[I’ve done a ton on this topic since and gathered upwards of twenty rare papers on the topic. I’ve been too busy to index anything, but if you search through the posts they can easily be found.]
The previous post contains an account of making Jamaican rum from a 1911 text on Cane Sugar from a renowned sugar technologist at the experiment station of the Hawaiian sugar planters association. The account very briefly explains the various cisterns used for preparing all parts of the sugar wash and uses the (new to me) term muck hole as opposed to the term dunder pit which many rum talkers like to throw around. True, Jamaican rums had dunder added [and this it turns out is ripened with bacterial fermentations], which just implied stillage, but they also had a quotient added called flavour, which is the legendary re-fermented portion. Not all of Jamaica made heavy, flavoured or German rums, they also made clean rums. Many people today are confused on what style of rum is represented by Wray & Nephews OP or Trelawny OP. They are unique relative to other clear rums, but probably do not see any of the flavouring technique.
“If common clean rum is being made, stick to common clean and never allow things to drift in the directions of making flavoured rum in the pious hopes that you may wake up some day to find that you have become famous by making flavoured rum where it was never made before. You are much more likely to find an enfuriated Busha awaiting to tell you that your services are no longer required on that estate.”
Searching google books for “muck hole”, many great explanations of Jamaican rum production come up as well as one particular old text that is basically the holy grail tell-all of Jamaican rum making at the beginning of the 20th century. I do not not believe this text is known to popular culinary or even the new distilling scene.
The text is pretty amazing and has staggering amounts of data on experiments conducted. The PDF was scanned poorly and is not searchable, but the content is so historically significant I might be tempted to re-type parts of it over so they are easier to use. Previously, I did not believe there were any works this scholarly being done at this time period concerning rum. It almost seems more advanced than works concerning whiskey or brandy and isn’t listed in any bibliographies that I know of. There is even an appendix of “Lectures on fermentation in relation to Jamaica rum as delivered at the Course for Distillers at the government laboratory in 1906 by Charles Allan, B.Sc.” (PDF p. 284). A likely reason for the advanced nature of the content relative to works of the same time by Scottish researcher S.H. Hastie is that Allan had carte blanche access to whatever he wanted with no legal restrictions unlike Hastie who was severely constrained by the rules of the excise officers.
The text is a compendium of three sections written over three years and at the end of each section rum production is discussed and the author’s handle on the subject gets better and better until finally he pretty much unlocks the secrets of muck hole bacterial fermentations.
Solids from the dunder go into the muck hole. These solids, which are pretty much completely composed of high acid spent lees, undergo a particular bacterial fermentation which produces increased amounts of fatty acids, notably butyric. The muck hole is essentially a pH sensitive bio reactor that is started and stopped constantly by the addition of alkaline lime marl. Besides stalling out with too low a pH, if the muck hole was neglected, the prized fatty acids would continue to break down into simpler molecules like ammonia, but when lime is added and the pH rises, fatty acids are also locked up as salts. Muck can be drawn off or more dunder solids added and the process restarted. Many rum talkers claim the content of the pits could be decades old but I suspect the break down of chemical compounds into undesirable forms like ammonia would not permit this and the contents rather were/are at most only from the previous season’s production.
A wash for a Jamaican rum is composed of sugar cane skimmings, dunder, acid, molasses, and flavour. Deconstructing all these terms is tricky and here is my best shot. Sugar cane skimmings could imply fresh sugar can juice [it is really the raft of coagulated proteins that float to the top with other stuff when you boil cane juice], which was known to be added to Jamaican rums. Dunder here is stillage from a previous distillation similar to backset used in the sour mash process and it often goes through bacterial fermentation as its held during the season. Acid, believe it or not, implies sugar cane vinegar and its role is a clever chemistry trick I’ll discuss next. Molasses is the molasses you’d expect, and flavour, finally, is the muck.
The muck is full of lime marl / fatty acid salts which are essentially locked up in a non-volatile form and needs the acid (again also said as sugar cane vinegar) to unlock. I learned about this concept intimately when creating the Tabasco aromatized gin recipe for my Distiller’s Workbook. The acetic acid in the Tabasco needs to be locked up as a non-volatile salt using baking soda so it does not carry over into the distillate. The chemistry concepts are also masterfully explained in Peter Atkins book Reactions. In the Jamaican rum context, the addition of acetic acid to the muck changes the bonds between the lime marl and a portion of the other fatty acids releasing them to participate in future reactions such as acid catalyzed esterification. So the most common shortest chain fatty acid, acetic, trades places with the longer more noble fatty acids created in the muck hole and become linked up as salts with the lime marl [I think over the years, acetic acid use went away and sulfuric acid became more popular].
The author gives the proportions of sample mashes but doesn’t explain how they are assembled. The muck and sugar cane vinegar could be thrown in with all the other components or left to react independently and then the newly formed lime marl / acetic acids salts separated and the more noble mixture added to the skimmings, molasses, and dunder. The latter option makes the most sense from a chemical perspective.
“Distillery work”, PDF page 471 is also worth a look.
Using google books, five more references were easily findable describing the muck hole and the use of lime. For some reason, none of the PDFs are searchable nor can text be copied and pasted from them. The two 1913 sources and the 1920 seem mostly plagiarized from each other.
The Chemical Age Volume XVIII July-December 1913
The School of mines quarterly A journal of applied science vol. XXXIV 1913
Food Products by Henry Clapp Sherman 1920
British and Foreign Spirits by Charles Tovey 1864
West Indian Bulletin Great Britain Imperial Dept. of Agriculture for the West Indies Vol. VI 1906 (this book looks especially cool!) The manufacture of Jamaican rum is discussed on PDF page 584 and is a summary of Charles Allan’s work in Jamaica which is quite good and fills in some pieces missing in the text from the experiment station. It gets interesting when he starts to paint a broader portrait and gives his opinions of the industry.
Once these imperialist chemists unlocked the secrets of the process, they also uncovered serious inefficiencies. Large amounts of sugar go wasted in each step and some processes were left to run away creating wastes. Spirits production was still very competitive back then and the authors discuss whether it was worth it to cut yields to make a higher ester product at the hopes of making a higher profit. It seems like changing distillery practices incurred more risk and often was just a break even proposition. Advances slowly moved forward over the years probably until we get to Raphael Arroyo’s work on heavy rums patented in 1945 where the techniques used today pretty much get settled.
To quote Arroyo:
It has now been found that heavy rums of excellent type and with high yields and fermentation efficiencies can be obtained by a procedure comprising:
1. The subjection of the raw material to a pre-treating operation which fits it for its intended use.
2. The selection of yeast and bacterial cultures adapted for symbiotic fermentation of heavy rum mashes.
3. The employment of optimum conditions for the production of alcohol and symbiotic fermentation for the production of aroma and flavor, wherewith to obtain high yields and fermentation efficiencies with a rapid fermentation, and a high quality of final product.
4. The employment of a proper distillation method for the resulting beers.
In the Arroyo technique, no dunder or muck hole is used but rather controlled inoculation of selected bacteria in the main ferment coupled with other tightly controlled fermentation variables. Looking at the balance between tradition and innovation it wouldn’t be surprising if for the sake of tradition Jamaica used a modified version of the Arroyo method where the bacterial fermentation was relegated to some sort of tightly controlled cistern / muck hole / dunder pit. One interesting thing to note in Arroyo’s technique is the way he uses alkaline lime during production.
“The addition of the milk of lime during the initial stage of the pre-treatment process has three main purposes:
1. It prepares the medium for the development during fermentation of the most important ingredient in the aroma of heavy rums, being the essential oil or mixture of essential oils known as “rum oil.”
2. It neutralizes the free fatty acids which are always present in molasses, thus eliminating the danger of their volatization during the heating operation which immediately follows, but permitting the reliberation of these fatty acids from their calcium salts upon the sulphuric acid addition to the already cooled thick mash in the second stage of the pretreatment, so that they are then available for the formation of valueable esters later during the fermentation period and under the catalytic action of the esterase produced by the yeast.
3. The disturbance produced in the medium through the alteration of pH value occasioned by the milk of lime causes a copious precipitation of organic bases, molasses gums, and mineral ash constituents of the molasses, and this precipitation is enhanced by the action of the heat applied.
The works of the sugar cane experiment station have been of immense value and it wouldn’t be surprising if other similar works exist for the other islands, particularly those colonized by the English. Maybe there is a text out there that explains the significance and ins & outs of wooden boilers as opposed to copper [I just found this in a Barbados document].
A completing scanning of Raphael Arroyo’s rare text Studies of Rum (spanish) can be found here.
More from the Journal of the Society of the Chemical Industry, volume 26, 1907 which features a very interesting comment section.
The first named needs no special description. “Skimmings” consist of the scum which rises during the boiling of the cane juice. Before they are allowed to undergo acid fermentation, either alone or in presence of the crushed canes (or “trash”). “Dunder” is the spent wash from the stills.