2026.06.21
コラム
High-Ester Rum and BAGASSE MACERATION™
What Is High-Ester Rum?
As you dig deeper into rum and taste your way through different bottles, you eventually begin to hear the word “high ester” here and there.
Esters are compounds formed when various organic acids bind with ethanol. Often described as fruity in aroma, they are among the most important elements in the aromatic profile of alcoholic beverages.
Depending on the category of spirit, esters may be highly valued or treated as something undesirable, which makes them rather mysterious in their own way. In rum, however, they are considered extremely important. A style in which these esters are pushed to an especially high level is known as high-ester rum.
In recent years, high-ester rum has been gaining popularity. Centered mainly around Jamaica, a handful of distilleries produce highly distinctive rum marks using their own closely guarded methods. One well-known approach involves collecting the liquid left after distillation and using it to cultivate organic acids and microorganisms, creating what is often called “dunder.” This dunder is then added to the fermenting rum wash to encourage the formation of esters.
BOSO Rhum Mer-海- Cuvée Spéciale ESTERY2024
At Péninsule de BOSO, we have been researching high-ester rum since our founding, alongside our agricole rum made from sugarcane grown through our own original farming methods.
The first Japanese high-ester rum produced using the dunder-based method described above is BOSO Rhum Mer-海- Cuvée Spéciale ESTERY2024.
ESTERY2024 is a style rich in acetate-based esters. Because its sharp, stimulating aroma stood prominently in the top notes, we bottled it after aging it for about three months in a bourbon cask.
The cask used for this maturation was later sent to Saburomaru Distillery through a cask exchange, and is now being used there to mature whisky.
Beginning Joint Research with Professor Seigo Amachi of the Research Group of Microbial Engineering, Faculty of Horticulture, Chiba University
Although we had managed to create a rum that could somehow be called “high ester,” we were still facing several issues, including low alcohol yield and instability in the esters being produced.
More importantly, when we looked toward our original goal of creating a uniquely Japanese high-ester rum, we felt that our in-house knowledge and research alone were not enough to see a clear path forward. So we applied for a joint research project through the industry-academia collaboration office at Chiba University.
The person they connected us with was Professor Seigo Amachi, who researches microbial engineering at the Faculty of Horticulture, Chiba University.
I am not sure how much I am allowed to say about Professor Amachi, but during our very first online consultation, he started pulling out various bottles of rum from his laboratory. He is, without question, a true lover of alcohol.
On the very same day as that consultation, he came all the way to BOSO Oigura Distillery. Later, the two of us ended up drinking together at the bar in Chidori Bekkan. That encounter became the starting point for the restart of our high-ester rum research.
The first step at Chiba University was to analyze and compare the aromatic components of ESTERY2024 and high-ester rums from other producers using gas chromatography-mass spectrometry, or GC/MS.
The results clearly showed that ESTERY2024 was strongly focused on ethyl acetate, and that it contained fewer types of esters compared with other high-ester rums.
That said, it is still a fascinating rum in its own right, and I personally continue to enjoy drinking it even now.
My hypothesis was that the pits used in Jamaica to cultivate dunder — essentially reservoirs — are large in scale, and that the accumulation of time, together with the local climate, allows them to generate a wide range of microorganisms and organic acids.
The question was how we could translate that idea into a small-scale system that could actually be operated at our distillery.
The approach we adopted was an application of mizumoto, an ancient Japanese sake-brewing method.
We also received support here from Tecchan of Pukupuku Brewing.
Our mizumoto rum method involves first generating various organic acids in the water used to dissolve the molasses, which is the raw material. We then use that water to prepare the rum wash and ferment it with yeast.
As we repeated trials, fruity esters that had not been present in ESTERY2024 gradually began to appear.
However, we were still left with a major challenge: organic acids that had not fully undergone esterification remained in the wash, resulting in a significant amount of off-flavors.
An Original High-Ester Method Born from Chance
To digress slightly, around the time we were researching mizumoto rum, we were also facing another issue.
The specially made domestic molasses that we had been using as the raw material for our traditional rum, BOSO Rhum Fleur-花-, was discontinued, forcing us to change the specification.
The new molasses we switched to had a very straightforward and clean flavor. As a rum, it produced a clear spirit with very little funk, but it was the complete opposite of the distinctive character that Fleur had previously possessed.
We wanted to create a new Fleur that would still feel unmistakably BOSO, even with this new molasses.
The method we adopted was one we had already been testing: using bagasse, the fibrous residue left after pressing sugarcane, as one of the raw materials, in order to add the terroir of our own sugarcane to a traditional rum.
In our trials, we found that this approach produced a rum with a heavier, more grounded character, quite different from the previous Fleur.
The remaining challenge was how to preserve bagasse, which deteriorates very quickly, so that it could be used year-round for a regular product in our brand lineup.
We tried every storage method we could think of.
Then, after the mizumoto rum development had reached a certain point, we went to check the bagasse we had been storing in order to begin trials for a bagasse-based rum.
To our surprise, one of the stored batches had developed an intense ester aroma.
Still shocked, I immediately called Professor Amachi at Chiba University to explain the situation.
The very next day, he came to the site, collected samples, and analysis began.
A few days later, we learned what had happened: among the yeasts living in the bagasse, there were strains with a high ability to produce organic acids. Under certain conditions, those yeasts became dominant, generating organic acids while also contributing to ester formation.
By combining what we had learned through mizumoto with this new discovery, and repeating trial after trial, we gradually became able to produce raw rum containing a wide variety of esters with a high degree of reliability.
At the same time, we also learned that some of the esters produced by this method can become off-flavors if they are too strong.
In other words, depending on how it is used, this method can push the rum in a very positive direction — but it can also push it in the wrong direction.
Release of BAGASSE MACERATION and Future Developments
Looking ahead, our goal is to share this method not only within our own distillery, but also with other rum producers, and to spread it to the world as a high-ester rum production method born in Japan.
However, before doing so, we believe it is essential to establish the method to a certain degree and become capable of consistently producing an original Japanese high-ester rum.
For that reason, we named the method BAGASSE MACERATION and filed for trademark registration and patent protection.
The raw rum produced in last year’s trials was stored in stainless steel tanks for eight months, allowing the esters to come together and settle. It will soon become the first release in our Mer-海- category.
Meanwhile, Fleur-花- has also been reborn, moving away from its former category as a traditional rum.
It is now a house blend called ASSEMBLAGE MAISON, built on a base rum made from the new molasses, blended with Soleil-太陽- and BAGASSE MACERATION to layer in the terroir of our own sugarcane.
This season, we plan to begin another BAGASSE MACERATION production around summer.
Compared with last year, we will expand both the scale and the number of refinements, deepening our research in pursuit of an even better result.
We hope to continue sharing the evolution of BAGASSE MACERATION, a Japanese-born high-ester rum method, with as many spirits lovers as possible.
And of course, we are also aging some in casks — so please look forward to that as well.
