Days of Dunder and What the Muck?

Days of Dunder and What the Muck?

Dermott DowlingNov 30, '23

Unpicking the Magic of High Ester Jamaican Rum

For the second in our series of exploring the art and craft of Molasses Rum we thought it would be appropriate to dive deep into Jamaican rum which is renowned worldwide for its powerful, easily identifiable pungent fruitiness, often described as hogo or funk. This unique character is largely attributed to two mysterious elements in its production process: dunder and muck.

Days of Dunder: The Base of Funkiness in Jamaican Rum

Photo credit: Spirits News

Dunder is the spent liquid left in the still after the rum wash has undergone the primary distillation (beer-stripping). For each litre of Rum produced there are up to 16 litres of dunder. In other distilling contexts such as US Sour Mash Whiskey making this is referred to as “backset”when it is used in future batches or as “residue”when it is simply discarded or used for livestock feed.

Dunder is used in subsequent fermentations, similar to the sour mash process in Bourbon whiskey. Dunder contains many by-products that aid future fermentations, including dead yeast cells, an excellent yeast nutrient. Dunder contains rum flavours that can make positive contributions to future batches and also helps to acidify the next batch since it has a pH of about 3.3.

However, safe disposal of Dunder has become an issue in larger Rum distilleries because of high levels of biochemical oxygen demand and chemical oxygen demand which must be reduced before it can be safely discharged into the water supply. Dunder, in countries that have vast areas of sugarcane production (like Australia) can also be used for ferti-irrigation since it contains many of the nutrients required for sugarcane cultivation. The use of dunder in future wash fermentation must also be carefully controlled. Stress on the yeast can cause mutations, altering the flavour, alcohol content, and overall consistency of the finished product. Typically, no more than 20% to 35% of a new wash is dunder from previous washes.

What the Muck? Taking Days of Dunder to the Next Level

Muck grave at Hampden Estate. Photo credit: courtesy of Dan Biondi

If dunder is just stillage, where does the cesspool of vileness that folks erroneously call "muck" come into play? Giant tanks of dark, thick dunder liquid, 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.

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 (A calcium carbonate or lime-rich mud which contains clays and silt). When ripe it contains large amounts 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

It is only natural to ask why on earth anybody would put muck into their fermented molasses wash. What purpose does it serve? The answer is from Alchemy to Science. In a nutshell, and simplifying somewhat, the majority of flavours 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.

High Ester Rum: The Result of Days of Dunder and Mucking around with your Rum Wash

Hampden Estate | The 8 Marks Collection | Rum Pack

High ester rum fermentations are usually associated with long and slow fermentation processes. The presence of high esters is also associated with the presence of higher alcohols, usually spicy in nature. Rums with high esters are more full-bodied.

Ester levels vary in different rums from as low as 0-10 ethyl acetate to as high as 1600 ppm ethyl acetate. For ester rums at the lower end of the spectrum (< 200 ppm), normally fermented wash is allowed to age by transferring to a separate holding tank and leaving for at least a week. Fresh cane juice can be added to the wash mixture, giving a different bouquet to the rum.

Higher ester Jamaican Rums are usually associated with longer fermentation times (15-21 days), more acidic media (pH as low as 3), and encouraged bacterial contamination.

There are two main categories of flavour active esters:

Acetate esters

  • Ethyl acetate (solvent like)
  • Isoamyl acetate (banana like)
  • Isobutyl acetate (fruity)
  • Phenylethyl acetate (floral)

Medium to long chain fatty acid esters

  • Ethyl hexanoate (apples and pears)
  • Ethyl octanoate (apple)
  • Ethyl decanoate (floral)

The key constituents of high ester rum include:

  • 97% Ethyl acetates
  • 2% Butyric acid Esters
  • 0.75% esters of high molecular weights
  • Traces of hexanoic alcohols
  • Trace of higher alcohols of an aromatic nature

Ester levels vary in different rums from as low as 0-10 ethyl acetate to as high as 1600 ppm ethyl acetate.

How lower ester rums are made:

For ester rums at the lower end of the spectrum < 200 ppm normally fermented wash is allowed to age by transferring to a separate holding tank and leaving for at least a week

Fresh cane juice where available can be added to the wash mixture and this will give a different bouquet to the rum.

You can achieve up to 200 ppm esters by using this method. Alcohol levels will fall depending on how long the fermented wash is held in storage.

How higher ester Jamaican Rums are made:

Usually associated with longer fermentation times 15-21 days.

Media is more acidic - pH as low as 3.

Composition of media is critical and bacterial contamination is encouraged.

An example of the kind of wash to produce a High Ester Jamaican Rum 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 “flavour”)

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 humming along or dormant, as necessary.

Nowadays, most Jamaican distilleries do not use muck. Instead, they produce their higher ester count rum via a very long (weeks) fermentation process. Also, it is important to note that Jamaican distilleries make different marques (recipes) and they may not use dunder or muck in every marque.


The use of dunder and muck in the production of Jamaican rum contributes to its unique, robust flavours. While the process may seem unusual, the result is a spirit that is beloved worldwide for its distinctive taste. As we explore the potential of these techniques in our own Aussie Craft Rum Distilleries, we open the door to new possibilities in rum production. Australia is blessed with a large sugarcane industry and an associated bounty of high quality Molasses. Australian Craft Distillers focused on Rum production and close to the cane fields have a unique opportunity to explore Dunder and Muck in their on-going wash productions and still runs.

From an environmental perspective reuse of Dunder saves valuable water and spraying it on canefields reduces potash and fertiliser for future cane harvests. Muck is a unique bioreactor of bacteria and acids that when combined with a rum wash can supercharge ester production in your stills. High ester rums might not be for everyone but as Aussie Craft Distillers explore ways to stretch drinkers minds beyond the humble Bundy Rum they will need to challenge their taste buds and minds and playing with Dunder and Mucking around could be a couple of handy keys in their Distillers Toolbox!

cheers #distilsafely always

Dermott Dowling Managing Director,

+61 490 501 392  |

Unit 1 / 1-3 Disney Avenue, Keilor East, VIC 3033

Bibliography and Knowledge Sources:

High Ester Rum Production with Vivian Wisdom WIRSPA Publications

Boston Apothecary (2014) “Muck Hole” Not “Dunder Pit” July 4.

Jamaican Rum - Spirits Beacon

Comparing Continuous Fermentation in Baijiu with Sour Mash Whiskey and Jamaican Muck Pits - Alcademics

Matt Pietrek Days of Dunder: Jamaican Rum’s Mystery Ingredient

Matt Pietrek Ground Zero of Jamaican Funk: Going Deep at Hampden Estate

Smiley, et al The Distiller’s Guide to Rum, American Distilling Institute.

Hogo: Rum’s Most Potent Flavor Profile, Explained