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Recently, I created an index of all the first person accounts of Jamaica rum production, but never found the time to write any synthesis posts. Afterwards, I found myself taking a detour through the Bourbon literature when two of my rarest book bounties came in:
After reading those with a critical eye, when it rains, it pours, and a unique unpublished Seagram manuscript crossed my desk titled “Distillery Practice” and likely written in the late 1930’s. I was saturated in sour mash whiskey!
What became apparent are remarkable parallels between heavy rum production and sour mash whiskey production. The sour mash process has also received a staggering amount of scientific attention to scale up massively while both reducing risk and being energy efficient. Production has been broken down into rational chunks with literature and guidelines for every aspect. The production scale of Bourbon and/or Rye, both spirits we can safely call heavy, vastly dwarfs production of heavy rum the market craves. Rum can learn a lot from the success of Bourbon on quite a few levels, and the Bourbon market is currently training rum drinkers to both pay the price for quality and to crave a full flavor, but what we are going to consider here is of a purely technical nature.
For starters, the sour mash process features both the use of backset which parallels heavy rum’s dunder as well as the deliberate souring of the “yeast mash” which matches poorly articulated processes grouped under the guise of “spontaneous fermentation”. To keep going before we backtrack; sour mash whiskey also features no mineral acids, similar to historic criteria for heavy rum as well as (at its best) no added pure chemical nitrogen source such as ammonium sulfate or DAP. All nutrition requirements should be furnished by particular features of the yeast mash substrate and dunder.
What we’ve glossed over so far is the specialty yeast mash used for growing yeast to seed the main ferment and especially for contributing fine aroma. When a Bourbon mash bill features, corn, rye and malted barley (i.e. 75%, 20%, 5% respectively), a portion of the so called “small grains”, rye and malt, are traditionally (at its best) front loaded into the yeast mash, and treated in special ways to optimize yeast growth, reduce contamination, and develop high value aroma. In this story of comparisons, corn is molasses and whiskey people know you can only expect so much from corn. Yes, “corn whiskey” is made, but has a poor reputation and does not do well in the market. Rye and barley are dramatically more nutritious for yeast than corn and they are also more hospitable to the bacteria that sours the yeast mash (in an allotted amount of time).
Much of Arroyo’s work which Cory Widmayer, Callum Upfold, and I have widely explored duplicating has been focused on maximizing the potential of poor quality molasses and it has been like seeking blood from a stone. Extraordinary things can happen, but they become increasingly technical and special effects must be layered atop which we will discuss later.
Thus, heavy rum has a yeast growth phase with parallels to the yeast mash of the sour mash process and approaches can be borrowed from Bourbon to reduce the risk of the old fashioned spontaneous process. Heavy rum also has substrate parallels where molasses can be safely thought of much like corn (which presents quality and performance limitations) and small grains may parallel other less refined sugar cane products including fresh juice and skimmings. It should also be mentioned that heavy rum has an equivalent of Bourbon’s seldom practiced sweet mash which operates at a higher pH and this best compares to Arroyo’s approach, typically involving no backset, but not always. The high pH approach is more technical, but the reward can be extraordinary.
Soured yeast mash ——> Spontaneous fermentation
Small grains——> Fresh cane juice, “rum canes”, cane vinegar, select varietal juice, skimmings, panella, etc., **add a few secret options
Rum’s spontaneous fermentation typically happens with juice souring at the same time ambient yeast start to multiply while under the soured yeast mash system, the process is made more linear with a pasteurization step to decrease risk and standardize parameters. However, if you go back to the 19th century, soured yeast mash used to look more like spontaneous fermentation where bacterial activity was allowed to overlap with yeast growth. Both heavy rum and the sour mash process embrace degrees of activity by bacteria, but have the same chief enemy which is heterofermentative lactic acid bacteria. Quite a lot could be said here, but this particular form of LAB can basically destroy high value aroma like damascenone and produce acetic acid as well as lactic acid. If sufficient populations develop, it can overcome low pH such as when dunder is introduced with molasses so sanitary conditions are still a must. Volatile acetic acid levels as high as 8.0 g/L can be produced halting even a fission yeast in its tracks at ABV’s below 3.0%. Many 19th century distillers thought they had aerobic acetobacter infecting their ferments, where in many cases they likely had this nasty type of LAB producing acetic acid.
The sour yeast mash process heads off heterofermentative LAB by deliberately using pure culture innoculated homofermentative LAB which only produces lactic acid to drop the pH to levels also associated with heavy rum. The bacteria is chosen to be non spore forming and is sufficiently destroyed by a pasteurization step before yeast are introduced.
Something that should be noted is that the vast bulk of activity by bacteria in heavy rum fermentation does not produce aroma itself; it is typically just LAB producing bulk junk molecules like lactic and acetic acid. In many cases, when activity by bacteria is thought to be aroma beneficial, what is happening is bulk lactic acid is displacing other volatile acids (that we hope become esters) locked up in the heavily buffered substrate. Lactic acid is special relative to mineral acids, or even organic acids like malic or tartaric, because of its low impact on pH per unit of acid. Lactic acid can displace the most volatile acidity in the buffer without a pH drop that effects yeast performance. Heavy rums, just like Bourbon, can have an extremely high titratable acidity while not exactly being low in pH relative to other substrates like grape ferments. If we better understand how lactic acid displaces other desirable acids from the buffer, we can create just enough room for it to be beneficial without any surplus that saps economy.
Heavy rum featuring a fission yeast has the opportunity to use either a homofermentative LAB like Bourbon or even traditional cane vinegar (given certain qualifiers) with volatile acidity present at yeast growth in the range of 3-5.0g/L+ (those numbers are considered huge and are particular only to heavy rum with fission yeasts). Many volatile acids have anti-bacterial properties to some degree independent of their influence on pH so that fermentation pH can operate at levels slightly higher than typically conducted which increases aroma beneficial enzyme activity and allows yeast to work under less stress at higher temperatures. Under optimal parameters, taking advantage of volatile acidity tolerant fission yeasts permits raising pH to a degree that allows true heavy rum ferments to achieve ABV’s significantly beyond what has been achieved with traditional spontaneous processes (5% ABV is traditionally considered exceptionally good performance, but a reliable increase to 6-7% would be a dramatic change on efficiency. The ceiling for ferments considered low risk, harnessing optimal values is not known).
A heavy rum yeast mash may see double heating just like a grain based yeast mash. Grain is heated for saccharification before the souring process and then is heated for pasteurization after souring before yeast are added. Similarly, cane juice may be heated with lime to prepare it to release bound aroma as well as decrease bacteria to favor an inoculated pure culture. Specific grain mashing parameters also split protein so that it may be used as a yeast nutrients which also has significant impact on aroma formation. Cane likely has an equivalent process that can maximize the potential for yeast assimilable nitrogen in the cane while also developing aroma. Nitrogen may have to be quantified and any additional amount supplied from processing recovered yeast (because in the yeast mash, dunder has yet to be added). Fission yeast have the benefit of being below average fusel oil producers and any use of a yeast extract does not cause detrimental levels of fusel oil formation as with a budding yeast.
The yeast mash is pumped to a fermenter and molasses & dunder are eventually introduced similar to the introduction of corn mash, the remainder of the small grains & backset. Formulas exist to calculate the amount of backset to introduce, taking into account both pH and titratable acidity (including VA), which can be adapted to rum production assuming a greater buffer capacity. Due to the nature of the substrates, rum also has the ability to progressively introduce the feed of molasses to reduce the stress on yeast as well as mitigate opportunity for contamination. Use of aged dunder may be considered a special effect and its chemical potential governed by how many precursors from non-molasses products such as fresh cane juice that have accumulated. Dunder derived only from molasses may have low chemical potential for aroma beneficial activity by bacteria.
Many new concepts have to be accepted such as fission yeast needing to be grown from pure culture and screened from a collection for desired characteristics instead of accepted as the product of spontaneous fermentation. Top fermenting fission yeasts are preferred over bottom fermenters. Not all bacteria is desirable and yeast growth & fermentation must be conducted to avoid all possibility of heterofermentative LAB which may be the leading cause of intermittency in open culture heavy rum production.
Production parameters must also be optimized to take advantage of four ester categories:
Ethyl acetate (with awareness of ethyl formate from the point of view of bacteria selection and distillation)
Cane derived esters (vesouté)
Yeast body derived esters (Bauer oil esters)
Ethyl butrate (and other special effect esters like ethyl tiglate)
Heavy rum is acknowledged to have elevated levels of ethyl acetate beyond that of other heavy spirits such as Bourbon. Bourbon accumulates a fair degree of ethyl acetate during barrel maturation in new oak while heavy rum achieves it often by the deliberate introduction of cane vinegar. Fission yeasts also have a unique relationship with acetic acid where they can tolerate it at levels well beyond a budding yeast so that it’s anti-bacterial properties can be harnessed. Fission yeasts may also be able to convert acetic acid to ethyl acetate as a bio transformation or even elongate it to other volatile acids. Formate is often produced by butyric acid bacteria, but beyond a certain level may represent a flaw.
Cane derived esters have been historically referred to as vesouté and are derived from acids inherent to the cane. When standing alone, such as in undefecated fresh cane juice rums, the character can seem hollow and lack persistence leading to polarizing opinions. Many traditions process the juice to remove the character and it is also stripped from molasses. The aroma has parallels to that of Tequila or certain grape brandies like Pisco and chemical comparison could be articulated. Historically, the precursors may have been concentrated in cane byproducts like skimmings and when accumulated creating density of aroma, may have had a different perceived character. These esters may also be significantly valourized by the presence of damascenone which contemporary rums with vesouté do not take advantage of.
Yeast body derived esters contribute generic persistence and fill out the sensory matrix. Their character may have parallels to that of Cognac or any alembic spirit distilled on the lees. At one point in history, they were referred to as Bauer oil after a scientist who developed a yeast extract process. It is possible this ester category overlapped with damascenone to encompass what was frequently called rum oil in the old literature starting with Karl Micko. The addition of muck may elevate this character as well as the use of fission yeast which theoretically produce more aroma from their yeast bodies under certain conditions. Rums harnessing this character can, like Cognac, become overly fatty from free fatty acids and care must be made during distillation for any spirit that is not a blending concentrate to avoid excess which can obscure a spirit’s olfactory clarity (Cognac literature explains this fault well). Free volatile acid associated with yeast bodies is known to have an anti-bacterial effect beyond its impact on pH and can also impact yeast performance (a tell of accumulation may be that the common mycoderma-vini does not grow). Accumulation of these acids must be considered when recycling dunder so that they have no undesired inhibitory effect. Historically, dunder saw extra evaporation to remove these acids or they were salted out with lime and it may be possible to speed up the processing of aged dunder by starting the material at a particular level.
Ethyl butrate can be categorized as a special effect or what I have previously called a complication. Presence of this ester is a historic ideal of heavy rum, and believed to be more prevalent than it is, but likely is only successfully harnessed by a small amount of producers and no doubt subject to frustrating intermittency. Its character may also be traded for a few alternates that are likely easier to control. Many think butyric acid simply has to be generated in the ferment and then becomes ester in either the still or during extended maturation, but that often does not happen to any meaningful degree, despite being an easy to explain pathway. Butyric acid can very probably only benefit a spirit if it becomes ester in the ferment (or offal) as a bio transformation. There are many ways to skin the cat, but one organism likely creates the acid, another the ethanol, and yet another the enzyme that brings everything together (under narrow conditions).
Arroyo, whose method, sees no current commercial practice, used a two organism system where a very specific butyric acid bacteria (we know where he got it) was inoculated into a fission yeast ferment and operated during a carefully structured window of opportunity which allowed ester to be formed as a bio transformation, likely by enzymes from the yeast. It is not definitively known, but Jamaican systems may operate on a three organism system where there is a bacteria, yeast, and possibly a mold acting in concert to produce ester. It is also possible that Jamaica rums may experience multiple variations that create successful bio transformation to butyric ester.
As an anecdote of what can be observed, I was sent a pack of moldy hot dog buns from a collaborator that were reported to smell like pineapple; and they certainly did! There was a white organism, likely a yeast, growing on the surface of the buns as well as a green mold. Neither could produce the aroma on malt agar or when isolated alone (no ability to produce the acid). Nor could the white yeast in the presence of liquid media containing butyric acid. However, the green mold in the same liquid media seemed to produce aroma. What likely happened is the buns were faux brioche and adulterated with butyric acid (to reinforce a buttery character). The yeast produced ethanol and the mold converted that with the acid to butyric ester. There very likely was no butyric acid bacteria present. If the mold could be adapted to fermentation media, it may bring about esterification as a bio transformation at the end of fermentation. There are no certainties this mold could be productive in a rum ferment, but it can offer hints about what to look for when prospecting and experimenting.
An alternative to butyric ester as a special effect may be ethyl tiglate or other similar aromas produced by mildew yeasts. Ethyl tiglate can be apple-like in character but also sometimes like strawberry. Many of these unique yeasts metabolize protein to produce aroma compounds and do not necessarily produce any appreciable amount of ethanol. Arroyo explored one example collected from what was likely the sap of a rubber tree. There is no precedent for a commercial spirit featuring a mildew yeast. We have observed a lot of variation in the ability of these yeast to adapt to rum substrates. Better understanding of the cane equivalent of small grains may be the key.
Many rums feature only one or two of the major ester categories, but a few Jamaican marks have been able to take advantage of all of them at the same time. Arroyo’s heavy rums minimized ethyl acetate, probably saw very little cane derived esters, and then finally were able to take advantage of both yeast body derived esters and butyric ester. Arroyo’s heavy rum also had the advantage that it was able to harness damascenone at levels that other estery rums are not always able to achieve. Damascenone is key to maximizing the value of esters due to radiant perceptual phenomena best described by perfumers.
An open question is where damascenone is produced in a heavy rum process that parallels Bourbon production. Experimentally, we have produced significant amounts of damascenone in poor substrates at high pH using what could be categorized as a sweet mash. It is possible that a substrate with more precursors may produce damascenone at a lower pH, reducing risk. Damascenone has also been known to be produced by very particular late stage LAB and there are hints in the literature that it can be produced by butyric acid bacteria, but all options may depend on ideal substrate and preparation of the precursor. Rum would become incredibly competitive with other categories of heavy spirit like whiskey if it was better known how to control damascenone formation through multiple avenues and collective research should be undertaken to explore the science.
There are a lot of details in between all the concepts presented here so a lot more could be said. If cane juice is presented as being equivalent to a small grain, something that needs more attention is an explanation of why rums produced exclusively from fresh cane juice never achieve the quality levels of rums produced from cane juice in combination with molasses. The answer likely has to due with the lack of dunder in most processes as well as favoring a direct pitched budding yeast over a slower growing fission yeast. Many cane juice rums also choose to defecate the substrate to minimize vesouté because they do not produce a broad enough sensory matrix to valourize the character. Surveys of cane juice spirits have also found them devoid of damascenone that can create radiant character. However, it appears that access to fresh cane or minimally refined products is paramount to achieving increases in quality above molasses alone. Anyone may have access to organisms capable of producing heavy rum, but quality may be capped unless there is access to cane products we can compare to American whiskey’s small grains. Growing yeast in the equivalent of a traditional yeast mash may be integral to aroma develop in ways that cannot be achieved by direct pitching yeast. Distilleries need to invest in the basics of microbiology, but luckily there are strong reasons why fission yeasts are easier to work with than budding yeasts from the point of view of maintaining purity and control of an organism. With pure culture yeast mashing, risk can be decreased and performance increased over a spontaneous process while maintaining all the active principles of highly marketable heritage processes consumers appreciate.
I’m sure this will get rewritten in the future, but I hope this is a good starting point for comparing heavy rum to Bourbon and planning investment and experimentation as well as drawing production insights from another well established body of literature. I could make endless citations from both the rum and Bourbon literature and may write something else that brings in more references.