Jamaican Rum on Acid: How Vinegar Brings the Funk

The learning never stops.

For several years I’ve endeavored to understand the flavor science and history behind Jamaican rum’s unique hogo. As part of that I’ve visited every Jamaican rum distillery, save one, and documented what I learned. One such article I wrote was perhaps the first to clearly define what dunder is, and differentiate it from muck, a related but very different ingredient.

To briefly recap, both dunder (what remains in a still after a distillation run, aka stillage), and muck (a putrefied dunder with other ingredients added) are substantial parts of the fermented wash of the next distillation run. You might call them the “special sauces” of Jamaican rum making. I refer to them frequently when educating people about why Jamaican rum is often so flavorful.

Note: Not all Jamaican rums use dunder or muck. It’s only the strongly flavorful ones that do.

I recently realized yet another special sauce is in use; one right in front of my eyes for quite some time. I’d failed to connect my understanding of historical texts with what I personally saw in Jamaica. It all came down to a simple labeling issue.

This third special sauce is vinegar – a special distillery crafted vinegar made from cane juice. You might be wondering why on earth you’d add vinegar to molasses for your fermentation. The answer: To make more flavor! But probably not the flavor you’re picturing.

Not what the Jamaican rum makers use

Vinegar is a diluted form of acetic acid. This common acid, when combined with ethyl alcohol, creates the ester known as ethyl acetate. Ethyl acetate is by far the most common ester found in rum, making an aroma typically described as fruity. Others say it brings to mind nail polish remover.

During fermentation, yeast consumes sugar and emits ethyl alcohol. By adding acetic acid to the mix, the distiller creates more opportunities for the alcohol to combine with the acid, birthing an ethyl acetate ester molecule. Simply put, it supercharges flavor creation!

Let’s wonk out just a bit deeper on this.

Charles Allan, in his magnificent 1906 lectures for the Jamaican Sugar Experiment Station, writes:[i]

Acetic acid is the acid of vinegar. The method in which vinegar is made is interesting as, in some measure, the same process is followed in making acid on estates making flavoured rum. The wine which is to be converted into vinegar is placed in casks, half filled, at about 30 degrees C. to which air has moderately free access. The formation of acetic acid takes place in consequence of the liquid being gradually covered with a film consisting of the mother of vinegar. In other countries the German quick “vinegar process” is employed in which the growth of bacteria suspended in dilute spirit mixed with vinegar, is accelerated by coming into intimate contact with the air. This is brought about by allowing free access of air, by dividing the liquid into small drops and distributing these over a large surface (such as beech shavings.)

Acetic acid must be produced in large quantities in the distilleries of this island but especially in those making flavoured rum. Indeed in them special processes have been evolved to produce this acid as well as others. The part of the process which is mostly concerned in the pro duction of acetic acid is the fermentation of what is called rum cane juice. This juice is generally poor in sugar and what sugar there is, is mostly glucose which would not crystallise out even the juice. It is however, in a suitable state for being fermented. A weak alcoholic solution is formed. This liquor is thrown over cane trash and allowed to stand. The result is that the alcohol is turned into acetic acid. You will note how closely this process corresponds to that of making vine gar. Only in the case of vinegar-making a freer access to air is given.

Note: Flavoured rum in this context doesn’t mean anything like Bacardi’s Dragon Berry rum. Rather, it refers to the high ester rum exported to Germany to make rum verschnitt, and to Amsterdam for blending and flavoring purposes.

So, now we know more about how the Jamaicans were making vinegar a century ago. The next question is: How much vinegar were they using each time? A cup? A gallon?

More than you might think. This same document referenced earlier provides a recipe for a typical wash to make a highly flavored rum:

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 (aka “muck”)

The acid in the recipe is cane vinegar.[ii] In this particular example, it’s over two hundred gallons, or 11 percent of the wash.

Cane acid (vinegar) tank at Long Pond distillery, Jamaica

Unfortunately, I’d not yet read the above lecture excerpts before visiting Long Pond. When I saw a tank labeled “acid” in the fermentation area, I blithely assumed it was for waste, or maybe for a cleaning solution.

In a nutshell, I took a very long time to connect the dots between the acid referenced in the 1906 text, and the acid tank I saw in Long Pond’s fermentation area. Instead, I (erroneously) assumed that the use of vinegar was an old practice that had fallen out of favor.

It wasn’t until I hosted an online session with National Rums of Jamaica that it clicked – The “acid” tank I’d seen at Long Pond was in fact a cane vinegar tank, clear evidence that acid is still used by some of today’s Jamaican distilleries. It’s not a forgotten practice after all! From that session, we also know it’s used at the Clarendon distillery as well as at Long Pond.

In short, when talking about the special practices that make Jamaican rum so ester-rific, don’t forget cane vinegar!


[i] Allan, Charles; LECTURES ON FERMENTATION IN RELATION TO JAMAICA RUM.  As delivered at the Course for Distillers —AT THE— Government Laboratory in 1906.

[ii] I find it rather starting to contemplate this old wash recipe. Only ten percent of the recipe is molasses, a far cry from most rum making today which is usually entirely from molasses. In those early recipes, dunder, skimmings and vinegar makes up more of the recipe than molasses.

3 thoughts on “Jamaican Rum on Acid: How Vinegar Brings the Funk

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  1. Some people refer to this heavy rum style as the “vinegar process”. Keep in mind, data is presented by Kervegant that this vinegar isn’t exactly the same as culinary vinegar and may have only been 4% acid (as opposed to balsamic’s 6%) and of that, 50% of the acid was lactic which is not a volatile acid. Kervegant defines a grand arome ferment as having 1.5% acidity (15 g/L) of which 1/3 is volatile acidity and the rest is fixed acids, but virtually none of which is strong mineral acids (so no sulfuric). Some ferments could be as high as 3% (30 g/L) and supposedly Hampden still pulls that off.

    A theory is that these distillers aren’t particularly after ethyl acetate but have to drag it along to get other stuff they want. However, ethyl acetate can become valuable later as a blending component. Part of what the acetic acid does is help select for a volatile acid tolerant fission yeast. Budding yeasts can tolerate a ton of lactic acid (think Bourbon process) but max out quickly and cannot grow in the presence of volatile acids like acetic or butyric.

    Yet another concept is that mixed bacteria cultures in these ferments can elongate both lactic and acetic acid into more desirable forms like propionic and butyric acid so a percentage of this acid can create more desirable forms and I just demonstrated this recently. Fission yeasts are also capable of chain elongation, but it is hard to pin down which characters do what and when.

    H.H. Cousins had a plan to modernize vinegar production for heavy rum, but it is hard to say if that ever came to be and if it would have compromised any other aromatic character. We recently demonstrated that action by bacteria can produce rum oil (carotene based aroma) just like fission yeasts in pure culture at high pH (think Batavia Arrack and Rafael Arroyo).

    In the early 20th century, vinegar was connected to muck. Long chain acids in the muck would be salted out with lime and this lime salt would eventually be transported to the acid cistern where acetic acid would switch places in the lime salt releasing the long chain acid. This means that vinegar added to the ferment also contained aroma from the muck pit. My understanding is that this is no longer practiced and other variations of muck use are favored.

    The strands of the Jamaica process become incredibly holistic and in a presentation a few years back, I referred to it as biochemical horology because it is complex like a clock mechanism. Rum is known for production complications and many others besides the vinegar process are documented but currently dormant.

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