The Chemistry of Rum

Arrrr! The chemical makeup of rum can be complex and therefore very unique to its brand. Different countries have different standards that rums must meet therefore significantly changing the chemical makeup of the final product. But despite the differences in types of rum, there’s still a lot of chemistry in common.

Rum is believed to originate from sugar cane or molasses in the Caribbean. Back in the 1600s, there was no real use for molasses, but the colonists quickly discovered that, if water was added, the resulting material could easily be fermented to produce alcohol. Suddenly, a useless byproduct was a valuable ingredient to make rum.

The process for making rum is also quite variable, depending on the type of rum being produced. Initially, the molasses, having been mixed with water, are fermented, with yeast being added to kick off this process. After this, the rum is distilled to concentrate the alcohol and aroma/flavor compounds. After this, the rum is aged, in barrels. These are often charred oak barrels from the bourbon production industry, as regulations surrounding the production of bourbon dictate that barrels can only be used once.

The ageing of rum is usually somewhat shorter than that of whiskey. This is in part due to the climate in areas where rum is commonly produced. Warmer climes lead to greater evaporation and faster flavor development of the spirit as it ages. Whiskey experiences this too, with the amount that evaporates being about 2% per year (the ‘angel’s share’). By comparison, rum’s ‘angel’s share’ can be up to 10% per year.

On a chemical level, the aroma can mainly be contributed to the ester compounds which are created by reacting an organic acid with an alcohol. Some smell fruity, medicinal, or others like glue. The range of esters adds the fruitiness to rum’s aroma. The most important contributors are ethyl propanoate which contributes a caramel-like, fruity aroma, and ethyl isobutyrate which has a butterscotch-like aroma. Phenethyl alcohol also adds a floral aroma and is also found in the aroma of roses.

These compounds are present in rum after distilling. During the ageing process, compounds from the barrels will also infuse into the rum and alter its flavor. These include several polyphenolic compounds, which can impart medicinal and smoky notes. They also include compounds like vanillin, the major flavor and aroma component of vanilla. Oak lactones are also found in rum, though to a lesser extent than in whiskeys which tend to be aged for longer.

Dark rums tend to contain more flavor and aroma compounds than lighter rums. White rum is filtered, often through charcoal, to remove compounds that cause coloration. However, this filtration can also remove compounds that impact the rum’s flavor, so caution is advised.

The appeal of this spirit’s particular mix of chemical compounds made it a big hit when it was first produced in the Caribbean and today is still one of the most popular spirits available. Cheers!