When Tim Faith, the research and development brewer at Goose Island Beer Co., wanted to make a beer using only Illinois ingredients, he knew he could get local malt and water. Hops would prove a little more difficult (less than 50 acres of hops were grown in Illinois in 2017), but not impossible. Yeast, the fourth ingredient necessary for beer, posed the biggest challenge. Faith and his team would have to collect the yeast on their own. They headed to Shawnee National Forest in southern Illinois in October 2017. Fall is the sweet spot for wild yeast when it’s not too cold, yet not hot enough for bacteria and mold to take over. The brewers came armed with petri dishes, sterile swabs and beer. The goal: Collect wild yeast samples from every hike and campsite they visited. Around 40 samples were deemed good enough to bring back to the lab, and four of those were safe, produced enough alcohol and fermented with pleasing aromas. Only one, a yeast from a pig nut found on Little Grand Canyon trail, made it into the final beer, called Scavenger. Scavenger uses all Illinois ingredients, but it’s the wild yeast that sets it apart. It’s the rarest ingredient and what makes the beer taste of a place. Or, to borrow an often misused and slightly pretentious term from wine, Scavenger’s yeast expresses terroir. “Obviously with wine and beer, we’re dealing with an agricultural product,” Faith says. “Yeast is probably the one thing that’s most derived from agriculture.” He adds that “yeast is probably the majority of the flavor expression in beer.” Still, calling flavor expression “terroir” can be tricky for brewers. The French word usually applies to wine, and generally translates to a product with a distinct taste of where it came from. Brewers have chased terroir in different ways over the years. Jimmy Mauric, the brewmaster at Spoetzl Brewery in Texas, told DRAFT that terroir comes from the water, while Dogfish Head founder Sam Calagione said in 2006 that it comes from the people involved in making beer. Mikkeller has a whole hop terroir series that operates under the notion that “Just as grapes are not just grapes, hops are not just hops.” But wild yeast comes closest to expressing terroir in the same sense that it’s used for wine, because wild yeast is the major ingredient that’s acutely specific to where a beer (or wine) is made.

What the store-bought yeast doesn’t have is the natural expressions of the place it’s from.”

The Oxford Companion to Beer defines wild yeast as “any species of yeast in fermentation other than the pitching yeast, often derived from the environment in or surrounding the brewery.” Most wild yeasts will spoil beer. The ones that don’t are most often variations of Saccharomyces and Brettanomyces found in and around overripe fruit. Brewers who want to use wild yeast must capture a strain that’s safe and makes a drinkable beer, which is a little like trying to catch a wild Gyarados in Pokémon. You’re going to catch a bunch of dud Magikarp, then out of nowhere you’ll randomly snag a Gyarados that’s just right. The number of duds brewers find when hunting yeast can be staggering. Urban Artifact in Cincinnati, Ohio, uses a custom wild yeast strain for its barrel-aging program that took around 18 months to develop. In that time, lead brewer Joshua Elliott says, brewers collected between 350 to 450 yeast samples. Four made the cut. “You have to think about where domesticated yeast has come from, where it originated and how much it has mutated to meet our demands with beer,” Faith says. “A lot of yeasts and funguses out there aren’t ready for a beer environment.” Then there are the yeasts that can technically make beer, but taste awful. Elliott found loads of yeasts that yielded plastic notes, and Faith and his team captured yeasts with cabbage, green vegetable and tomato soup aromas. Even viable beer yeast can’t always complete the job. Scavenger’s prized yeast strain, which targets long-chain polysaccharides and gives the beer a lemony melon taste, needed help. Faith added a French ale yeast to give the extra oomph the wild yeast needed to complete fermentation. Today, cultivating wild yeast is an unnecessary labor of love. Thanks to modern brewing technology, brewers don’t have to fish for their own Gyarados anymore. International companies like White Labs in San Diego and smaller companies like Wyeast and Imperial Yeast sell yeast with known flavor profiles and brewing abilities. A world of yeast from around the globe is available with a few clicks. It’s the greatest possible version of what used to be the beer industry’s American dream: reliable, accessible, and easily acquired yeast that brews a consistent and replicable beer. What the store-bought yeast doesn’t have is the natural expressions of the place it’s from. It wasn’t always like this. Belgian lambics are the most famous example of beers with yeast that expresses terroir. For centuries, Belgian brewers have kept a less-than-pristine brewing environment to maintain the same yeasts around their open-air barrels of fermenting beer. When regulators forced Brouwerij Boon to paint their walls with a food-grade paint in 2000, they compiled, then promptly sprayed the walls with beer to bring the wild yeasts back. This type of brewery management may seem counterintuitive, but it works.

There’s yeast all around us. It’s just a matter of capturing the yeast at the right time and the right place.”