[If you are a distiller, make sure you know about the birectifier]
In Arroyo’s 1945 Studies on Rum, he presents two different paths for symbiotic fermentations to produce full bodied rums. The first path uses a bacteria while the second path claims to use an Oidium (a mould), but recent research shows it may actually turn out to be a type of alt yeast.
Where did he get the idea anyhow? Arroyo has pretty much no bibliography other than the classic rum texts, but appears to cast a wide net and is well versed in emerging ideas in bio technology. He mentions finding this “mould” on the sap of a tree in a shade grown coffee plantation (shade grown coffee is really interesting). He does not say exactly what specific tree so it is hard to pin down because shade grown coffee plantations are known for spectacular diversity.
Besides, a mould of the Imperfecti group, Oidium Suaveolens, was also found very well adapted for the production of a special type of heavy rum.
Pages later he tell us more:
Another special type of heavy rum was produced during our studies and experiments. This time the raw material used was sugar cane juice. The yeast strain used was No. 764 and the auxiliary ferment was a member of the Fungi Imperfecti, Oidium Suaveolens. The Oidium was found and isolated by the writer from the sap of a tree much used in Puerto Rico for shading coffee plantations.
A study of this Oidium revealed that it would grow very fast in cane sugar juice media with the production of a thick film over the surface of the liquid. It was further discovered that it hardly touched the sugars in the medium, but that it was a good producer of esters and organic acids from the proteins of the raw material. A fragrant odour, very similar to that of ripe apples was the predominant aroma observed.
This Oidium was used as an auxiliary ferment for the production of heavy rums from sugar cane juice in two different ways: (a) a sterilized sugar cane juice mash of from 12.0 to 15.0 per cent total sugars was inoculated firstly with the Oidium culture. After the Oidium film was formed on the surface of the medium it was allowed to act up it for a period that could vary between 24 and 72 hours or more if desired. Then the mash was inoculated with an active footing of yeast No. 764, and the fermentation was carried to completion. (b) In the second method the yeast was allowed to operate alone in the substrate, and towards the finishing of the alcoholic fermentation the Oidium culture was inoculated. The Oidium fermentation was allowed to act then for variable number of hours, as desired.
Both methods worked satisfactorily in the creation of a new variety of heavy rum out of sugar cane juice mashes; but the rums obtained differed somewhat in each case, those produced by method (a) being of intenser taste and higher aromatic tone.
Many databases exist like the Global Biodiversity Information Facility that have listings for the organism, but they contain no spelled out history of discovery that may elude to where Arroyo got the idea (A Russian database points us to Geotrichum Frangrans which can be purchased here from the American Type Culture Collection ATCC). Arroyo did name the organism correctly as Suaveolens, and calling it an Oidium may point to the Russian biologist Krzemecki who possibly discovered it in 1913 based on language included in the GBIF entry. Arroyo may have also been hip to the organism by reading the mycologist Christine Marie Berkhout. Or maybe Arroyo never read her 1923 doctoral thesis that (quoting wikipedia) was later described as marking “the beginning of the rational systematics of the anascosporogenous yeasts” (I’m way out of my depth, but trying to bridge the gap between Arroyo’s mould and the recent researcher’s recategorization as yeast. The difference between a mold and a yeast is that molds grow with multi celled hyphae while yeast’s are single celled).
We are building up to some links to great modern research papers, but we should pause for a moment. The whole point of this exercise is to (a) celebrate how fucking cool Arroyo was, (b) help modern rum writers who may talk to producers and find evidence of these techniques used in a production, (c) help new producers jump off on this, and lastly (d) celebrate the contemporary researchers who will help us bring more of Arroyo back to life and create new exciting styles of rum.
Contemporary research on this organism and my realization that Arroyo may have been incorrect about it being an Oidium are lead by Thomas Petit and Eric Grondin working on the island of Reunion off the coast of Madagascar.
This brief info graphic style paper by Petit mentions participation in a European COST (cooperation in science & technology) bioflavour project. After scouting yeasts, Suaveolens, came up as their most significant flavor producer, specifically producing the valuable ester, ethyl tiglate (a known semio-chemical #pheramone).
To back track a bit, from an old text that summarizes abstracts, we can glean a little bit of the interest from 1923.
A study of Ester-Forming Yeasts.
Ulrich Weber, Biochem. Ztshr., Berlin 129:208, April 19, 1922.
Experiments are described that sought to determine the conditions under which the formation of fragrant esters takes place in some lower fungi. The question was dealt with by physiologic experimental methods. There were employed Willia saturnus Klöcker, Oïdium suaveolens Krzemecki and Siachsia suaveolens Linder. These organisms were raised in pure cultures in nutrient glycerin and mannite solutions under different conditions.
Results showed: In the observed yeasts and imperfecti fungi the ester odor typical for normal cases is not developed under all conditions. Cases occur in which, in spite of the most abundant development, no ester formation takes place, as in the case of growth in a carbon dioxide atmosphere. Esters are formed only when the simultaneous fermentation of carbohydrates assumes the role of sugar fermentation and liberates the energy requisite for the decomposition of albumin. Addition of alcohol enables a qualitative alteration of the ester odor to be attained. The employment of different nitrogenous nutrient media achieves an alteration of the odor only when other amino-acids are thereby presented simultaneously. Following addition of leucin a distinct odor of amylester is perceived. The ester odor of the species here investigated, which is always observable under normal conditions, is therefore capable of being influenced experimentally both qualitatively and quantitatively, as it is possible to alter both the character of the odor and also to prevent its occurrence in spite of the best development of the fungus.
The Yeast, A Taxonomic Study has some useful information on understanding what exactly the Saprochaete of Saprochaete Suaveolens entails.
Swedish Wikipedia provides a great bibliography.
And finally we come to a few spectacular modern research papers: