Case Study: Black Frost and the Pursuit of “Rum Oil”

When Black Frost Distilling opened in late 2022, cofounder Jace Marti envisioned a distillery that put the flavors of “northern terroir” on display. However, during downtime in the initial stages of the coronavirus pandemic, he began exploring the science of rum—not exactly a big thing in Minnesota.

So, Marti looked to the Caribbean—in particular, to chemist Rafael Arroyo’s research in the early 1930s and ’40s in Puerto Rico. That study led Black Frost to concoct two different rums, both steeped heavily in science and made distinctive by using the distillery’s wooden, open-top fermentors.

Marti is a brewer, and his family still runs August Schell’s Brewery in New Ulm—the second-oldest family-owned brewery in the United States, after Pennsylvania’s Yuengling. Schell’s began using local barley in its helles lager about a decade ago, and that’s when Marti began to wonder about using local crops in the production of spirits.

Marti revels in fermentation and spearheaded the mixed-culture program at the brewery’s Starkeller taproom. Growing up in New Ulm allowed him to befriend local farmers—including Nate Gieseke, a sixth-generation farmer whose background neatly parallels Marti’s status as a sixth-generation brewer. Given their interests, cofounding a distillery just made sense—and Gieseke could grow whatever they needed.

“Looking back, it sounds weird, but it was COVID times,” Marti says, “and we had to find something to do.”

As Marti dove into Arroyo’s research, he began planning how he wanted to produce heavy, high-ester rums. [PAYWALL]

Wildcatting for Rum Oil

Marti was drawn to Arroyo’s research for a couple of reasons. The first was that he wanted to pursue “rum oil,” which Arroyo viewed as a desirable aromatic compound that was rum’s defining trait. The second was that Marti learned that they could produce a more mature spirit before barrel aging by allowing a longer fermentation using a half-wild yeast in their open fermentors.

Whether it was Marti’s background in mixed-culture brewing, his appetite for technical knowledge, or his interest in just doing things differently, he found himself drawn to the idea of producing funky rums—and to using wild yeast in those open, wooden fermentors.

Marti learned a lot about heavy rum and rum oil from Stephen Shellenberger’s Boston Apothecary blog, where the author digs into Arroyo’s rum research. Marti’s big takeaway was to tap into Schizosaccharomyces, a genus thought to have split off from Saccharomyces cerevisiae hundreds of millions of years ago. German microbiologist Paul Lindner first isolated Schizosaccharomyces pombe in 1893 from a sample of East African millet beer; pombe is the Swahili word for beer. Notably, the yeast also can be found in wooden fermentors used to make rum in the Caribbean.

Schizosaccharomyces is a fission yeast, not a budding yeast such as Saccharomyces cerevisiae. Rather than reproduce by budding, a fission yeast elongates and separates. Black Frost ferments all its washes with Schizosaccharomyces pombe and japonicus.

“All this science back then said Schizosaccharomyces was superior, but the downfall was it grew slower,” Marti says. “If you think of that time period—wood, wild, no regard for sanitation whatsoever—it would naturally become the dominant strain because it had a higher tolerance than Saccharomyces.” The yeast had an advantage in that high-stress environment.

Schizosaccharomyces produces less of a burn flavor in higher-alcohol content, Marti says. It also produces more beta-Damascenone, an aromatic compound seen as a prized quality marker in spirits. Extended aging and evaporation can accentuate it, but this type of yeast overproduces it during primary fermentation. That combination is what leads to a more mature-tasting spirit at a younger age. The tradeoff is a longer primary fermentation—but that’s just two weeks, while it may shave months or years off barrel maturation. That was Arroyo’s argument: The tradeoff is worth it, especially given the added production of beta-Damascenone—or, as he coined it, rum oil. 


Developing a House Culture

Beta-Damascenone is also present in roses and used in perfumes, giving off a bright and persistent aroma, and it gives rum extra depth. “You target rum oil, and everything else falls into place,” Marti says. “Any yeast will produce it—this yeast just produces it in a large amount. … Arroyo was trying to use science to target this compound directly.”

Unlike Arroyo, Marti isn’t using stainless steel to ferment. Using the hot dunder—the stillage that’s left in the boiler after distilling—he refills the fermentors to sterilize the wood as well as possible using heat. If Arroyo’s method was all about pure cleanliness, Black Frost’s is as close to clean as they can get using porous wood that’s prone to bugs and bacteria. They don’t use artificial cooling. Open fermentors have been part of spirits production in the Caribbean in the past, but Marti didn’t know any distillers who were using them in the United States. He was stepping onto some unfamiliar ground.

The biggest challenge, he says, was tracking down the right yeast strains. He scoured every source and online yeast bank he could find. There are ways to capture Schizosaccharomyces in the wild, but he was more intentionally pursuing the most desirable strain he could find. Marti found 50 strains of it—he says that sounds like a lot but isn’t—and the early trials were pretty basic: Does it ferment in a reasonable amount of time, and does it smell good? That was the early pass/fail system for those 100-milliliter fermentations.

“That was a really long journey, trying to find those original yeast strains for these experiments,” Marti says. “I started with 50 cultures that I ran through trials to get down to 10—really ran through those in fermentation trials before settling on four.” 


One strain became the house culture, and Marti uses it in their two rums, which are both made from Louisiana blackstrap molasses: the white Black Frost Rum, at 94 proof and the Black Frost Overproof Rum, 114 proof and rested on Brazilian amburana wood. There are four more Schizosaccharomyces strains in rotation at the distillery, with about a dozen more on standby. Using those strains on standby, they have been conducting monthly tests to see what the yeast strains can bring to a whiskey, especially because Black Frost blends different fermentations to create its whiskey. 


While Marti says he had to unlearn a lot of his brewing background to learn distilling, you can see a lot of his brewing expertise and passion for fermentation in the process at Black Frost. The interest in “northern terroir” began at Schell’s, and he brings techniques and ideas from mixed-culture brewing to help Black Frost shape its own unique identity.

“We wanted to box ourselves into the corner and explore what we could do out of it,” Marti says. It’s not being different for the sake of being different.

“Being different with a purpose is the best way to describe it. Use what is grown on the farm. Use open-top wood fermentors.” And now, use the 90-year-old research of a Puerto Rican chemist to successfully combine Caribbean flavor with northern terroir.