Rum - How is rum made?
Words by Simon Difford
Rum is produced from Genus Saccharum officinarun, the giant grass that’s better known as sugar cane. The plant takes between ten months and two years to reach sufficient maturity to harvest and extract its sugars. There are many different varieties of sugar cane and the variety and the region where it’s grown significantly affect the profile of the rum produced from it.
In the Caribbean the cane is usually cut once a year, while in South American sub-tropical climates it is possible to crop twice a year. Like most grass varieties, sugar cane thrives on being cut and simply starts growing again after cutting, this cycle only needs to be interrupted due to diminishing nutriments in the soil. After six years or so it was common to plant another crop to reinvigorate the soil but modern fertilisers are now often used to stretch a few more years of cane growth.
Once a field of cane is ready to be harvested, that particular field is sometimes burnt to remove the leaves (and scorpions and such like). The cane is left standing and is only singed by the fire due to its high water content. Once the cane is burnt it must be quickly harvested and then milled within 24 hours to prevent deterioration of its sugars and bacterial infection.
Traditionally, cane is harvested by cutters wielding machetes, who cut the cane close to the ground as this part of the stem has the highest concentration of sugars, before lopping off the leafy tops. A good cutter will average three tons of cane per day but this is a tiny fraction if what a machine can cut and (gradient allowing) mechanised harvesting is now used.
In Jamaica it is common for the cutters to leave an odd cane or two standing at the edge of the field. These are tied in elaborate shapes to represent a watchman, which the cutters believe will ward off the 'duppies' or mischievous spirits.
Sugar cane contains about 75% water, 10-16% sugar and 10-16% fibre. The harvested cane is washed, chopped into short lengths and milled (pressed) to extract the water and sugar juice.
Rum can be made by distilling the beery type liquid produced from fermented fresh sugar cane juice. This method of rum production is common on the French islands, particularly Martinique where it is called 'rhum agricole'. Elsewhere, it is rare to find rums made directly from sugar cane juice.
By far the majority of rums are produced from molasses - known as 'rhum traditional', but also sometimes rather unkindly described by producers of rhum agricole as 'rhum industriel' (industrial rum).
Some newer rums are a blend of agricole and traditional
Rum can also be made from cane syrup, made by boiling cane juice to remove some of its water content. ('Fancy molasses' is a term for 'inverted sugar' syrup where sucrose has been converted to glucose and fructose with acid or enzymes.)
The sugar found on your supermarket shelf, whether white or brown was produced from sugar cane juice and regardless of its end colour was originally brown - white sugar is the result of a further industrial process.
The process of extracting sugar from cane juice produces a by-product called molasses and this is what most rum is made from.
At the sugar mill, the cane is chopped and crushed to extract the juice which is boiled to reduce the water content and leave a syrup known as 'wet sugar', comprising approximately 30% sugar. This syrup is clarified and mixed with sugar crystals, which provide a core for the dissolved sugar in the syrup to crystallise on. This mixture is boiled and then cooled to encourage the sugar crystals to enlarge. It is then spun in a centrifuge to separate the crystals from the liquid.
This process is repeated a couple of times and the sugar produced sold on the world market. What's left is the thick black liquid by-product known as molasses. This is fermented and then distilled to make rum (or the neutral alcohol on which some liqueurs are based).
The sugar and molasses produced by the first process are termed A-grade and the second B-grade. The third batch of sugar produced by this process is known as Low-grade sugar and this is used to mix with the next batch of syrup to start the process again.
For rum production ideally molasses should have at least 52% sugar content but sadly for the distillers there is no specification for molasses as this is a waste product of sugar processing. Thus a good sugar factory will produce bad molasses as it will have efficiently extracted most of the sugar. As the sugar processing industry becomes more efficient so the amount of rum that can be produced per ton of molasses is failing.
Whatever sugar cane derivative base ingredient is chosen (molasses, cane juice or cane syrup) this is fermented with water and cultured (almost always) or naturally occurring yeast to produce a beer like 'wash' of 5-10% alc./vol.. The resulting 'wash' can then be distilled to make rum.
Put simply the yeast eats sugar and in doing so produces alcohol, heat and carbon dioxide. In addition, yeast also initiates chemical reactions in the wash to create compounds such as aldehydes, esters and acids which are collectively known as congeners. The compounds are flavoursome and depending on the type of rum to be produced, their formation will be encouraged or discouraged by the type of yeast used and the temperature of the fermentation. It's worth remembering that distillation can only separate and remove flavours while fermentation generates flavour in the first place.
Molasses are so rich in nutriment that the yeast needs to be propagated and slowly introduced to progressively higher concentrations of molasses as its cell numbers increase. It is typical for rum distillers to talk about three or four-step fermentation in reference to the number of ever larger vessels used between propagation and the fermenters. The type of yeast used varies tremendously from country to country and distiller to distiller. This can be commercially cultured yeast or natural ambient yeast found on the leaves of the sugar cane.
The rate of fermentation and the alcohol level produced is partly governed by the levels of non-sugar dissolved solids, being mainly minerals and Potassium Chloride, high concentrations of which inhibit yeast growth. A longer, slower fermentation will result in a heavier, more acidic wash due to other contaminating bacteria also given time to reproduce during the process. Fermentation can be a quick as 24 hours or as long as a fortnight.
The pH of the molasses will also affect fermentation and ideally will be in a range between 4.4 to 4.6 and this may be adjusted but the addition of the acidic residue (lees) left in the still after an earlier distillation. Dunder is the term given to lees which have been left in open dunder pits to concentrate the ester content and the acetic/butyric acids.
Light or heavy rum
Rum is termed 'light' or 'heavy' depending the level of flavour components or 'congeners' - products of fermentation that are not ethyl alcohol. The level of these (esters, aldyhydes and lower alcohols) is dependent on the length of the fermentation and the purity to which it was distilled. When alcohol is concentrated during distillation, the levels of congeners are reduced. The fewer congeners, the lighter the rum, the more congeners the heavier it will be.
Rum produced from a pot still or single distillation column is usually described as heavy. Multiple-column stills can produce both heavy and light rums depending on where the spirit is removed from the still.
Light rums tend to have a short fermentation while heavy pot still rums are usually distilled from a wash formed by a long fermentation.
The odour, texture and taste of light rums are more subtle and refined than those of heavy rums, which have a heavy, syrupy flavour to match their dark colour. The level of impurities in light rum is less than a third of those found in heavy rums. Distilleries producing light and heavy rums often blend the two to produce a rum having characteristics of both.
Light rums tend to originate from countries originally colonised by the Spanish, such as Cuba, the Dominican Republic, Puerto Rico and Venezuela. Heavy rums traditionally come from former French and English colonies, including Haiti, Jamaica, Martinique, Barbados, Guyana and the Virgin Islands.
Distillation of rum
Distillation works on the principle that alcohol boils at a lower temperature (78.3°C /165°F) than water (100°C /212°F). So if you take a mixture of alcohol and water, boil it, collect the vapours given off in batches throughout the boil, cool and so condense (turn vapour back into liquid) these vapours back to liquid, the liquids collected at the start of boil will be alcoholic and those towards the end will be water.
In practice distillation is a lot more complex with numerous variables affecting the final distillate produced - mainly the different boiling points of the various different kinds of alcohol and their particular flavour compounds. The skill of the distiller is to use the distillation process to separate and collect the alcohols and flavour compounds (congeners) wanted in the finished rum.
The lightest of these, the 'low wines' (those with a low boiling point) will be given off first and many of these most volatile compounds are harmful.
Ethanol alcohol and other desirable compounds are less volatile (slightly heavier) and so follow the low wines. Often described as the 'cut', it this proportion of the run which will be used to make the finished rum.
The heaviest compounds, the 'high wines' (those with a high boiling point) come off the still last. Some of these heavier congeners are oily and are referred to as fusel oils.
As touched upon earlier, particularly when discussing the difference between light and heavy rums, there are basically two types of still used in rum production: pot (alembic) stills and column (Coffey) stills. The way the distiller can influence the type of distillate produced varies according to which of these two types of still are used.
In general, heavily-bodied rums are those with more congeners and they tend to be made in pot (alembic) stills.
Pot (alembic) still rum production
Pot stills are the simplest and the original type of still. Extensively they are glorified copper kettles - indeed in some countries such as the Netherlands even call them 'kettles' rather than stills. These are the kind of stills used in Scotland to make malt whisky and France to make cognac.
The still is charged with the wash and then heat is applied to bring to the boil. The volatile 'high wines' or 'heads' will be given off first and set aside. These are undesirable compounds, partly due to being tainted as these first vapours effectively clean the still from the previous distillation.
Next follows the desirable part of the run, 'the cut', as the alcohol level of the distillate collected starts to fall, and the 'low wines' or 'tails' arrive and are set aside. As in Cognac and Scotland it is usual for pot still rums to be double distilled - put through the pot still twice with the distillate collected from the first distillation producing a distillate with an alcohol strength in the low twenties and the second distillation typically being over 70% alc./vol..
A mentioned above the distiller must judge when to make the cuts during distillation so controlling what congeners are retained and discarded. Some stills are very simple, while others have devices which allow the distiller more control.
In rum making one of the common additions to pot stills are retorts. The distillate from the pot still is directed into the first of two retorts, a copper chamber filled with low wines from the previous distillation diluted with water (to approx 50% alc./vol.). The hot vapour causes the liquid in the 'low wine retort' to boil and so concentrate the strength of the vapour which then moves on the second retort. This is filled with high wines from the previous distillation, again diluted with water but to a higher strength. As in the first retort, the vapour causes the liquid to boil and the alcohol strength of the vapour is boosted a second time.
It is common for stills with retorts to have tanks under each retort where the low wines and the high wines are sent in preparation for charging the retorts above for the following distillation. The liquids placed in the retorts will have a dramatic affect on the finished distillate. For example the first retort may contain low wines mixed with fermented wash, dunder and even some high wines.
Some distillers use chilled heads on their retorts which condense the vapours as they rise up causing them to fall back into the chamber. This is called reflux and it can also be achieved by the way the head on the pot still is shaped. Reflux increases copper contact and effectively raises the boiling point so producing a lighter distillate.
Column still rum production
Column stills are also known as 'continuous stills' because, as their name suggests, they can be run continuously without the need to stop and start between batches as in pot stills. This, coupled with the higher concentration of alcohol in the final distillate, makes column stills much more economical to operate than pot stills. They also allow the production of lighter, cleaner rums.
Simple column stills (like that designed by Aeneas Coffey) consist of two tall columns, one called an 'analyzer column' and the other the 'rectifying column'. Perforated copper trays or 'plates' sit horizontally in each, like the floors in an skyscraper. Put simply steam is introduced at the bottom of the still and the wash mid way up. The hot steam rises through the still with each floor or plate acting to distil the wash with heavier compounds unable to rise to the next floor so falling while lighter compounds vaporise and ascend the still. The two columns are linked, the second further purifying the vapours from the first while at the same time heating the wash that will charge the first (analyzer) colum.
The taller the stills, the more plates they contain and so the purer the alcohol (lighter rum) produced. More modern three and four column stills allow for more efficient rectification. Cooling jackets can be used to increase reflux and there is also a move to use vacuums in column stills which reduce the boiling points and so the temperature the still runs at. Modern column still technology allows light and extra-light rums to be produced efficiently and cheaply.
The origins of a rum and its 'terroir', will affect its style, not only due to local production method traditions, but also the micro-climate, origin of the sugar cane juice / molasses and location and style of ageing warehouses. Thus each island and country where rum is produced tends to have its own style, favoured by the native population who have grown-up drinking that style of rum. Thus Trinidadian rums taste very different to Jamaican rums or Martinique rums.
Like all distillates, regardless of whether they are distilled in pot or column stills, all rums are clear when they condense after distillation. Colour in rum is the result of ageing in oak casks (and also the possible addition of caramel colour). White rum can simply be a sugar cane distillate watered down and bottled.
The fact that ageing in oak barrels improves the raw rum was discovered when ships carried rum on the long passage to Europe, it arrived darker in colour and with an improved flavour.
Due to their plentiful supply, rum is most commonly aged in American oak casks which have previously been used to age American whisky (bourbon). This is due to the rules of bourbon dictating that the whiskey must be aged in new white oak casks so once used they are useless to the bourbon industry, other than as a commodity to be sold to other distillers around the world. The inside of these casks are charred at the cooperage when first made. This caramelises natural sugars on the wood's surface increasing the vanillins. Casks may be scraped clean to remove any previous charring, and/or re-charred before being filled with rum: the quality of these casks, what they previously held, how many times they are refilled and their treatment dramatically affects the character they impart to the rum stored within them.
Some distillers increase the surface area of wood in the cask by inserting oak or charred oak chips. This reduces the ageing period required to gain a similar effect and is viewed by many (including me) as being something of a shortcut, as although it imparts a lot of flavours from the oak, the effects of extended oxidisation and evaporation are not replicated.
Whether a cask is a 'first re-fill', meaning the cask was previously used to age another spirit and this is its first time it has been used to age rum. Or it is a second or third re-fill will make a huge difference to the effect the cask has on the maturing spirit.
Whatever the origins of the cask, ageing spirits in the humid/tropical climatic conditions typical of the Caribbean and South America will have a very different affect to that of somewhere like Scotland. Some say that every year of 'tropical ageing', where the oak's pores are wide open so allowing the rum to soak deep into the wood, is equivalent to three years in a cold warehouse were the pores will be more closed.
Losses in volume to due evaporation are also more exaggerated in hotter climates (around 6% per year as opposed to 3% in Scotland) and high humidity can mean an almost equal loss between alcohol and water, meaning that although the volume is lost the strength remains fairly constant. To prevent casks destined for extended ageing from gradually emptying over the years, it is common for casks to be topped up with rum from other casks in same batch. Thus, you might start with ten casks of rum from a particular batch and ten years later be left with only five casks.
Charcoal filtration of rum
Thanks to Don Facundo Bacardi's lead it is common to use charcoal filtration during the production of light rums. This filters harsh and undesirable components from the rum and different types of charcoal can be used to remove different substances and so "smooth" the rum.
Although a closely guarded secret it is known that Bacardi use a mix of tropical woods and coconut shells to filter their charcoal. Charcoal filtration can also remove the colour imparted by age so a three year old rum may be crystal clear when bottled. Charcoal filtration may be used before and after ageing.
Blending is the final process available to the distiller to alter a rum's character. Many rums are blends of light and dark rums of different ages. Rums may be 95% column still with just 5% or even less pot still to add character and flavour.
It is at this blending stage where some of rums controversies occur. Many rum producing nations have precious few rules governing age statements and additives, and I am not being controversial by saying that some producers take advantage of the lack of regulations or governing bodies. The Caribbean Rum Marquee for example, presently proves little else than that a rum originated in the Caribbean.
Additives used to boost a rum's flavour include prune wine, chaptalized fruit juice, boisse, sugar, spice and flavourings. As for age statements, in many cases they are just that, statement and not fact. In the case of the best producers and regulated markets, these age statements (rightly) represent the age of the youngest rum in the blend. Others portray the average age of the rums in the blend and some are simply meaningless.
It is common for caramel to be added to aged rums to 'correct' the colour, and lets be honest, often to darken the colour so potentially giving the rum an older appearance. Conversely, some aged rums are charcoal filtered to remove any colour and are bottled completely clear. So with rum what you see is not necessarily what you get.
Before bottling, quality rums are left to marry in tanks - this allows the different flavours and bouquets of the many rums in the blend to fuse together.