I think it is finally time to delve into some yeast topics regarding rum. I’ve sort of avoided it though Arroyo states you should always start with yeast selection. Factory design and equipment purchases, Arroyo explains, are dependent on yeast choices. How fast they ferment and their alcohol tolerance dictates how many vats. Do they produce aroma worthy of batch distillation or should you go continuous? It all begins with yeasts.
When I first read the works of Percival Greg in Jamaica and his descriptions of yeast no. 18, I became wildly curious about Schizosaccharomyces Pombe. Pombe is a fission yeast as opposed to a budding yeast so they divide differently. All brewers yeasts are budding yeasts and pretty much no beverage uses Pombe yeast but rum (industrial wineries are exploring Pombe to reduce malic acid because they can metabolize it into alcohol). The name Pombe is Swahili for beer and it was first identified in African millet beers in the late 19th century. It has been explored in conventional beers, but is known to produce off aromas in the context. Not even all rums use Pombe yeasts, but typically those of the grand arôme tradition do (I use that term without nationality because I like the ô). Geneticists have been studying them as a model organism for looking at mutations because of how they divide, so when searching for information you have to wade through a lot of genetic science.
Yeast type seems to be something beverage people have forgotten about and connoisseurs don’t even know to make the distinction. If used properly, Pombe can make the heaviest, most suave of rums, but they can also be used to make straight light rums. Many producers inoculate with them and many other producers accidentally end up with them. Among the serendipitous category, some end up producing a heavy type of rum while most others sort of sabotage their chances of grand arôme by other contingent practices. Typically, producers of heavy rums, are keener on their science and with great care, coax their Pombe yeasts into producing tons of gorgeous aroma.
[Fermentor at Hampden estate in Jamaica, likely a top fermenting Pombe yeast and closely matches the description of Percival Greg’s. Image credit Rum Gallery]
Schizosaccharomyces Pombe yeasts have sub categories just like budding yeasts and there are top fermenting and bottom fermenting varieties. As a rule of thumb top fermenting yeasts produce a fuller bodied aroma and ferment even slower. Pombe yeasts do not produce all the aroma themselves, and the slowness of their ferment means that favorable bacteria can take hold and produce desirable aroma compounds. It also leaves room for undesirable bacteria and Pombe yeasts typically require more care which is why many distilleries avoid them.
Pombe yeasts relate to the dunder process because as a rule of thumb they have significantly more osmotolerance than budding yeasts. This means they can withstand the stress of high acidity, high initial sugar contents, and high contents of dissolved minerals that accumulate in dunder. Jamaica and many other areas likely ended up discovering Pombe yeasts via sloppy fermentation practices where things weren’t measured properly. Empirically it was revealed that dramatically different results could be gotten. There was a time where distillers were just learning about bacteria and how it was inhibited by acidity, so to protect yields they were heavy handed and eventually ended up with a rare yeast type. When acidity is high, Pombe will take hold as the dominant yeast, but bacteria will not and this is a way to get particularly light rums as a result of using Pombe. This idea may have great bearing on older Haitian rums pre-1980’s and many small production Cachaças.
It was initially thought that high acidity correlated to fuller flavored rums but that isn’t exactly true, the Jamaicans were sort of wrong and Arroyo straightened it all out. More on that later, because it gets really profound.
Another point I should make is that fission yeasts relative to budding yeasts have a low frequency of occurrence. That is why they are typically not dominant, but can be made to be dominant as part of a community by messing with fermentation kinetics. The low frequency of occurrence, coupled with being abandoned by the industry, and the rise of in house research by certain sophisticated firms means there are no industrial catalogs of Pombe yeasts to choose from like there is with budding yeasts. There is a Pombe society, but it is a group of geneticists and not a group of rum aficionados.
If you want to mess with fission yeasts, you need to round a bunch up and have them go through a fermentation Olympiad. This is what Jamaican had done very early on and their Agricultural experiment station would send a new yeast out at the beginning of the season. Later in the season they would find that it often wasn’t still dominant. Arroyo conducted his own Olympics with some extra wild results, and the French have held their own games for themselves. Arroyo doesn’t say much on the actual methods of discovery, but he does describe his pentathlon better than anybody.
The big test is the yeast’s ability to produce rum oil, which is the mysterious prized metabolite and something fission yeasts produce more of than budding yeasts. Pombe yeasts have to be found at random (a strategic random), purified, then run through feats of strength. After they pass the rum oil tests, they have to go to the fusel oil event. For grand arôme rums, fusel oil has to be minimized during fermentation because fusel oil and rum oil fractions overlap in the still. If you use a fusel oil separator in a continuous still, or a decanting technique in a batch process, prized rum oil will be lost, so unlike other spirits, rum has to be that much more in tune with its yeasts. You can ferment molasses with yeasts, distill it, and not get rum, (but my God will they try to call it rum!) They will also probably try to adulterate it instead of learning more about fermentation. Rum is that much more profound than other spirits.
I have long wanted to bring fission yeast to New England and I’ve slowly been hatching a plan. My initial plan had to be scrapped as I learned much more. You cannot exactly get a pombe yeast from the catalogs. I mean, you can for $350USD, but it has yet to be vetted and win a gold medal, and it has to compete against a lot of other candidates so ultimately you would need a few thousand dollars just for your first cultures and they’d come with no story. Rum has to have a story, and typically they all do, more so than any other spirit.
I plan on taking a slightly different tack, I am going to do discovery myself, and then hold my own Olympic trials. I will try wide ranging local discovery with various fruits, but it doesn’t look promising (I need to verify that hunch is correct regardless). Isolating budding yeast is easy, but even when understanding their typical kinetics, isolating fission yeast, with their low frequency of occurrence is hard. I have hatched an extra novel plan which would get me a standing ovation with the yeast biologists if I can execute it. I have an extraction protocol with a track record of success and I’ve already been collecting exciting sample media from around the world. Arroyo isolated near all his contestants from sugarcane, but I do not think that is the only way. I’ll have contestants coming from as far away as Sardinia and I have eight potential contestants from within Minnesota alone, that with genetic testing I can give first names to. Will any be the same or will they all be unique? And what will my distribution of tops and bottoms be? Or am I completely wrong and all the gold medal winners must come from sugar cane because they are best adapted? There is only one way to find out! I do have a yeast biologist partner in crime for this so I’m hoping the protocol will be successful.
My initial plan was to understand Pombe kinetics enough to create dominant cultures in varying mediums like apples, raspberries, foraged blueberries, etc very much like what is rumored to happen at Hampden estates. This would also come with free riding bacterial communities. This plan was dashed because though I could possibly construct a starter one way and build a large footing to optimize aroma and fusel oil production. The rest of the wash would be very different and the community would change significantly. An entire community from a starter would bring an elegant sense of terroir, but it would not be correlated to the most extraordinary of sensory values. Until I learn more, my initial best bet is Arroyo style pure culture fermentation.
The majority of the rum industry that found themselves with Pombe yeasts have abandoned the practice and in many cases because it didn’t meet their objectives or technical abilities. For a long time, and for much of the world without any established reputation, rum sold largely on price. Being most competitive required continuous stills which need continuous streams of wash to run. Vats require capital expenditure and to feed the hungry stills, a quick fermenting budding yeast could turn around a vat much faster, thus reducing expense. In many scenarios Pombe yeasts also have less alcohol tolerance than budding yeast which means more energy to separate all the extra water. Arroyo and a few other consultants tell tales of massive distillery wastes due to incomplete fermentations. Pombe yeasts are never singled out as at fault, but these distilleries were never testing the boundaries and limits of their workhorses. Biological control requires chemical control and if you don’t even have that, you just cannot mess with fission yeasts.
We are going to see a fission yeast renaissance. The first step is recognizing their current usage on the market and their usage historically. Did New England rum at Felton & Son’s ever use a fission yeast? They did make very full bodied rums and were using dunder. There is one untapped resource left to possibly find this information (and I’m working on it!). Is Hampden estates using the most noble of grand arôme fission yeast processes? Does Cape Verde end up using a fission yeast? In the previous post I attributed much of their character to cane juice not being defecated or clarified, but if a cachaça producer could end up with a fission yeast, why couldn’t a grogue maker? Answering that question may be as simple as asking how long their fermentation lasts.
The next step is bringing it to the new distillery movement. There are countless new distilleries producing rum but do any of them have progressive yeast labs and is anyone holding systematic trials for their yeasts? (Hint: partner with a university!)
Yeasts we will see are where it all begins, but they guarantee nothing if not followed up with chemical control and more in depth understanding of their specific metabolisms. Next, and after I gather a little more information, we will move down the road and look deeper at comparative fermentation options for rum making.