The Components of a True Rum

By Hendre’ Barnard, Training and Marketing Manager
Distillique Beverage (Pty) Ltd.
https://distillique.co.za

PLEASE NOTE: This article is aimed more at Commercial Distillers than Home Distillers. If your curiosity about spirits and the search for recipes and techniques begins and ends with adding yeast to diluted molasses and then letting it sit in a Barrel for a couple of weeks, months and years (depending on the barrel size) then you should probably stop reading right here.

While barrel aging is a critical part of creating and imparting the flavor of aged spirits, there are other factors that are equally or even more important.

The creation of an aged, distilled spirit is a complex, multi-part process, and with Rum even more so than most. The common perception is that Rum is easy to make, and for the most part that is true – we are guilty of projecting that myth during training as well, as we have often said that Rum is a forgiving Spirit, in that the Molasses overpowers most of the other flavors, and what isn’t right, the wood will fix.

This might hold true for the Home Distiller, but not the Crafter.

The overarching goal is to create a tasty blend of water, ethanol, and those organic compounds which we collectively refer to as Congeners (those of you who have done our C1 – Introduction to Distilling Course will know all about them).

Subcategories of Congeners include methanol, acetone (nail polish remover), esters, and aldehydes – the latter two categories are sometimes collectively referred too as Fusel Oils. You can reference our Article on “How do I get rid of Fusel Oils in my Spirits?” for more information on them as well.

Although their actual Chemical names may sound foreign, unpronounceable and too scientific – Propionic Acid, Guiacol, Isovaleraldehyde and Syringaldehyde (too name but a few) – these congeners are what our nose and tongue reports back to our brain as flavors – Pineapple and Kiwi, Tabacco Smoke, Peaches and Chocolate, Cinnamon Smoke, and so on.

By the time you have blended in your chosen tail cuts (in the case of a White Rum) or when the spirit emerges from a barrel (in the case of a Gold Rum), it (hopefully) has a lot of the good-tasting Congeners and relatively few of the bad tasting ones. Fractional Distillation obviously plays a role in this (refer to our Article on “The Effect of Boiling Rate on Fractions”), but maturation and aging can also play a role, as well as wooding.

The two primary categories of congeners responsible for the flavors associated with distilled spirits are Ester and Aldehydes, and in this article, we will try to explore them a little more in detail.

What choices affect my Spirit Flavor?

A distilled spirit can contain between 300 to 500 esters and aldehydes in its makeup, each contributing in a greater or lesser extent to the final flavor profile of the finished product.

Every single choice the Distiller makes in the Spirit Production process contributes to this final Flavor Profile and Make-up of the product, but the Primary Contributing Factors include:

Raw Material Selection: This refers to the Raw Material that provides the Sugar Source for Fermentation – either directly as in a natural source of sucrose, fructose or glucose, or in the case of Grain or Potatoes, in the form of Carbohydrates which may be converted too Fermentable Sugars. This Raw Material obviously creates ethanol, but in the process, it also creates congeners, and the type of congeners created is impacted by the compounds present in the Raw Material. This is pretty obvious you might think – Whisky tastes like Whisky because of the Barley (or Grain), Bourbon like Bourbon because of the predominance of Corn, Brandy like Brandy because of the Grapes or Fruit used, etc. The issue with Rum however is that if we use Cane Juice (Rhum Agricol), we get a completely different flavor profile than with Molasses (Rhum Industrial), but to make it even more complicated, the flavor profile can and will differ between different cultivars of Sugar Cane, and between different Grades of Molasses.

Yeast Selection: Yeast is what converts simple sugars – Fructose and Glucose present in the Raw Material or Converted from it (Starch Conversion) – into ethanol. And for most Home Distillers that is it. That is all that is important and all that they care about. What is however equally (if not more) important, is the assorted esters and precursor acids generated by the yeast during propagation, respiration and fermentation. The strain of yeast used influences the type and quantity of esters and acids created during these processes, but even within a single yeast strain you get many derivatives. For years we have been limited in South Africa to only being allowed to use two strains of yeast – Saccharomyces Cerevisiae and Saccharomyces Bayanus. This is due to an interpretation of the National Liquor Products Act that we are only allowed to use Yeasts used in the Wine Industry. As the Wine Industry has changed and developed, so too has their use of yeast, and now our choices as distillers have opened up as well.

Backset: Commonly used in Sour Mash Whisky and Rum, the addition of Backset (or what Distillers refer too as Stillage) to a new Fermentation allows for the incorporation of a LOT of additional flavor and aromas into the Fermentation. It greatly increases the concentration of Congeners, but also adds new Congeners or the building blocks for new Congeners. An example might be Caramelized Residual Sugars which would normally not be present in a fermentation just containing Water and Grain or Water and Molasses.

Dunder: Unique to Rum Fermentations, Dunder can basically be described as an overdose of bacteria that’s added to the Raw Material and Yeast during Fermentation to create even more esters and acids. Many stories, legends and myths surround Dunder and its use, some greatly exaggerated or misinterpreted, but at its most basic, Dunder is the intentional or unintentional inoculation of Bacteria into a Fermentation. This is similar to the use of Malo Lactic Bacteria in Wine production, to a certain extent the process of Sour Mash (if the Mash is left to grow a bit of bacteria prior to use)  and the use of Wooden Open-Top Fermenters (Wash Backs) where the open nature of the Fermentation allows for wild yeast and bacteria to be inoculated.

Fractional Distillation: If you have done training with us, or if you have done some proper research about Distillation, you would already know the basics of Distilling – each component has its own boiling point, and components that boil at lower temperatures evaporate out of the fermentation first, while those with a higher boiling point evaporate last. This is true in theory at least – those of you that have done training with us would already know that due to certain affinities between certain compounds (i.e. Methanol and Water affinity) not all compounds come out at their expected temperature. But the theory holds for most compounds. The choices the Distiller makes in terms of the alcohol percentage he or she distils at (determined by the amount of reflux applied), which components (specifically from the tails) gets blended in, the stabilization time of the column, the temperature of the cooling water in the condenser, and the speed at which the still is run (to name but a few), all impacts on the Compounds allowed into the final distillate, and thereby determines the Flavor Profile of the new-make Spirit, which is either consumed as is, or then aged.

Still Type: Spirits in general, but for the sake of this Article, Rum specifically, can be distilled in Pot Stills or Continuous Stills. Now, keep in mind that when we say Pot Stills in Legal Sense, this refers to Batch Distillation, opposed to Continuous Distillation. Already there is a big difference here, as the Continuous Distillation process is geared to the production of higher ABV% (high purity) spirit, and this immediately means a low flavour profile – Light to Extra Light Rums. Under Pot Stills (in the sense of Batch Distillation) we find Alembic Stills, Adjustable Reflux Column Stills, Fractionating Reflux Column Stills, Multi Column Stills and Hybrid Stills. In an Alembic Still the shape of the Vapour Chamber, the height of the Vapour Path, the Angle of the Lyne Arm or Swans Neck, and even the design of the condenser can all impact on Spirit Flavour Profile. See our Article on “Different ways in which Still Shape Impacts Spirit Quality”. In Adjustable Reflux Columns the height of the column and the Distillate Flow Control affects the Flavor Compounds. In Fractionating Reflux Column Stills, Multi Column Stills and Hybrid Stills the number of plates and the rate of Coolant Flow affects the Flavor Compounds.

Cuts: Along with Fractional Distillation and Still Type, when Distillers apply the right techniques in running their stills to the theory of Fractional Distillation, this grants them the ability to select what Distillate to collect and what to discard. These selected Fractions, also known as cuts, inherently include some congeners and exclude others – hence the basis of their selection. These personal choices exercised by the Distiller is the reason why, using the same equipment and same fermentation, 20 different distillers will give you 20 different Rums. The Craft of Distilling at its purest.

Barrel Aging: The important term to note here is the wording – Barrel AGING. Those of you that have done training with us would know that we continuously harp on the difference between WOODING, and AGING or MATURATION. The purpose of Barrel AGING is not just about adding Wood flavors and color to a spirit. That is Wooding. The purpose of Aging is to transform precursor acids into simple esters, and simple esters (typically those with fruity notes) into more complicated esters with notes like honey and spice, or even phenols – buttery, leather and old book flavors. Generally speaking, the longer the product stays in a barrel, the more ester transformations can occur, up to a point – after a certain amount of time there is nothing left to happen, normally around 8 years. In cases where people can actually pick up the differences, this is the reason that a barrel-aged spirit tastes different than a younger barrelled spirit, but for the most part these nuances are not noticed by the majority of consumers.  To make things even more complicated, the type of wood used adds new acids to the mix, which in turn change into esters, and the previous use of the barrel – i.e. a fortified wine barrel, whisky or bourbon barrel, wine barrel, etc. would also add more Congeners into the mix.

These are only 8 of the most common variables that will affect the Congeners and therefore the flavor profile of a Rum (or any other spirit type). To this we can include variables such as Fermentation Temperature, Aeration Levels, Yeast Stress, pH, etc. etc. – all of which affects the final product.

What are Esters?

Scientifically speaking, esters are formed due to the chemical bonding of an alcohol or phenol to an acid.

Numerous alcohol variants (ethanol, methanol, isoproply, amyl, etc.) can help form an ester, alongside several phenolic variants, while the acid component (at least in spirit production) must be a carboxylic or phenolic acid. Considering all the possible combinations of acids and alcohols that is therefore possible, thousands of different esters can be created, each with its own particular smell and flavor.

We also need to keep in mind that normally what we associate or assume to be the smell of a single compound, let’s say Peaches or Chocolate, is in actual fact a combination of different compounds.

The following is merely a small selection out of the hundreds of esters and other flavor compounds found in Rum:

Research shows us that only a relatively small portion of the compounds that comes off the still during distillation, even with our careful control and cuts and blending, ends up in the finished spirit in their original form.

The majority of the Post-Distillation Congeners (those we can detect in our New Make Spirit) are what’s known as Precursor Molecules – Acids and Alcohols that combine over time into esters. In traditional rum this occurs during wood aging, but it is also similar to the process of Batch Maturation which is applied to Gin in order to have the flavors and aromas “settle” prior to bottling.

The problem, or should I rather say challenge is, that some of these precursor molecules can have nasty flavors (e.g. human vomit, or butyric acid) before they get combined into esters. This means that most distillers would choose to not include them in their selected fractions, unless they fully understand the role they play in creating the final product. It must however also be understood that without the correct period of Barrel Aging – long enough to turn this unwanted acids into desired esters – choosing to not include them in your selected batch is the right choice.

The challenge is as well that due to the fact that AGING cannot be accelerated as with WOODING, doing product development becomes a serious challenge for the small Craft Distiller.

A notable exception to the rule above is high ester (often described as “funky”) Jamaican rums, many of which are bottled straight off the still or after spending only a short period of time in the barrel, but still carries a strong fruity aroma.

This aberration can be attributed to the addition of a large concentration of esters due to addition of dunder during fermentation. The result of using dunder in the mash is to “supercharge” ester production during the fermentation process.

Now, as if things were not complicated enough already, different esters have different Aroma-Detection Thresholds – that is, the parts per million necessary for the human nose to sense them.

This means that even though there may be lots of a particular ester in your distillate or spirit, it may not contribute much to the overall taste. A good example of this is ethyl acetate, which is associated with the aroma of green apple, pear, and nail polish remover. Ethyl acetate is commonly found in distilled spirits and can originate from fermentation or be created during barrel aging. In fact, the longer a spirit is aged, the more ethyl acetate it will likely have. Despite the frequent abundance of ethyl acetate, its high aroma-detection threshold means it typically doesn’t dominate a flavor profile. It is more commonly perceived as a subtle undertone.

What are Aldehydes?

Aldehydes are organic compounds that also contribute flavors to spirits, but unlike Esters aren’t easily described with a simple word.

Like esters, some aldehydes are formed prior to distillation in the fermentation process, while others form during aging. In most Distilled Spirits, the primary desired aldehydes that distillers and blenders target are acetaldehyde, vanillin, sinapaldehyde, and syringaldehyde, all of which are extracted from the oak.

During the discussion of Esters we focused on the more fruity flavors and aromas we find in Rum, but now will focus more on the darker, more complex flavors. These flavors typically come from an important subcategory of Aldehydes known as Phenolic Aldehydes and are responsible for non-fruity flavors like wood, smoke, tobacco, cinnamon, pine, and so forth. The base Phenols that contribute to Phenolic Aldehydes form in the barrel wood as its lignin decomposes during charring. Lignin is an element of the wood’s rigid structure – similar to Pectin in a Fruit Cell.

Phenolic Aldehydes are formed entirely during barrel aging, rather than fermentation. The notable exception smoky and peaty Scottish Whiskies, where the kilning process and use of peat effectively coats the barley grains with high concentrations of peat smoke.

Here is a small selection of various aldehydes found in Barrel Aged Distilled Spirits (not just Rum):

What is Jamaican Funk?

Jamaican Funk. Sometimes called Hogo. Most commonly described as the smell of overripe banana.

For those that know and appreciate Rum, Jamaican rum is uniquely beloved due to its powerful, easily identifiable pungent fruitiness. Connoisseurs of Jamaican Funk regularly quote phrases like “High Ester Count” and “Long Slow Fermentation” when discussing the concept, but nothing gets them more excited than when you bring up “Dunder” – that mysterious and magical ingredient that allegedly makes Jamaican rum “Extra Funky”.

Exactly what goes into Dunder is shrouded in mystery, legend and myth, but even the slightest bit of research will yield references to goat heads, dead bats, human waste, and more, bubbling and broiling in an earthen pit, presumably somewhere near the distillery for easy access. Throw a few scoops of this black death into the molasses wash, and voila! Instant Funk.

Or so many people think.

The reality is far less simple, and way more interesting.

First and foremost, you need to realize that Dunder isn’t what most people think it is. At every distillery you visit, and pretty much everywhere online (especially on Social Media), the term Dunder is used to refer to what’s left in the stills after a distillation run. The Distiller’s term for this leftover liquid is stillage.

Most distilleries take their 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, some distilleries retain their stillage (which THEY then refer to as Dunder for use in the next fermentations.

Now, adding stillage to the next fermentation is not unique to rum. The process is also used in the Whisky / Bourbon world to produce Sour Mash.

So, if Dunder is just Stillage, where does the bacteria and fecal matter and dead bats and goats and pigs come in? 

In some of the more traditional Jamaican Distilleries you might come across a tank filled with dark sludge, and bubbles popping on top every now and then. It might bring back memories of MacBeth in High School: “Double, double toil and trouble; Fire burn and caldron bubble.”

This sludge is called Muck – a giant mass of bacteria that creates a soup of carboxylic acids. This Muck and its vast quantity of acids go into the fermentation along with several other interesting ingredients you might not expect in rum making.

As mentioned earlier, the addition of a large dollop of this muck to a Fermentation stimulates 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 might have an ester count of 100 to 200 PPM. Muck treated Rums tally at over 500 PPM. Taste it, and you’ll have no doubt what funk is. And then you also get rums with an ester count in the 1600 PPM range, essentially undrinkable and used primarily for food flavoring (although it is rumoured that some Blended Rums add these products to Light Rum in order to create their signature blends).

How do I use Muck?

Now, if instead of searching for information on Dunder, you search for information on Muck, you tend to find a lot of gems of information. One of these Gems is the 1906 document, titled “Report on the Experimental Work Jamaica – Sugar Experiment Station”, written by Charles Allen.

Available on Google Books, and available to order online, it can be a challenge to read as the terminology is quite dated and unfamiliar. But for its time the report is very detailed and contains quite a few surprises.

Here is an excerpt from the Report:

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

Now, some of that terminology might be unknown to you, so here is a short Glossary:

German Rum: Essentially rum concentrate. Exported to Germany for dilution with German-made neutral spirits, producing what is known as a Rum Verschnitt (those of you that have done our Rum Training will be familiar with this). This style of rum is differentiated from the “common clean” style, which uses mostly the same ingredients but doesn’t include muck, and therefore has a lower flavor profile.

Acid: Acetic acid, in the form of Cane Vinegar (more on this a little later)

Marl: Calcium carbonate (traditionally a lime rich mud)

Skimmings: The debris that collects on the top of the boiling fluids during sugar cane juice processing and is skimmed off during the production process.

Although it may take several read-throughs, the information this recipe reveals is quite fascinating. While we typically think of rum as made entirely from water diluted molasses and yeast, here we learn that, historically speaking at least, Jamaican rum is a lot more complex (the following percentages are rounded):

• 30% Molasses Skimmings
• 40% Stillage from Prior Runs
• 10% Acetic Acid in the form of Cane Vinegar
• 10% Molasses
• 10% Muck

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 (yeast and non-fermentable compounds sedimented)
• Semi-solid materials at the bottom of the Dunder, i.e. the wash after it’s distilled (solid sedimentation at the bottom of the boiler after the stillage has been drained or pumped out)
• Cane trash – the field residue remaining after harvesting the cane stalk (cane grass leaves and stalks)
• Lees – In the context of this document, lees is the unutilised tails

So technically speaking, Muck is a Bio-Reactor for generating acids that eventually turn into esters. The fuel for this Reactor is the waste and refuse from various parts of the Rum Production Process – from harvesting to molasses processing to fermentation and distillation. And its pH level is carefully through the addition of an alkaline to keep it in humming along (or keep it dormant), as necessary.

In some modern Jamaican distilleries, the ratio’s might be different, and the process slightly altered, but the ingredients are still the same. The plant material, sediment and tails are churned into a mixture, and continuously aerated through the injection of air. At the appropriate time (just prior to distillation) added to the fermented molasses. In one specific case, 11 parts of diluted, fermented molasses to 7 parts of the aforementioned mixture.

It is however important to note that most Jamaican Rum Distilleries no longer use muck. Instead, they produce their higher ester count rum via a very long (couple of weeks) fermentation process – similar to a Wine Fermentation.

What is Cane Vinegar?

Vinegar is a diluted form of acetic acid. This common acid, when combined with ethyl alcohol, creates the ester known as ethyl acetate. As mentioned before, 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 – otherwise known as acetone.

During fermentation, yeast converts sugar to 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. Therefore, by dosing the fermentation with acetic acid (vinegar) you are greatly increasing your chances of forming desirable fruity aromas and flavours.

In the previously mentioned report written by Charles Allan, he writes:

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 Celsius 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.

Now, Flavoured Rum in this context does not refer too Spiced Rum or Coconut Rum. Rather, it refers to the high ester rum mentioned previously.

So, now we know how the Jamaicans were making vinegar a century ago. But in terms of Rum Production, how much vinegar were they using each time?

More than you might think.

If we refer back to the earlier quoted 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

The acid in the recipe is Cane Vinegar

In this particular example therefore, the amount of added Vinegar equals several hundred gallons, about 11 percent of the total Fermentation Volume.

How do I use Muck and Sugar Cane Vinegar in my Rum?

Let us look at Sugar Cane Vinegar first.

Now the immediate question many of you might ask is “Seeing as how I cannot get hold of Sugar Cane Juice to make my own Sugar Cane Vinegar, could I just use normal Grape or Apple Cider Vinegar?”

Technically speaking you could, as a Home or Hobby Distiller, but not as a Commercial Distiller, as the National Liquor Products Act prohibits the mixing of raw materials in fermentation, hence you would not be allowed to add in Grape or Apple Cider Vinegar into a Molasses Fermentation, but you could add in Sugar Cane Vinegar, as the law allows Rum Production form Molasses, Sugar Cane Juice and Sugar Cane Syrup.

Furthermore, an often ignored part of South African Rum Legislation also plays a role now.

Let us look at South Africa’s Rum Spirit Category Legislation:

Rum shall –

(a) be produced by the distillation of –

(i) fermented sugar cane juice;

(ii) fermented, undiluted sugar cane molasses, or fermented sugar cane molasses, which has been diluted with water; or

(iii) fermented, undiluted sugar cane syrup, which has been produced in the manufacturing of cane sugar, or fermented, with water diluted, sugar cane syrup, which has been produced in the manufacturing of cane sugar,

at less than 96 per cent alcohol per volume, irrespective of whether sugar cane leaves or fruit have been added thereto.

The second part of the last sentence is important here: irrespective of whether sugar cane leaves or fruit have been added thereto

This means that not only can we make Muck in the traditional sense using the leaves and stalks from the sugar cane harvesting process, but we could even use OTHER fruit leftovers.

Now, this might be a too technical interpretation, but in my personal opinion DAFF (Department of Agriculture) will not allow the addition of Fruit Juice to a fermentation as that would be seen as mixing Sugar Sources, but the Law clearly allows for the addition of Fruit – which would be whole or pulped or squashed – and this would work perfectly as a substitute for or ingredient in Muck.

As Sugar Cane Vinegar is in essence fermented Sugar Cane Juice, it is also legal as per point (a) of the law. But unfortunately, the fruit addition section will still not allow for the addition of Grape or Apple Cider Vinegar to the fermentation, as those would be fermented products and therefore be seen as mixing sugar sources.

This article might have confused you even further now about Rum production, and quite possibly left you with more questions than answers, but at the very least we hope it has provided you with more variables to try and play around with.