are a few variables, the two main ones being time the wort spends in the open tank, and what temperature the wort is sent into the tank at.”

Coolship beers so far: “We typically run eight brews through it a year. Up until now, it has been one beer we call Sonambic, which is our 100% spontaneously fermented beer. Multiple batches of Sonambic get blended—disproportionately—to make a beer called Beatification.”

Coolship beers coming up next: “Right now, we only ferment Sonambic in our horny tank. The blending of multiple batches of Sonambic into Beatification only happens once a year and is not on a strict schedule—the beer tells us when it is ready. We just had a Beatification release in late December, so we are a long way off from another release—probably next winter.”

Anchorage Brewing Company

From: Anchorage, AK
Founded:  2011
Coolshipping since: November 2012

What prompted Anchorage to get a coolship?
Owner/brewer Gabe Fletcher: “I’ve just always been fascinated with them and the challenges that come with producing beer in them. Of course, visiting Cantillion didn’t help any! I’ve tried capturing wild yeast and bacteria up in Alaska before. Sometimes it works and sometimes it doesn’t. I think that’s why I love it so much: There are many ways you can use a coolship and it’s very unpredictable. But when you get it right, it’s such a great feeling to know that you have captured a flavor that no one else has captured before. It’s not the main focus at the brewery right now, but it is definitely a long-term project that I can’t wait to focus more attention on.”

What have you learned?
“One of the biggest things that I learned is that it has a lot more to do with the room that the coolship is in than the outside air. You should have your coolship in a room that is covered in wood, especially the ceiling. The more flora that is in the wood, the better results you will have, and the more the flora will grow. It’s no different then having a barrel that has yeast and bacteria embedded in the wood. When you put wort or finished beer in the barrel, everything comes to life and inoculates the beer or wort, just like when you put that hot wort in the coolship, it heats up the wood on the ceiling and begins to condensate, dripping that flora right back into the wort.”

Coolship beers so far: “We have, unfortunately, only been able to make one batch in it so far. That beer did start fermenting by the next day, but the flavors haven’t quite come together yet. Right now, it is on a cradle with heavy casters on it, so it can be wheeled around to different spots in the brewery and then tucked away when we’re not using it. We are working on building a new facility, and that will include a dedicated coolship room, so we can really get the flora going for a hopefully consistent result.”

Coolship beers coming up next: “I plan on using it some more before the room is finished, but at the moment, I don’t have any definite date when that will be done and what the beer will be. That’s the thing about these beers—until you have a consistent technique down, you never quite know when they will be done. Taste and wait, taste and wait.”

Jester King Brewery

From: Austin, TX
Founded: 2011
Coolshipping since: February 2013

What prompted Jester King to get a coolship?
Co-founder/owner Jeffrey Stuffings: “What turned us on was drinking authentic lambic. The depth of flavor and overall enjoyment from drinking authentic lambic is unrivaled by any other style of beer, in my opinion. The challenge of making our own spontaneously fermented beer was really appealing to us. Also, we’re a brewery that thinks it’s important to make beer with a sense of place, meaning that our flavors are unique to, and derived from, our location. Using a coolship to catch wild yeast from the Texas Hill Country for fermentation is one of the best ways we can think of to make beer with a sense of place. “

What have you learned?
“We’re still getting used to using our coolship. There’s a lot of good tips we’ve learned from the authentic lambic brewers and blenders in Europe, as well as a few American brewers making spontaneously fermented beer. We’ve learned that the biological momentum of the coolship room picks up over the course of a season. Our spontaneously fermented beer takes off faster and ferments more vigorously upon each subsequent filling of the coolship. We’ve also learned more about working with aged hops, which have been fun to experiment with.”

Coolship beers so far: “As of now, we’ve inoculated four 15-barrel batches of wort in [our coolship]. We’ll continue to inoculate more wort during the winter of 2014 and will do so every winter. The beer we’ve made so far—assuming it matures well—will see a number of different uses. Some will be blended with younger beer spanning back a number of years, some very mature beer will be bottled still, and some will be refermented with fruit.”

Coolship beers coming up next: “It’s too soon to say at this point. Our best guess right now would be 2016. We’ve followed a very traditional path by incorporating things like a turbid mash, extended boil, aged hops, 100% spontaneous fermentation, and long-term fermentation and maturation in large (400L to 600L) oak casks. We plan to blend various vintages stretching back across a number of years, package some of our favorite mature stock still without bottle refermentation, and referment mature spontaneously fermented beer with fruit. We may get more creative from there, but we love the classics and are excited to take a classical approach using the microorganisms native to our region.”

Side Project Brewing

From: St. Louis, MO
Founded: 2013
Coolshipping since: April 2013 (Side Project operates as a small, startup brewery within Perennial Artisan Ales)

What prompted Side Project to get a coolship?
Head brewer Cory King: “I love the history and tradition of the styles that used a coolship, and I knew that I wanted to try to capture native yeast and bacteria from my family’s farm in hopes of creating my own house flavor over time. My first coolship batch was a 20-gallon batch that I racked hot into four buckets, left them outside overnight to cool, and then transferred to an oak barrel at the brewery. Two years later, it’s nice. My brewery is tiny and on a tight budget, so my coolship is actually an old dairy tank. It’s a balance tank: open top, shallow, stainless with TC fittings. Works perfectly for my size.”

Coolship beers so far: “So far, all of my coolship beers have been blonde- and lambic-inspired (pilsner and wheat). I will start experimenting with the base recipe here soon.”

Coolship beers coming up next: “I have a beer releasing in early February that was part of my first coolship batch. The beer is called Pulling Nails Blend #1. My intention with the rest of the coolship beers that I currently have is to wait until I have several years’ worth and decide whether I am going to fruit any of the batches or create my own blended sour with them. It’ll be at least another year—maybe two—before a release.”

New Glarus Brewing

From: New Glarus, WI
Founded: 1993
Coolshipping since: Installing now—Carey designed the coolship himself, then had a Wisconsin manufacturer make it out of stainless steel. He also built a cellar where the coolship will live, alongside oak tanks.

What prompted New Glarus to get a coolship?
Owner/brewmaster Dan Carey: “We have made spontaneous sour brown ales since 1994 and we started to make [lambic-style beers] five years ago using our lauter tun as a coolship—boiled the wort, pumped it into the lauter tun, let it sit overnight, then pumped it into the barrels to ferment. It was an introduction and got us going making this blonder, lambic-type of beer. It took us five years to figure out how to do the process. I didn’t want to go out and spend the money when I didn’t know what I was doing. We’ve been slowly building up our cellar of beer, mainly in wine and bourbon barrels.”

Coolship beers coming up next: “We will make spontaneous, American wild ale, which is like a lambic. We’ll make that, and we’ll continue to make our sour brown beer style. We’ll make a brown beer and a blonde beer, as well as fruit beers.”