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It is looking more and more like Rafael Arroyo’s mythic rum oil is nothing more than rose ketones. Luckily, the rose ketones are among the greatest beauty the world has ever known. Your immune system bends around them. I hypothesized that their presence reduces hangovers. Arcadi and other perfumers with intimate experience have hypothesized mildly psychotropic effects.
The global rum industry saw $2.6 billion in sales in 2015 while the perfume industry saw five times as much with $12 billion (in only U.S. & E.U.). These aroma compounds are big business and should become a spirits industry target at both the level of production and marketing.
After wandering through the perfume literature [1,2], probably the best primer on these compounds comes from Alvin Williams of Firmenich, Rose Ketone: Celebrating 30 Years of Success. Truly a beautiful must read.
And remember, this glorious rose petal smell was found recently in Hampden’s Rum Fire. Back in October, Vivian Wisdom, distiller of Rum Fire, told us in Jamaica: “If its missing, its not a Hampden rum.” In the last year or so Hampden has been continuously and deservedly ascending as an industry darling. No doubt, the radiance of rum oil is at the heart of their success.
What I’ve learned is that many spirits have these compounds, but an exemplary rum can possibly have four times as much as a single malt, bourbon, or Cognac (regarding damascenone).
These compounds are derived from carotene which is ubiquitous in substrates. Even corn has it, but the unique processing of molasses as well as careful production, holistically sets rum up to max it out.
- Rum oil, and more likely its precursors, can be formed during molasses processing. These days, re-processing may maximize the potential. We know precise liming and time under heat are key variables. Possibly even cane varietal.
- Fermentation may unlock rum oil from precursors. Unique enzyme activity by yeast is the key. Schizosaccharomyces Pombe is thought to be the queen yeast, and besides enzyme activity, Pombe may also have other key features such as high osmotolerance, low production of fusel oil, and tolerance of symbiotic bacteria. Many bacteria are also known for contributing enzyme activity.
- Resting periods, which may also be called staling, are key. Rum oil can be unbound from precursors by heat, but also by cold processes in the presence of acidic conditions (buffered but well below 5). Staling integrates with high acid fermenting techniques. Slower, top fermenting Pombe yeast tolerate low pH and benefit from staling before distillation. Acids produced by bacteria further aide the staling process. A portion of what we think are long duration ferments is the staling phase. Risk likely escalates during staling, but no doubt new strategies can be developed. Staling is also expensive because of the capital costs for all the vat infrastructure as well as loses due to evaporation. Staling is the most poorly understood channel for rum oil production and possibly the most important phase.
- Rum oil is formed during distillation and catalyzed by high acidity. Ester formation benefits in parallel. If you target rum oil, you will also accumulate ample esters. Everything is holistic and interrelated. Time under heat, just like during molasses processing, creates more precursors that can be unbound during the next fermentation cycle. Pombe yeasts often multiply in dunder as it ripens and those new precursors may benefit from enzyme activity. The dunder process is key to maximizing rum oil, however, Arroyo did not use it.
- During maturation, rum oil may be concentrated by the angels share. It may also be revealed as more abrasive facets of the sensory matrix erode. It is not formed during maturation and may even come out of solution as high proof spirits are cut to bottling proof. To limit aroma breakage, distillation needs to proceed at as low a proof as possible (fermentation variables have to be there to justify this). Some exemplary Jamaican producers distill at fairly high proofs, but they avoid breakage by diluting not with water, but with lighter bodied spirit.
- Lastly and critically, the fate of rum oil is bound to fusel oil because it is less volatile. To push the rum oil into a spirit, fusel oil must be managed at multiple junctures. Always think holistically.
Production is holistic and there are multiple avenues to accumulate rum oil. The extended time under heat of molasses production may get the ball rolling, accumulating precursors, and may explain why cane juice rums do not yield appreciable rum oil. Some channels of formation are likely more economically viable than others and there is a bang for your buck.
Many heritage techniques in rum production we attribute to seeking esters likely target rum oil. Murkier processes, such as extremely long duration ferments at extremely low pH, may even work against ester formation, but absolutely favor rum oil production. Accepting the rose ketones as a new set of congener targets may help us truly understand processes which, from the sidelines, we may have been over rationalizing as for esters.
In many cases, it may be possible and viable to track the formation of rum oil at different production junctures. It may however be daunting (possible but not viable) to track the precursors.
These compounds are the drivers of modern perfume and known as radiants with incredibly low thresholds of perception. Rum is therefore the radiant spirit. These compounds have perceptual abilities to tie together other odorants into more extraordinary percepts. Sensory quality may depend on these perceptual bridges. They are not merely rosey. Isolated rose ketones can resemble tobacco, leather, and complex dried fruits. They can prevent esters from, on their own, being perceived as plebeian and ordinary (the grape drink / cheap candy trap).
Fractional distillation by birectifier has been shown to be the most economical way to work in this territory while in chromatography it has taken exotic highly involved forms beyond what the distilling industry uses day to day.
Rum making is the act of seeking out these astoundingly beautiful, nearly divine, odorants. You may have the same ingredients, but if you don’t try for them, you’re not making rum. This concept is even tied to the etymology of the word rum and its very early legal regulation. The Caribbean has long had rum with rum oil, while New England and continental European producers did not. Many felt that legally nothing without the correct characteristics should be called rum. We are getting closer to returning to that framework while also getting closer to democratizing the techniques that produce authentic, radiant, rum.