Three Nashville brewers sipped beer from small plastic cups in the backroom of Bootleg Biology, Tennessee’s only yeast lab. The lab’s garage door, framed by an old red, white, and blue brewers’ conference banner, was open despite the early February cold. The beer they were sampling was brewed with a wild yeast collected from a sunflower. Surrounded by brewing and lab equipment, they compared tasting notes. They pronounced it “Belgian saison-y,” with flavors of bubblegum and banana.
Chad Mueller, head brewer at TennFold—who, with his colleagues, had begun their hunt for this yeast six months earlier—was surprised by the banana flavor. “I’m pretty interested,” he said. But he wasn’t sold. Did the yeast have a gene that could lead to exploding cans and ruin the taste of his beers? They'd need to test it.
The yeast hunt began after the Tennessee Craft Brewers Guild and the Tennessee Department of Agriculture announced they wanted an official yeast for their Farm to Tap collaboration, an initiative to promote the use of Tennessee agricultural products in local brewing. They sought something special, something wild. Domesticated yeasts, like certain strains of the familiar brewer’s yeast Saccharomyces cerevisiae, are commercially available and have been passed down through generations. But there are also wild strains of yeast, which brewers sometimes call bugs, that exist naturally all around us.
These bugs are tantalizing variables in the brewing equation; if they’ve never been captured, no one knows how they’ll react during fermentation, but they offer brewers the chance to stand out by featuring unique flavors in their beers.
To help them in their quest, the Guild and the Department of Agriculture enlisted Jeff Mello, the founder of Bootleg Biology. Bootleg assists commercial and homebrewers with all things microbial, supplying them with unique yeasts and bacteria, hosting a culture bank, and testing for contaminants.
In early August 2023, Mello met Guild and Department members at Nashville’s Ellington Agricultural Center, the Department of Agriculture’s headquarters. With Mello’s guidance, they set out for an unusual harvest in the Center’s gardens and greenhouses. Wearing blue latex gloves, they picked small buds and pieces of blooms from crops, filled small plastic test tubes with their finds, and sealed them. They sampled a variety of plants, including tomato, okra, cucumber, elderberries, and, of course, sunflower.
Mello took the samples back to Bootleg’s office, part of a commercial office building just southeast of downtown, where no big sign advertises its presence—just a small outline of the Bootleg logo, a boot and a flask, on the front door. The first stop was the lab.
Bootleg’s lab is a far cry from Mello’s initial setup. In 2013, he was a homebrewer living in Arlington, Virginia, who, looking for a change, had left his job in nonprofit fundraising. A trip to Brussels had introduced him to brewing with unusual yeasts and bacteria, and he decided to experiment. He filled a mason jar with wort, covered it with cheesecloth, and left it overnight in his yard to feed any microbes that might take up residence inside. From that wort, he isolated his first yeast strain and named it S. arlingtonesis.
“These bugs are tantalizing variables in the brewing equation; if they’ve never been captured, no one knows how they’ll react during fermentation, but they offer brewers the chance to stand out by featuring unique flavors in their beers.”
For most of that year, he worked from his home kitchen with the most basic equipment, even using a paper clip as a loop, a standard lab tool to transfer cultures. He had plenty of mold-covered failures and had to learn basic lab techniques like streaking a culture onto a petri dish. He brewed beer with his captured yeasts, and a celebrated beer writer even wrote that he enjoyed the brew made with S. arlingtonesis. “This was huge,” Mello recalled. “The proof of concept worked. It shouldn't have. But I didn't know enough to be daunted. And that's been a theme for Bootleg ever since.”
Mello likens his first wild yeast capture to “[a]lmost like Prometheus personally handing you the gift of fire,” referring to the Greek myth. The comparison is apt. Mello recognized that the ability to harvest wild yeasts opened up a world of opportunities, as infinite as the microbes around him. He named himself Bootleg’s Chief Yeast Wrangler, emphasizing his role—not as a creator, but as a trusted guide, connecting brewers to the microbes that will help them succeed. “We're trying to innovate with yeast cultures and to give brewers more tools to make better beer,” said Mello.
Mello was introduced to one of those tools in Amsterdam in 2018. At a wild-yeast beer festival, a Norwegian brewery gave him a mixed culture beer. Back at his lab in Nashville, Mello and the team were curious about the microbes it contained. They isolated a yeast strain and discovered it could ferment beer at higher temperatures—around 95 degrees instead of the usual 65—without affecting the beer’s flavor. Because the yeast itself had a neutral taste, unusual for a wild strain, this was a boon to brewers who could now brew a lager-like beer in just a couple of days instead of weeks. As a nod to the original brewery’s location, Bootleg named the yeast “Oslo.”
With Oslo, or any wild yeast capture, the Bootleg team’s first task is to identify and then isolate a pure yeast strain. They began this process with the Agricultural Center harvest. Inside the small lab, its floor a hodgepodge of green and brown shapes, the staff poured some low-gravity wort into each sample tube taken from the Center and left them out at room temperature. Eventually, this liquid took on different shades depending on its contents, some light-colored, some darker, some purple. After several days, staff opened each one, listening for a hiss of escaping gas, a sign that the contents had been fermenting.
“I didn’t know enough to be daunted. And that’s been a theme for Bootleg ever since.”
For each tube that hissed, a lab tech sterilized a loop, holding the thin metal wire with its circular tip over an open flame until the metal burned bright orange. The tech then dipped the loop into the tube’s liquid. Next, he “streaked” a petri dish, rubbing the loop coated in wort across the gelatinous surface of the dish. Each plate is made in-house from a rehydrated powdered media and left to chill until needed. Only certain yeast and bacteria will grow on certain media. Because they were looking for a yeast in the Saccharomyces genus, they used WLN media, an abbreviation of the brand name Wallerstein Media Nutrient.
After spending time in small countertop incubators kept in upper 90-degree temperatures, the dishes were checked for colonies of yeast and bacteria. Because they needed a pure culture, the staff repeated the process for each promising dish. A tech plucked a button from each plate with a sterilized loop and transferred it to a clean plate, dragging it across the media surface to give it room to grow. Only one strain of yeast appeared in the sunflower sample; it had streaked to a dark seafoam green, sometimes appearing as round buttons and sometimes like smooth streaks of paint.
When the tech was satisfied that only the desired strain was present, the team proceeded to the next step. Of the samples collected at the farm, Bootleg moved forward with three isolates: yeasts collected from the sunflower, an artichoke plant, and a blackberry plant.
In the next stage, Bootleg staff had to propagate, or grow, these isolates to a sufficient volume with which to brew test batches of beer. For each of the three isolates, a tech scooped a colony of yeast from the petri dishes and dropped each into a 40-milliliter plastic tube with fresh wort. The tubes were then taken to the “prop room,” a room behind the lab that’s kept at about 85 degrees year-round. Placed in small plastic racks on the top of a wall shelf, they remained in this balmy environment for 24 hours.
Then a staff member took them to a hood and poured, or pitched, the contents of each tube into a 400-milliliter glass flask with more wort and returned them to the prop room. The yeast ate the sugar in the new wort and reproduced, eventually appearing as a thick sludge, called a slurry, at the bottom of the flasks.
Each 400-milliliter flask was then pitched into two 20-liter glass flasks with fresh wort to reproduce. The flasks spent 24 hours on stir plates in the prop room. The magnetic bars placed in the flasks were spun by the plates they sat on, agitating the slurry to prevent the yeast from flocculating, or falling to the bottom of the flask and ceasing to ferment. The stirring also helped to expel carbon dioxide and pull in the oxygen necessary for yeast cell reproduction. Staff inserted a chunk of foam in the opening of each flask to seal it, allowing in oxygen while blocking other contaminants. For each 20-liter flask, only about one liter is usable slurry, but the actual number of yeast cells present differs based on the specific yeast culture.
After the three isolates were propped up, Bootleg brewed a one-gallon test batch with each and invited Guild members to taste them. The group thought the beer with the artichoke yeast tasted like “hot alcohol,” or nail polish remover. The beer with the blackberry yeast tasted good—“jammy,” or like apples—but it didn’t fully attenuate, meaning the yeast didn’t consume as much sugar as expected. The sunflower yeast beer tasted like a saison. It was the most promising.
Bootleg brewed another test batch of beer using the sunflower yeast, five gallons this time. Since the staff wanted to taste the yeast, they brewed a basic mash. They used 80% pilsner malt, 16% wheat, and 4% Vienna malt; added two ounces of green, fragrant Mount Hood hop pellets; and boiled the wort for 30 minutes, aiming for a beer with 20 IBUs, on the higher end for lagers but lower than a pale ale or IPA. After this wort cooled, staff pitched the re-propped sunflower yeast into it, and it fermented for three weeks until the three Nashville brewers tasted it.
The sunflower yeast had proven to brew a delicious beer, but the brewers wanted to know if it was diastatic. A diastatic yeast has a gene, STA1, which can, but not always, cause it to attenuate a beer beyond a non-diastatic yeast’s ability. Bootleg staff suspected the sunflower yeast had this gene since it tasted very dry. The problem with this type of yeast is that brewers can package their beer believing it’s done fermenting—but it can continue to ferment, causing bottles or cans to explode and wasting brewers’ product, time, and money. It can also cause the beer to develop unwanted flavors. It only takes a few diastatic yeast cells to cause these problems, and many brewers refuse to work with yeasts that have the STA1 gene to avoid the risk.
“The group thought the beer with the artichoke yeast tasted like ‘hot alcohol,’or nail polish remover. The beer with the blackberry yeast tasted good—‘jammy,’ or like apples—but it didn’t fully attenuate, meaning the yeast didn’t consume as much sugar as expected. The sunflower yeast beer tasted like a saison. It was the most promising. ”
Bootleg tested for this gene by streaking the beer on a particular type of petri dish. If the yeast was diastatic, it would consume the media in the dish. The sunflower yeast beer did not; it was negative.
But there was still much left to learn about this yeast. What might it contribute to a finished beer? How would it compare to other yeasts? Nashville’s Black Abbey Brewing, a Guild member, volunteered to test it further. Using the same wort from their flagship beer, a Belgian-style blonde ale called The Rose, they’ve brewed three identical five-gallon batches. The yeast is the only variable: One batch has a Belgian Ale yeast, one a California Ale yeast, and the third the Farm to Tap yeast. If the beers differ in any way, they can attribute the variations to the yeasts used.
After that, the sunflower yeast’s future is uncertain. But TennFold’s Mueller has already suggested a name: “Tennessaison” yeast. He’s a passionate supporter of the Farm to Tap program. “It would be a really cool way for Tennessee beer to define itself,” he said.
For Mueller, having Bootleg Biology in town is vital to that endeavor. Mello couldn’t agree more. “If you want to think of food as being local, you should also think of yeast that way,” Mello said in a news interview about the movement. “And that’s kind of been our mission since day one—is to get people to think about microbes the same way they might think about wheat or barley or hops.”
Words by Maggie GigandetIllustrations by Colette Holston
