Rum is a spirit often renowned for its complex and diverse aroma, a result of the interaction between various volatile compounds. Among these, esters play a crucial role in shaping the fruity, floral, and sometimes funky character of rum. In fact, esters are the second most abundant compound class in rum, after alcohols (Pino et al., 2012)1.
What Are Esters?
Esters are flavorful organic compounds that result from the reaction of acids and alcohols during fermentation. In rum, they are primarily formed when fatty acids react with ethanol, producing a wide range of aromatic compounds that contribute to the spirit’s overall flavor and aroma.
Some of the most common esters in rum:
- Ethyl acetate – Fruity, solvent-like (most abundant ester in rum)
- Ethyl butanoate – Pineapple, tropical fruit
- Ethyl hexanoate – Green apple, banana
- Ethyl octanoate – Fruity, waxy
- Ethyl decanoate (aka ethyl caprate) – Brandy-like, floral, apple
- Ethyl lactate – Creamy, slightly sour
- Diethyl succinate – Mildly fruity, grape-like
- Ethyl vanillate – Vanilla, caramel
How Esters Influence Rum’s Aroma
The concentration of esters contributes significantly to how a rum smells and tastes. One key measurement used to quantify ester concentration is grams of esters per 100 liters of absolute alcohol (gr/hlAA). This measurement accounts only for the alcoholic component, ignoring dilution from added water.
High-Ester Jamaican Rums
Jamaica is known for producing some of the world’s highest ester rums. By law, rums in Jamaica cannot exceed 1600 gr/hlAA. Three distilleries are known to produce rum at this upper limit:
- Hampden Estate (DOK marque)
- Long Pond (TECC marque)
- New Yarmouth (NYE/WK marque)
Marques
A marque (also spelled “mark”) is a classification system used by rum distilleries, particularly in Jamaica, to categorize different rum distillates produced based on their ester content and aromatic profile. Each distillery has its own unique system, with some marques being publicly documented while others remain proprietary. These classifications help define whether a rum is destined for blending, ageing, or direct consumption.
For more information Jamaica’s ester limits, check out this detailed breakdown on RumWonk2.
How Are Esters Created in Rum?
Esters in rum develop through a combination of fermentation, distillation, and ageing,
- Fermentation
The precursors to esters – organic acids – are produced during fermentation by yeast and bacteria. During fermentation, yeast converts the sugar found in the raw material, such as molasses or sugar cane juice for rum, into various alcohols. Bacteria present in the fermentation create acids such as acetic acid and lactic acid. Different yeast strains can influence ester production, with some promoting fruitier esters, while bacteria (like lactic acid bacteria) contribute additional complexity.
Key factors that affect ester production during fermentation include:
- Raw materials: Sugar molasses or fresh cane juice
- Yeast strain: Some strains favor ester formation
- Fermentation time: Longer fermentations allow more ester development
- Aeration: The presence of oxygen affects yeast metabolism
With these alcohols and acids now present, a chemical process known as esterication starts to occur, where alcohols and acids react to form esters and water. This reaction starts in the fermentation stage but continues occurring in the following stages.
- Distillation
Now that the fermented “wash” is complete from the fermentation stage, we move on to distillation. The type of distillation impacts ester concentration. Pot still distillation doesn’t strip out heavier compounds and tends to retain more esters, resulting in funkier rums. Column stills, on the other hand, strip out these heavier compounds and produce a cleaner spirit with fewer esters.
- Ageing in Oak Barrels
Barrel aging transforms a rum’s ester profile over time. Some esters, like ethyl vanillate and oak lactones, develop as the spirit interacts with the wood, adding rich vanilla and woody notes. At the same time, evaporation–known as the “angel’s share”–concentrates the remaining esters by reducing the alcohol and water content. As aging progresses, certain esters mellow out and break down, while new ones continue to form through chemical reactions between alcohols and acids.
1. Pino, J. A.; Tolle, S.; Gök, R.; Winterhalter, P. Characterisation of Odour-Active Compounds in Aged Rum. Food Chemistry 2012, 132 (3), 1436–1441. https://doi.org/10.1016/j.foodchem.2011.11.133.
2. RumWonk: A fantastic resource for anybody interested in rum.