Hogo. Funk. The smell of overripe banana. Jamaican rum is uniquely beloved in the spirits world for its powerful, easily identifiable pungent fruitiness. Connoisseurs of Jamaican funk utter phrases like “high ester count” and “long fermentation.” But nothing gets the rum nerd more enthused than talk of “dunder,” the mysterious ingredient that allegedly makes Jamaican rum extra funky.

Exactly what comprises dunder is shrouded in mystery, but dig around a bit and you’ll find references to goat heads, dead bats, and worse, churning in a lethal, volatile mass of evil death stored in an earthen pit, presumably somewhere near the distillery for easy access. Heck, even I’m guilty of spreading these tales. Throw a few scoops of this black death into the molasses wash, and voila! Instant hogo. Or so many people think. The reality is far less simple—and way more interesting.

As part of an ACR tour of Rum Industry influencers, including Martin Cate, Camper English, Peter Holland, and Wayne Curtis, I received an intense, behind the scenes look at Hampden Estate, ground zero of high-hogo rums. Standing in the hot, dimly lit fermentation area with distillery manager Vivian Wisdom, we grilled him for nearly an hour on every aspect of how the wash that goes into Hampden’s stills is created. No detail was spared: Fermentation times, pH levels, quantities, we wanted to know all.

The first and foremost thing we learned is that dunder isn’t what most people think it is. At every distillery we visited in Jamaica, the term dunder was used to refer to what’s left in the pot stills after a distillation run. The common distiller’s term for these remainders is stillage. Most Jamaican distilleries take their dunder (stillage) and dispose of it, either spreading it on the cane fields under controlled conditions, or processing it to make it less environmentally harmful. However, Hampden retains their dunder in wooden vats for use in subsequent fermentations; I’ll come back to exactly how in a bit. It’s important to know that adding stillage to the wash for subsequent distillation runs isn’t unique to rum. There’s a close parallel in the bourbon world, where it’s called sour mash.

[Editors Note: Vivian recently allowed me to share a presentation he did on high ester rum techniques. You can find it here.]

Important: What follows may shock you. Rest assured, the act of distillation effectively separates the ethanol and flavorful organic compounds (i.e. the finished rum) from the scary things that go into the wash prior to distillation.

If dunder is just stillage, where does the cesspool of vileness that folks erroneously call dunder come into play? Standing on a platform in a room filled with giant tanks of dark, thick dunder liquid, we peered down into a large box eight feet below. Within the box was a dark sludge with bubbles on the surface. It absolutely gave the appearance of a box of bad, bad news, straight out of a horror movie. Vivian told us the ooze is called muck— a giant bolus of bacteria that creates a soup of carboxylic acids. The muck and its vast quantity of acids go into the wash along with several other interesting ingredients you might not expect in rum making.

Standing over the muck pit, I instinctively clutched my camera and cell phone a bit tighter. The smells from the dunder tanks and muck were off-the-chart malevolent. Any right-thinking person would have beat a hasty path to the exit, but nobody in our group was in a hurry to leave. Here, in the heart of Hampden, was the closest any of us had ever come to experiencing what a rum distillery might have looked like a hundred years ago or more.

It’s natural to ask why on earth anybody would put muck into their fermented molasses wash. What purpose does it serve? The answer can be found in my post From Alchemy to Science: Esters, Aldehydes, Mass Spectrometers and Hyper-Accelerated Aging. In a nutshell, and simplifying somewhat, the majority of flavors in distilled spirits come from esters. Esters are organic compounds with names like Ethyl Butyrate, Amyl Acetate, and Ethyl Acetate. They form when alcohol molecules chemically combine with acids. For example, Ethyl Butyrate is formed when ethyl alcohol molecules combine with butyric acid. Fun fact: Butyric acid by itself has the smell of human vomit. But combine it with ethyl alcohol, and the resulting Ethyl Butyrate molecule smells of fruit and pineapple. Likewise, acetic acid, the primary ingredient in vinegar, combines with ethanol to form Ethyl Acetate, which has a sweet smell. Ethyl Acetate is by far the most dominant ester found in rums, but a typical bottle of rum contains hundreds of different esters.

As we talked with Vivian, we got into details like pH levels, acids, and when each component of the wash is introduced. I was particularly pleased when Vivian explicitly called out butyric acid as one of the components found within the muck. With my wee-bit of organic chemistry knowledge, I knew that meant lots of Ethyl Butyrate, i.e. pineapple, in the resultant rum.

Hampden’s addition of a big slug of muck to their mash acts to supercharge the ester production process. While all rums have esters, Jamaican rums are renowned for having very high levels of fruity esters, measured in the parts per million (PPM). A very fruity, funky Jamaican rum like Wray & Nephew Overproof might have an ester count of 100 to 200 PPM. Hampden’s Rum Fire Overproof clocks in at over 500 PPM. Taste it, and you’ll have no doubt what funk is. But Hampden blows that out of the water, making a rum with an ester count in the 1600 PPM range, essentially undrinkable and used primarily for food flavoring. I wrote about this rum, named “DOK,” in my post about E&A Scheer in Amsterdam.

Armed with the knowledge that “muck” is the magic term, I hit up Google and came across an absolute gold mine of a document. Beyond just talking about muck, it describes the Jamaican rum-making recipe in far more detail than I’d ever imagined. The 1906 document, titled “Report on the Experimental Work Jamaica. Sugar Experiment Station,”, can be a challenge to read; the terminology it uses is somewhat arcane. But it is detailed and full of surprises. In excerpting the original text below, I’ve slightly reformatted it to tease apart meaningful sections.

Before you read the text, here’s a quick primer on some terminology you may not be familiar with:

German Rum – Essentially rum concentrate. Exported to Germany for dilution with German-made neutral spirits. This reduced the high import duties that Germany imposed on foreign spirits. This style of rum is differentiated from the “common clean” style, which uses mostly the same ingredients but doesn’t include muck.

Acid – Acetic acid, in the form of cane vinegar. (As implied in other parts of the text.)

Marl — A calcium carbonate or lime-rich mud which contains clays and silt.

Lime — A soil additive made from pulverized limestone or chalk. The primary active component is calcium carbonate.

Skimmings – The debris that collects on the top of the boiling fluids and is skimmed off during sugar and molasses production.

Excerpted text:

Flavoured or German Rum. — These rums are made on estates having old fashioned boiling house plant where the manufacture of sugar is of secondary importance. The usual common clean materials are employed and in addition “flavoured.”

“Acid” is prepared from cane juice or skimmings in the usual way in a succession of trash cisterns.

A “muck hole” outside the distillery is the receptacle for the thick matter deposited from the dunder, and the wash (dead wash bottom) to which is added cane trash and lees. The matter consists to a large extent of dead yeast and is therefore highly nitrogenous. It undergoes slow fermentation and putrefaction and its acidity is kept low by the addition of marl. When ripe it contains large amount of butyric and higher fatty acids, both free and combined with lime.

It is added to a series of acid cisterns outside the distillery where the butyric and other acids are set free. This complex acid material is the “flavour.” The flavour enters the wash after fermentation has begun owing to the presence of acids in it which are injurious to yeast, the fermentation is prolonged and the sugar is never very completely fermented out.

Fermentation lasts 9 to 10 days and the dead wash lies for several days longer. An example of the kind of wash follows: —

Capacity of fermenting cistern 2,000 gallons.

Skimmings (fresh) 620 gallons at 12 brix

Dunder 760 gallons at 24 brix

Acid 220 gallons at 8 brix

Molasses 200 gallons

Flavour 160 gallons at 8 brix

Although it may take several read-throughs, the information it reveals is quite fascinating. While we typically think of rum as made entirely from fermented molasses and yeast, here we learn that Jamaican rum is quite a different beast altogether (percentages are rounded):

30% molasses skimmings

40% stillage from prior runs

10% acetic acid (cane vinegar)

10% molasses

10% muck (aka “flavor”)

In addition, we have some insight into exactly what makes up the muck:

Semi-solid materials settled at the bottom of the wash, pre-distillation.

Semi-solid materials at the bottom of the dunder, i.e. the wash after it’s distilled.

Cane trash–the field residue remaining after harvesting the cane stalk.

Lees – In the context of this document, lees is the residue at the bottom of the still retorts.

To sum up, muck is essentially a biological reactor for generating acids that eventually turn into yummy esters. It’s fed refuse from various parts of the rum production process, and its pH level is carefully nurtured via the addition of marl to keep it in humming along or dormant, as necessary.

Bringing this back to the present, the 1906 wash recipe from above generally lines up with what Vivian told us Hampden Estate uses. Vivian mentioned cane juice, water, dunder, and muck. He didn’t mention acid or vinegar, to the best of my notes and recollection. All the ingredients he listed are churned into a mixture and at the appropriate time (just prior to distillation) are added to the fermented molasses: 11 parts of diluted, fermented molasses to 7 parts of the aforementioned mixture. The mixing occurs in pits in the ground and is driven by injected air bubbles:

Mixing pit at Hampden Estate

So what of these stories about outdoor muck pits in the bare ground? I asked Vivian that exact question. His answer was that they do store a semi-solid version of the muck in the ground, as a form of long-term storage for the dormant bacteria. Distilleries commonly start and stop production. Left alone for long periods, the happy, flavor-causing acids in the muck pit will turn it into less desirable forms. By carefully adjusting the pH of the muck, it can be put into a semi-stable state, where it can be stored in an earthen pit. Vivian said the pit is slightly bigger than the size of a human grave. Although we did not see these dormant outdoor muck graves during our visit, my friend Nicholas Feris of the TheRumCollective.com did, and he shares a photo in his Hampden Estate write-up. In addition, my friend Dan Biondi gave me permission to include his muck grave photo from Hampden Estate here:

Muck grave at Hampden Estate. Photo courtesy of Dan Biondi

While I’ve only grazed the surface of the biochemistry of esters and muck, the BostonApothecary.com blog goes far deeper down the rabbit hole of the chemical reactions going on in the muck pit and rum wash. It’s worth checking out their “Muck Hole” Not “Dunder Pit” post for all the hardcore details.

All of the other Jamaican distilleries we visited told us that they don’t use muck. Instead, they produce their higher ester count rum via a very long (weeks) fermentation process. Also, it’s important to note that all distilleries, including Hampden make different marques (recipes,) and that Hampden may not use muck in every marque.

And what of those tales of dead bats and goat heads? We didn’t see any, but given what we saw, rotting goat heads would simply be yet another day the office for a muck pit. When I asked Vivian about the tales, he shook his head and smiled. His theory: Those tales likely originated back in the day when there were far more distilleries, big and small, legal and illegal. He believes that the stories may be just an attempt to throw off one’s competitors by sending them down a false path—one laced with goats and bats, apparently.

At the start of our Jamaican rum distillery expedition, our tour group adopted the name and hashtag DunderCats. At every distillery we asked about dunder, likely amusing the distillery staff who couldn’t understand our extreme interest in the post-distillation sludge in the stills. But at Hampden Estate we finally came face to face with what we sought, albeit by a different name. The reality of high-ester Jamaican rum turned out to be both very similar and very different than what we thought we knew. Leaving Hampden Estate, I realized how fortunate we are to still have a distillery firmly planted in the traditions of rum’s storied past, yet still making amazing, funktastic rums for the world to savor.