An Inside Look At Coolship-Brewed Beers

The thought of bugs flying into a vat of something you drink is enough to make most people shudder. Yet when you're brewing beer in a coolship—a flat, open vessel in which cooling wort is naturally inoculated with ambient microorganisms—that's just part of the deal.

Allagash Brewing should know. The Portland, Maine, brewery does some 10 coolship brews a year, all within a specific window in winter. For the process to work, the weather has to be cold enough so that the beer is not overwhelmed with microbes and wild yeast. The annual result is a few hundred barrels of what will either become a Belgian-inspired lambic (young, spontaneously fermented ale), like the cherry-aged Coolship Cerise, or a gueuze-style beer (a blend of one-, two- and three-year-old lambics), like the Coolship Resurgam.

Today, only around two-dozen other American breweries have coolships, but the number has been growing steadily since 2008, when Allagash became the first commercial American brewery to build one. Before then, the tradition was limited to Belgium, where breweries, such as Brasserie Cantillon in Brussels, have been spontaneously fermenting beer in open vats for centuries (in Cantillon's case, since 1900). Allagash and old-school Belgian brewmasters have had a tremendous influence on U.S. beer culture; beyond the nod to history and experimentation, though, a larger local and artisanal trend seems to be inspiring American coolship brewing as well. 

"Many brewers are interested in beers that are uniquely tied to time and place, and 100 percent spontaneous fermentation is a great way to go about this," Jeffrey Stuffings, founder of Jester King Brewery, explains. The Austin, Texas, brewery, best known for its wild ales, is now one of the largest producers of coolship-brewed beer in the country.

Dave Linari, assistant brewer for OEC Brewing in Oxford, Connecticut, agrees. "It adds to our house terroir," Linari says of their coolship, invoking a term usually used by winemakers when they reference the climate, soil and other local features that influence flavors and aromas. "Wild fermentation also allows for more creativity than clean fermentation techniques, and opens up a world of flavors and aromas," he adds. With coolship brewing, beer gains complexity as it ages, picking up flavors from the liquid the barrel previously held, encountering microorganisms in the wood, and slowly oxidizing over time.

Although coolships are simple to use and clean, exposed beer can spoil much more easily, which means brewers must be vigilant in tending to their product. Yet the extra time and attention is a source of personal pride for many coolship brewers, who would much prefer this method over quicker, cheaper techniques like kettle souring (manually pitching live bacteria, such as Lactobacillus, in a steel-brew kettle). "In my experience, there's just no shortcut when it comes to creating the complexity you get from spontaneous fermentation with wild yeast and bacteria," Stuffings explains. "The difference in flavor, aroma, mouthfeel is night and day."

As a fan of coolship beers, I wanted to get a better understanding of the process. So I headed to Allagash to coolship-brew with their team, tagging along from the very start of mash-in—the process of combining and heating grain and water—to the moment the beer is poured into the barrels for aging. Here's the play-by-play.