Birectifier Analysis of a Mildew Yeast Turbo Charged Rum

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What we have on our hands is an experimental rum from New Zealand by Callum Upfold that featured a staged fermentation of two days with a mildew yeast followed by 7 days of alcoholic fermentation with a pure culture S. Pombe fission yeast; all on molasses. A few of us have been exploring mildew yeasts, but Callum is known to have by far the best strain and results. Mildew yeasts grow as a film on top of the ferment and typically metabolize proteins or particular amino acids to produce aroma compounds while typically not producing ethanol. At their best, they can add a layer of high value aroma to a rum before alcoholic fermentation takes place. Because of what they metabolize before fermentation commences, they can also in theory reduce fusel oil formation in a budding yeast. However, this is a fission yeast fermentation which is already a low fusel oil producer.

One thing we can say about this rum is that Callum produced an ester profile associated with percentages of fresh cane juice usage while only using molasses. His mildew yeast somehow unlocked potential in the substrate for aroma formation is that is typically dormant. The rum also has a radiant feature that is a little bit hard to pin down chemically but easy to notice. The aroma can fill an entire room. Radiance is basically the idea that obscure congeners like rose ketones (and a few others), when combined with esters, can lift their aroma very significantly and make everything easy to perceive as more beautiful. The phenomenon is well described by perfumers. Congeners capable of radiance may also change thresholds of perception making it easy to feel the overall aroma of a rum is massive and exuberant. There is nothing finer than radiance in a spirit, but its ins & outs are still mysterious.

Despite being tightly wound in a 69.5% ABV, I knew right away this rum would have a very significant fraction 5, full of high value aroma. What was not apparent until I fractioned the spirit with the birectifier is that the rum would be fairly light in ethyl acetate. This puts it somewhat in line with Arroyo’s suavity aesethetic which he claimed made a spirit mature faster. Arroyo described his mildew yeast as producing the ester, ethyl tiglate, which I believe I have personally observed in experimental ferments. However, if this rum had it, it was not obvious with this means of analysis. Whatever the mildew yeast created was powerful, fruity and completely in line with current expectations of rum.

This rum and its mildew yeast technology begs to be brought to market and see full scale aging. No doubt it would pick up more ethyl acetate from the cask and fill out its sensory matrix from non-volatile cask extractives. The estery-ness of this rum is likely subject to aroma breakage from watering. The literature tells us it would benefit from reduction in ABV by maturation with the angels share. If it went in at 70%, how many years would it take to reach a drinkable 50% needing no dilution with water?

Special & extraordinary stuff.

Fraction 1: Not overwhelming or concentrated to the point of non culinary aromas. Safe to say this rum is light in ethyl acetate.

Fraction 2: Fairly neutral no doubt because fraction 1 was light in ethyl acetate.

Fraction 3: Very neutral as expected.

Fraction 4: Below average fusel oil as is typical of a fission yeast.

Fraction 5: Visibly louched. A very strong concentrated estery character that seems a little more radiant than usual. When you take the watch glass cover off, stuff is leaping at you. Nuance isn’t really perceivable and what you sense is one very strong concentrated tone. There is some kind of radiant effect going on, but you cannot parse the character like the menthe expression in fraction 6. Not acrid on the palate and almost oily relative to other fraction 5’s I’ve tasted.

Fraction 6: Estery character. Very slightly sweaty like a free longer chain volatile acid. Possible radiant menthe character like TDN.

Fraction 7: No significant gustatory acidity. Faint aroma. No negatives. What is there is appealing.

Fraction 8: No significant gustatory acidity. Faint aroma. No negatives. What is there is appealing. On the second day, when the air was very clear, a radiant estery character was easily perceptible in the headspace.

Stillage: Residual aroma, but no significant gustatory acidity.

3 thoughts on “Birectifier Analysis of a Mildew Yeast Turbo Charged Rum

  1. Could it be possible to give more information regarding the mildew yeast?
    Is it mold, yeast or is it what we call “mildiou” in french? I am currious how mister Upfold managed to inoculate his molasse mash and his source of mildew yeast.

    Thank you

  2. Hello FB,
    It is a mildew yeast. It produces a ton of enzymes that react with simple proteins, amino acids and alcohol in the wash to produce high value esters and congeners.
    It has been known to even produce ketones like those found in tobacco leaves and fine wines.
    My particular strain seems to be very versatile and robust. While many more common strains struggle in liquid media(especially something as harsh as molasses.) Mine seems to perform quite well in just about any wash or wort.
    If you would like to know more or if you are interested in discussing anything further you may email me at

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