What are esters?

When customers enter the shop looking for rum, we often ask if they want a "sweet-soft-comfort rum" or something more challenging. Within the challenging category, we can go in several directions: For example, the French style, which has a drier and more cognac-like style than other rums, or something with Jamaica-funk. Here we will often talk about esters and the ester level in the rum, especially in Jamaican rum, where it is significantly higher than in other rums.

But before we dive too deeply into esters, it is important to point out two things:

Esters are only one component of the taste and flavor experience of rum. Many people are obsessed with ester levels, but that's like only listening to the guitarist's performance in a five-piece band. Many of the wonderful aromas we associate with our favorite rums do not come from esters. The term "volatile compounds" gives a more comprehensive picture of a rum's aroma and taste. Esters are just one part of the group of volatile compounds.

The second thing is that it is not only in Jamaican rum that you find flavor bombs and high levels of volatile compounds. Those familiar with French AOCs and GIs will note that they have minimum requirements for the amount of volatile compounds at 225/250/325 gr/hlAA (grams per hectolitre of pure alcohol), and according to the French legal definition, "Grand Arôme" rum must have a minimum content of volatile substances, other than ethyl and methyl alcohol, equal to or greater than 800 gr/hlAA and a minimum ester content equal to or greater than 500 gr/hlAA. High-ester rum and "grand arôme" are just different names for the same style of rum made from molasses. Jamaicans and French distillers have adopted slightly different technical approaches, but ultimately both are about creating as many aromatic compounds as possible during fermentation.

More and more producers have also started to disclose the content of both volatile compounds, congeners, and esters, which is primarily a delight for enthusiasts who love to delve into what they are drinking. This applies, for example, to Velier and Plantation.

What are esters?

A 40% ABV rum contains 40% ethanol, approx. 59 percent water, and one percent or less of other organic compounds. For a typical bottle, these other compounds contribute about 7 ml or less. Water does not contribute flavor to a rum, and ethanol contributes very little. Therefore, almost all the flavor in a rum comes from a very small portion of its organic compounds. Call them flavor compounds if that makes things easier to understand. Everything we smell - whether it's strawberries, caramel, burning plastic, wet dog, or freshly cut grass - is due to organic compounds in the air reaching our nasal receptors. Our olfactory system (nose and tongue) can detect these compounds in extremely small concentrations, for example, one part per billion.

Esters are compounds formed by a chemical reaction between acids and alcohol. In wine and spirit production, they primarily arise as a result of fermentation and aging in a process called esterification. Esters formed during fermentation are influenced by many factors, including basic ingredients (grains, fruits, molasses, etc.), fermentation temperature and duration, yeast strain(s), climate, and other factors.

Esters can be classified into two groups: acetate esters (acetate plus alcohol) and ethyl esters (ethanol plus fatty acid). A distilled spirit typically contains hundreds of different esters in its composition, which help create its aroma. Many of the lighter esters from fermentation are removed during distillation. The amount of reflux and the size of the cuts made during distillation will affect the amount of esters remaining in the resulting spirit, as will the acidity of the type of alcohol; cognac, which has a high acidity level, bourbon, and many rums are typically high in esters. Aging also contributes esters by converting acids into simple esters and simple fruit esters into those with notes that include honey and spices.

Choices that affect the quantity of esters

Several steps in rum production affect the type and quantity of esters in a given spirit. The most important contributors are:

Choice of base material: The organic material that provides sugar for fermentation, which in turn creates alcohol. For single malt whisky, it is barley. For bourbon, it is typically a blend of grains, including corn. For rum, it is sugarcane molasses, syrup, honey, juice, or a mixture of these. This raw material naturally creates ethanol, but in the process, it also creates congeners, and the type of congeners created is affected by the compounds present in the raw material.

In rum, it is a bit more complicated because if we use sugarcane juice (Rhum Agricole), we get a completely different flavor profile than with molasses (Rhum Industrial), and to make it even more complicated, the flavor profile can and will vary between different species of sugarcane and variants of molasses.

Yeast: Yeast is what converts sugar in the base material into ethanol (the potable form of alcohol). However, equally important (if not more so) are the various esters and precursor acids generated by the yeast during propagation, respiration, and fermentation. The type of yeast used affects the type and quantity of esters and acids created during these processes, but even within a single yeast strain, many different chemical substances are produced.

Duration: Fermentation length will largely determine the acidity of the wash (The fermented liquid in the fermentation tank, which is then sent for distillation). The longer the fermentation, the more acidic the wash will be. With more acid, there is potential for the formation of more esters in the fermentation phase, along with other volatile compounds and flavorings that will develop in the rum.

Fermentation can vary in duration from 20 hours to several weeks. Many distilleries allow sugarcane to ferment for standardized lengths of time to produce a specific product. Shorter fermentations (20-26 hours) are used to produce lighter rums, while longer fermentations (>7 days) result in rums with more esters.

At, for example, Jamaican Hampden, fermentation for their heaviest marques can last over three weeks. This is because once the yeast is used in the original fermentation, their dead wash is left in the tank for a secondary fermentation process. Here, bacterial cultures begin to develop the acid compounds that evolve and create new ester compounds.

Backset/Dunder/Muck: These terms are commonly used in the production of sour mash whiskey and rum. The addition of backset, dunder, or muck to a new batch of fermentation means incorporating a LOT of extra bacteria and thus a lot of flavor and aromas into the process.

First of all, it is important to understand that dunder and backset are what remain in the stills after distillation. In whiskey production, the word backset is used, and in rum production, the word dunder is used. The distiller's term for this remaining liquid is stillage. Most distilleries take their stillage and either dispose of it by spreading it on sugarcane fields under controlled conditions or treating it to make it less harmful to the environment. However, some distilleries retain their stillage, which they then refer to as dunder for use in subsequent fermentations. The addition of stillage means an overdose of bacteria is added to the raw material and yeast during the fermentation process to create even more esters and acids.

However, it is important to distinguish between the different forms. It is easy to get confused and mix these terms up. There are also many stories, legends, and myths about dunder and its use, some highly exaggerated or misinterpreted, but in reality, those stories are about muck and not dunder. Dunder, as mentioned, is the same as stillage, and in its most basic form, is not much different from the use of malolactic bacteria in winemaking, to some extent the Sour Mash process (if the mash is allowed to cultivate some bacteria before use), and the use of open fermenters (Wash Backs), where the open nature of the fermentation allows wild yeast strains and bacteria to be created.

Then there is muck, which is primarily used in connection with the production of high-ester "funky" Jamaican rum. The story of muck is shrouded in mystery, legends, and myths, but even the slightest research will provide references to goat heads, dead bats, human waste, and more. It will often also refer to it bubbling and boiling in a hole in the ground, presumably somewhere near the distillery for easy access. But muck is merely an evolved form of dunder that is allowed to sit in open vats.

At some of the more traditional Jamaican distilleries, one might encounter a tank filled with dark sludge, with bubbles occasionally breaking the surface. This sludge is muck - a huge mass of bacteria creating a soup of carboxylic acids. This muck and its enormous amount of acids can also be added to the fermentation.

Distillation: The distiller uses heat to separate and concentrate the components of the liquid. Here, components like delicious fruity esters are good, while higher alcohols (also known as fusel oils) are bad. Essentially, distillation works by having all components in a liquid boil at different temperatures: Methanol boils at 65 °C, ethanol boils at 78 °C, and water boils at 100 °C. Each has its own boiling point, and components that boil at lower temperatures evaporate and leave the still first, while those with a higher boiling point come out last – this also applies to esters.

With alcohol produced in pot stills, the longer the neck of the still, the more separation occurs between lighter and heavier compounds.

With column stills (generally speaking), the more plates it has, the more separation occurs between different components.

When distillers choose which vapors to collect and which to discard, they include some congeners and esters and exclude others. These personal choices made by the distiller are why, even with the same equipment and the same fermentation, 20 different distillers will give you 20 different rums, whiskies, cognacs, or bourbons. The art of distillation in its purest form.

Cask Aging: The purpose of cask aging is not just to add woody flavor notes to a spirit; its other critical purpose is to convert acids into simple esters and simple esters (typically with fruity notes) into more complex esters with notes like honey and spices. Generally, the longer a spirit is aged in a cask, the more ester transformations can occur, and therefore it will have more esters.

It is for this reason that a cask-aged spirit tastes so different from its younger sibling. To complicate matters further, the wood adds new acids to the mix, which in turn are converted into esters.

The majority of by-products after distillation are what are known as precursor molecules - acids and alcohols that combine into esters during cask aging. These precursor molecules can have unpleasant flavor notes (e.g., human vomit or butyric acid) before combining into esters. Therefore, the importance of cask aging in converting these acids and alcohols into delicious esters cannot be underestimated.

How esters smell and taste

Many esters produce aromas similar to those we perceive as fruit and flowers. The most common esters include ethyl hexanoate, with an aroma often described as apple, and ethyl acetate, which can contribute aromas of both fruit and nail polish. Ethyl acetate is the lightest and most volatile. It is not very aromatic, but it always dominates, accounting for 90 to 98% of the total esters and acting as a vector for other, more aromatic esters. These esters - each heavier than the last - bring floral and fruity notes such as green apples and pears, and then more intense citrus, pineapple, and banana flavors (which come from ethyl butyrate and isoamyl alcohol).

The more substantial esters such as lactic acid and esters beginning with "capr-" can evoke red fruits, coconuts, sweet fruit, or notes of cream and butter. These esters are less volatile and usually do not evaporate, which is why we perceive them in the mouth rather than the nose.

Different esters also have different aroma detection thresholds - that is, how many parts per million are needed for the human nose to perceive them. While there may be plenty of a certain ester in a spirit, it may not contribute much to the overall flavor. A good example of this is ethyl acetate, with an aroma of green apples, pears, and nail polish remover. It is often found in distilled spirits and can come from both fermentation and be created during cask 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 typically means it does not dominate a flavor profile.

The scent of a spirit is the perceived sum of the diverse volatile compounds contained therein, to which the human nose is sensitive (sometimes called the spirit's "nose"). A distilled spirit typically contains 300 to 500 esters and aldehydes in its composition, each contributing a part of the final flavor profile.

With so many compounds and varying intensities, it's not hard to imagine that each distilled spirit has its own distinctive fingerprint, such that rum from one part of Jamaica can be distinguished from rum produced in another part of Jamaica.

And since people differ significantly in their ability to smell and identify many compounds, aroma in this context can only be an imprecise term: one person's "burnt rubber and tar" is another person's "pineapple and Jamaica-funk."

Conclusion

As rum producers begin to publish more technical information, it's important to understand the information they provide. You can't just take a measurement from one brand and compare it to another brand's measurement; you need to ensure it's an apple-to-apple comparison. Esters, volatile compounds, and congeners are not the same.

Two different rums with similar measurements of esters or volatile compounds can taste very different. To use an analogy, these measurements only tell you how loud the music is, but not what type of music is playing. Furthermore, rums that undergo aging can experience significant changes - both upwards and downwards - in compound levels. Comparing unaged distillates with their long-aged versions can be very misleading.