Table of Contents
Introduction
Fermentation
Distillation
Compounding
Aging
Blending
Conclusion
Introduction
Rum comes in many different flavours, colours and ages. The choice of ingredients and how each of the key chemical processes are carried out, results in a huge number of variants. The main processes involved in the manufacture of rum are fermentation, distillation and optionally compounding, aging and blending. These are covered in another blog post called "What is a Distillery, who works there and how is Rum made".
Although there are numerous types of rum, the most common ones are:
- White Rum: known for its clarity and mild flavour, white rum is distilled to a high purity and is often charcoal filtered to remove impurities and colour.
- Gold or Golden Rum: due to moderate aging in barrels, gold rum gains a warm, golden hue and subtle flavours.
- Dark Rum: aged longer in charred barrels, dark rum boasts rich, caramelised flavours and a darker colour.
- Spiced Rum: compounding infuses the rum with spices such as cinnamon or cloves, using the alcohol to extract and preserve the flavours.
- Overproof Rum: distilled to a higher alcohol content, overproof rum often has intense flavours and packs a punch.
- Rhum Agricole: made from fresh sugarcane juice, this rum is fermented and distilled quickly to capture grassy, vegetal notes, which sets it apart from molasses based rums.
- Flavoured Rum: After distillation, fruit extracts or syrups are blended, ensuring vibrant, consistent flavours.
From the above, it is easy to see how different rums can be created by the main chemical processes being carried out in different ways.
Fermentation
Rum fermentation begins with mixing water and sugar cane derivatives (typically either molasses or sugar cane juice). This mixture is known as the "wash" and is inoculated with either wild or cultivated yeast. The yeast consumes the sugars in the wash, converting them into alcohol, carbon dioxide, and a variety of flavour compounds, over a time period ranging from several hours through to several days. The temperature, yeast strain, and fermentation duration all significantly affect the character of the final product and are often a closely guarded secret by the distillery.
The primary chemical reaction in rum fermentation is alcoholic fermentation, where the yeast metabolises sugars, such as sucrose, glucose (C6H12O6), and fructose, into ethanol (C2H5OH) and carbon dioxide (CO2). This can be seen in the below formula:
C6H12O6 -> 2C2H5OH + 2CO2
Esterification can also occur where various esters are produced, giving the rum a fruity and floral taste. For example, ethyl acetate (C4H8O2) can provide a fruity aroma, ethyl isobutyrate (C6H12O2) gives a butterscotch-like smell while phenol compounds usually give spicy notes to the rum. Additionally, a secondary metabolite production by the yeast and bacteria can create higher alcohols, acids, and aldehydes, contributing to complexity of the rum's flavour. The alcohol and an acid combine during the reaction to create esters and water as a by-product. An example reaction is shown below where ethanol and ethanoic acid (C2H3OOH) create ethyl-ethanoate (CH3CO2C2H5) and water.
C2H5OH + C2H3OOH -> CH3CO2C2H5 + H2O
A number of undesirable chemicals can be formed during this process. The most dangerous is methanol (CH3OH), which is formed by pectin (C6H10O7) breakdown during the process and can be toxic. Sulphur compounds can also occur and produce unpleasant odours. A harsh taste can come from the presence of fusel alcohols, such as propanol (C3H7OH) and butanol (C4H9OH). As a result, master distillers focus on high quality ingredients with low pectin content, keep the fermentation temperature constant at around 25–30°C, monitor the sanitation to avoid unwanted bacteria and use a carefully selected yeast strain.
Distillation
Rum distillation separates alcohol and other compounds from the fermented wash to produce a concentrated spirit. The process begins by heating the wash in a still. Column stills are used for high volume, often white, rums while pot stills make a single batches of more flavoured rum. As the temperature rises, the alcohols and volatile compounds vaporise before water due to their lower boiling points. For example, ethanol's boiling point (78°C) means that it evaporates before water (100°C). These vapours are captured, condensed, and collected as liquid distillate. The process starts by adding the fermented wash to the still (known as charging the still) and applying heat to the still. As the temperature rises, the alcohol starts to evaporate. The process of collecting different compounds due to them vapourising at different temperatures is known as fractionation.
Fractionation can be split into three parts, known as "heads", "hearts" and "tails", in that order with each corresponding to a different temperature range, and therefore different compounds. Broadly speaking, the heads and tails account for about 10% of the volume each, with the remainder being hearts. Acetaldehyde (C2H4O) (boiling point of 20°C), methanol (65°C), acetone (C3H6O) (56°C) and ethyl acetate (77°C), can be found in the heads as that is collected first, has a harsh solvent like aroma and is therefore discarded. These compounds are known as "low boilers" as they appear below 78°C. The hearts are the most prized as they contain almost pure ethanol, while the tails (often containing fusel oils) may be discarded or retained for future distillation, in order to make other types or flavours of rum. However, isoamyl alcohol (C5H12O) (boiling point of 132°C) can sometimes be found in the hearts cut and can give an unacceptable odour and bitterness, which may require a further round of distillation to remove it. The tails consists of volatile fatty acids such as acetic acid (C2H4O2), propionic acid (C3H6O2), butanoic acid (C4H8O2) and 2- & 3- methyl butanoic acid (C5H10O2), which result in an offensive, plastic like aroma and an unpleasant flavour.
It is worth noting that since a wash is a mix of compounds such as 10-20% ethanol, water and congeners (other by-product compounds created during fermentation), the boiling point where the still starts to produce is higher than the 78°C boiling point of pure ethanol. For a 10% ethanol mix, the temperature could be as high as 92°C before production.
Compounding
Rum compounding involves blending rum with natural or artificial flavours, such as spices, or botanicals to create flavoured or spiced rum. This process enhances the spirit’s flavor profile and customises it for specific markets or consumer preferences. The first step is to select a base rum. This can be an existing, high quality rum from the distillery's portfolio and can be eiter aged or unaged. The next step is to select the ingredients and prepare them. Common ingredients include caramel, vanilla, spices (such as cinnamon, nutmeg or cloves), botanicals or fruit extracts. These can be steeped, macerated or blended with the rum so that the alcohol (acting as a solvent) extracts the flavours and aromas from the ingredients. The result is then filtered and allowed to rest so that the flavours harmonise, prior to bottling the result. It is important that the ingredients are not left too long otherwise bitterness can occur from over steeping. Additionally, oxygen exposure should be controlled so that off flavours from oxidation do not occur.
Aging
Rum aging is the process of maturing rum in wooden barrels to develop complexity and depth of flavour. The barrels are usually oak and can be new or previously used for bourbon, wine or sherry. The inside of the barrel is charred as it alters the physical and chemical composition of the wood by caramelising sugars, increasing vanillin content and removing undesired tannins. The filled barrels are stored in warehouses where the temperature, humidity and air circulation are carefully controlled. The barrels are periodically sampled or blended to ensure consistency of the rum. Additional aging in other specialty barrels may be carried out in order to achieve unique flavours, prior to bottling.
Compounds like tannins, lignin, hemicellulose, and vanillin are extracted from the wood, contributing flavours such as spice, caramel and vanilla. Oxygen can seep through the wood, softening any harsh notes and allowing chemical reactions between the alcohols and acids to form esters, that in turn provide fruity or floral aromas. The flavour can be concentrated due to evaporation (known as the "angel's share"). For example, in Scotland 2% evaporation is common whereas in the Carribean the "angel's share" is closer to 8%, due to the climate. It is important not to over age, use high quality barrels and provide an optimal storage environment. Failure to do so can lead to excess tannins or sulphur compounds, which in turn introduce bitterness or off flavours into the rum, thereby ruining the final product.
| Chemical compound | Flavour |
| Vanillin (C8H8O3) | Vanilla |
| Gamma-Nonalactone (C9H16O2) | Coconut |
| Linalool (C10H18O) | Orange peel |
| Ethyl butanoate (C6H12O2) | Fruity |
| 4-hydroxy-2,3-dimethyl-2H-furan-5-one (C6H8O3) | Spicy |
| 3 methyl butyl acetate (C7H14O2) | Banana |
| Phenylacetaldehyde (C8H8O) | Honey |
| 3-(methylthio) propanal (C4H8OS) | Cooked potato |
| 2-methoxy phenol (C7H8O2) | Smoky |
| Methyl propanol (C4H10O) | Malty |
| Phenyl acetic acid (C8H8O2) | Bees wax |
| Acetic acid (C2H4O2) | Vinegar |
| Butanoic acid (C4H8O2) | Sweaty |
| Decanoic acid (C10H20O2) | Soapy |
Table 1: Chemical compounds and their corresponding flavours in aged rum
Blending
Rum blending is the art of combining different rums to achieve a consistent and well-balanced flavour profile. Master Distillers choose rums with varying characteristics, such as light, heavy, aged, or unaged rums and analyse each rum for its flavour, aroma and chemical profile. The next step is to carry out a number of small test blends so that the final recipe can be honed to perfection, with the esters, aldehydes and higher alcohols all being in perfect harmony. During the blending process, the rums are mixed in the specific proportions in order to achieve the desired profile, and then allowed to rest so that the flavours are fully integrated. A final step is often to filter any impurities so that the rum is polished and clean. The key objective for rum blending is consistency, balance and depth of flavour and aroma.
Conclusion
As can be seen from the above, making rum gives rise to some complex chemical processes with numerous other chemical compounds involved in the production of a final product.