Rum’s character varies by region, traditionally, and much of what defines that character happens before anyone fires up a still. When it comes to Jamaican and Bajan rums, the yeast and fermentation are seminal—yet they’re also woefully misunderstood.
Craft distillers have been attempting to reconnect to yeast of all varieties, but it’s not as simple as pitching another strain. In the Caribbean, distillers have been honing their use of different types of yeast over centuries, and those practices are as integral to their distillations as the soil that grows the sugarcane and the sun overhead.
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A Distinctive Approach
Broadly, rum enthusiasts sometimes delineate Caribbean styles by colonial power: English, French, or Spanish. For English-style rums, including Bajan and Jamaican, molasses is the main raw material used. A major benefit to molasses is that it can keep in bulk for long periods of time without further processing, so—unlike fresh sugarcane juice—it’s less affected by the seasonal ups and downs of sugar production.
Besides glucose and fructose, molasses contains small amounts of many vitamins, nitrogen, phosphorous, and colloidal constituents, which support yeast growth and healthy fermentation. There are also trace amounts of acids, esters, phenolics, aldehydes, and ketones, which can all influence the flavor of the final spirit.
Before fermentation, distillers using molasses typically clarify it via chemical or physical means, adjust the pH, and apply heat for a mild pasteurization treatment. They also must dilute the molasses before fermentation and may add yeast nutrients, such as ammonium sulfate.
From there, outside understanding of Caribbean rum production—especially fermentation—can begin to falter. While it’s common to have conversations that revolve around molasses in rum from Barbados or Jamaica, they’re incomplete without mention of the role of yeast and the microbiome. Caribbean distillers approach fermentation and the microbiome very differently than a typical bourbon distiller, for example. The microbial ecology of fermentation is essential to the production of rum, and it’s driven by both the yeast and bacteria included in the process.
Unique Cultures
Bacteria get a bad rap, and in this case we aren’t talking about the kinds of bacteria that get you sick—the Bacillus species and lactic acid bacteria are essential components in microbial fermentation, and indigenous contaminants are an important element in the creation of distinct terroir.
Perhaps no country is more adept at incorporating and emphasizing the microbiome than Jamaica. Its rums are known for their complex processes and intense aromatics, often referred to by the umbrella term “funk.” However, that oversimplification does little to convey the complex list of interactions that take place, nor does it capture the breadth of fermentation prowess on display in these distilleries.
At Worthy Park Estate, for example—in Lluidas Vale, about 35 miles northwest of Kingston—they leave wooden collection vessels outside to capture the wild, ambient yeast.
Jamaican producers might also leave out fresh cane juice, says Maggie Campbell, estate rum manager at Mount Gay in Barbados, “to wild-ferment into cane wine, that then converts into cane vinegar as an acidulation adjustment ingredient that is itself filled with wild microbiome—to say nothing of dunder [stillage] that can be used fresh or is left to ripen.”
Distillers and consumers outside the Caribbean often misunderstand dunder—for example, by treating it synonymously with muck, which is a different ingredient in rum fermentation. While dunder is spent rum wash, leftover from distillation and rich in acids, muck is liquid sludge produced via a controlled, putrefactive fermentation process, during which it’s left to sour over a long period of time, further developing its intense microbial character.
Some of that character is cultivated, Campbell says, by adding a jackfruit, “which is naturally covered in pombe yeast, and when it decomposes [it] acts like a whale dying in the ocean, creating a large microbial feeding and burst of microbial life.”
Controlling fermentation with isolated yeast strains is a relatively new phenomenon. Until the early 20th century, microbes in the local microflora were responsible for all these fermentations—these contaminants could be found in the cane juice or molasses, or on the exterior of the processing equipment. Before scientists identified the organisms responsible, distillers and processors knew to add a portion of a previous fermentation into a new one, encouraging a faster onset. Greater understanding has led distilleries, over the past 70-plus years, to isolate and catalog the main yeast strains active in their distillations. From wild-culture collections, they can propagate those strains into cultures that can be pitched for fermentation.
Local Expression
Caribbean distilling never fully lost its relationship to native yeast or the local microbiome, always keeping them involved in production—even when consumer preferences shifted in the 21st century to more neutral styles of rum.
The degree of involvement can vary by style. Distilleries that make Spanish-style rum, emphasized as light and delicate, mostly have switched to inoculating with specially chosen low-ester yeast, which they pitch with pasteurized molasses in sterilized, closed-tank fermentors. Their aim is to get as mild an expression of the local area as possible, getting more of their character from specific notes that the yeast create in the raw materials, as well as from oak maturation.
That type of production sits on the opposite side of the spectrum from the methods in Jamaica, where producers were able to satisfy some of the mid-century demand for delicate styles even while maintaining their craft for esterification. Facets of Jamaican rum fermentations such as dunder, muck, and cane vinegar create unique flavors that are not only expressive of their place—they’re also distinct to that community and the region itself.
“In fact, a fermentation in Jamaica may be so microbially active [that] you hit, say, 7 percent ABV after three days,” Campbell says, “but by the time the extended microbial conversion part of the fermentation is done on Day 21, the ABV may be as low as 3 percent. When it is distilled the volume is low, but the intensity of character is sky-high, to be used as a minor blending ingredient in more moderate and neutral rums.”
As understanding of rum fermentation’s microbiology has grown, so has a divide in the research and distillation communities. Some have argued that rum distillation and efficiency would be improved by using selected yeast strains in more sterile environments, with less bacteria present. Other groups view the broad spectrum of bacteria as contributing a great deal to the expansive organoleptic qualities of certain rum styles, especially “heavy” or more flavorful rums from the Caribbean. Demonstrating that view in liquid form, many Jamaican estates have released different rums produced via ambient yeast and spontaneous fermentation, bottling them under unique marques that distinguish not only the individual bottling but the distillery that made it.
Perhaps neither group is entirely correct, and the truth lies somewhere in the middle. There is a place (and a preference) for styles of rum produced via “pure” fermentation. Yet the estates of Barbados and Jamaica have maintained an incredible wealth of knowledge and understanding about their methods, clearly evidenced by the terroir present in their offerings.
Still, this information eludes most distillers who haven’t worked in these places.
“There is a real lack of respectful, long-term relationship building that is needed if you want to get accurate information from a reliable resource,” Campbell says. “I get asked by Americans all the time if I can recommend a book, and I laugh internally. No, there is not a book, but there is an amazing community of great people who know more than I alone ever could.”
For anyone truly curious, she recommends attending the Barbados Rum Experience, held every November, or one of Lallemand’s Alcohol School sessions in the Caribbean. At either event, you can start to make those essential connections.
“You just need to take the long-term time to build meaningful, non-transaction relationships with them,” she says, “and then the real understanding will come.”
