Rum Crafting in Japan, Unlocking the Secrets of High-Ester Aromas

Rum, the Caribbean spirit crafted from sugarcane, is undergoing a transformation in Japan. At the helm of this innovation is Peninsule de Boso. Under Taisei Aoki's leadership, the business operates a distillery in Minamiboso, Chiba Prefecture. Partnering with microbiologists from Chiba University, the team aims to craft a uniquely Japanese rum featuring a distinctive ester aroma — compounds renowned for their enticing scent of ripe tropical fruits. 

This type of rum is rare, offering a chance for Japanese rum to gain international recognition.

Limited Distilleries

During the Age of Exploration, Columbus introduced sugarcane, rum's main ingredient, to the Americas. European settlers later brought their thirst for spirits across the Atlantic, spurring the creation of rum from local sugarcane.

Yeast ferments sugar to produce alcohol, and settlers recreated the flavors of their native spirits by using local sugarcane. Over time, distinct rum styles emerged. Spanish-style rum took inspiration from sherry, French-style from cognac, and British-style from Scotch whisky. 

Rum became synonymous with the seafaring lifestyle, as the British Navy and Caribbean pirates embraced it. Literary greats like Ernest Hemingway, author of The Old Man and the Sea, also enjoyed it.

Today, most rum sold globally is produced from molasses, a byproduct of sugar production, and is classified as traditional rum. In contrast, rum made directly from fresh sugarcane juice is known as agricole rum.

Among the diverse flavor profiles of rum, one of the rarest is high-ester rum, which boasts a strong, fruity aroma. Only a handful of distilleries — primarily in Jamaica and Reunion Island — produce this exceptionally rare style of rum. To date, Japan has yet to produce its own.

Unlocking the Secrets of Mash Bacteria

At the Boso Oigura Distillery in Minamiboso, brewers craft agricole rum using freshly harvested sugarcane juice from their own fields. The juice is fermented by yeast into moromi (fermentation mash), a low-alcohol mixture. Distilling this mash concentrates the alcohol to create raw rum, which producers then age in barrels to deepen its flavor and complexity.

The mash plays a pivotal role in developing the compounds that define high-ester rum's signature aroma. Bacteria within the mash produce organic acids such as acetic acid and butyric acid, which react with alcohol created by yeast. 

These interactions yield ethyl compounds — the building blocks of ester aromas. For example, acetic acid forms ethyl acetate, while butyric acid forms ethyl butyrate. Crafting high-ester rum involves carefully selecting the optimal combinations of alcohols and acids to create the most desirable aromas.

About Aromas

Professor Seigo Amachi of Chiba University, an expert in applied microbiology, leads this exploration. One focus of his research is the waste liquid left after rum distillation, which retains residual sugar. Peninsule de Boso provides re-fermented samples of this liquid to Professor Amachi's lab, where he carefully analyzes the bacteria present. 

By identifying the specific types of bacteria and their roles in ester formation, the collaboration aims to uncover the mechanisms behind crafting the complex aromas of high-ester rum.

One of the key tools they use in the lab is a gas chromatograph-mass spectrometer (GC-MS). This equipment analyzes scent components in samples. High-ester rum, known for its prominent aroma, is characterized by a high ester value — a metric quantified using this equipment.

The team then shares the lab findings with Peninsule de Boso, which then uses them to refine the rum-making process. This includes proposing various ester profiles to enhance the product's uniqueness. 

Professor Amachi, a self-proclaimed rum enthusiast, shared his passion for the project. "Until now, I primarily focused on basic research, so I'm thrilled to be contributing to the local community in this way," he stated. "We aim to create world-class high-ester rum that people around the globe will enjoy."

Boso's Unique Dunder

Re-fermented distillation waste, known as dunder, is a rich source of organic acids that significantly influence ester aromas when added to mash. Dunder is a cornerstone of high-ester rum production in regions like Jamaica, where distilleries fiercely guard its composition as a trade secret, making it difficult to analyze.

However, dunder's local environment shapes its composition. If the southern Boso Peninsula can cultivate ester aromas unique to its own dunder, it could pave the way for a truly distinctive rum.

Taisei Aoki, head of Peninsule de Boso, remarked, "It will take repeated trial and error." He added, "But we are determined to produce Japan's first high-ester rum and demonstrate its potential to the world."

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Author: Shinji Ono, The Sankei Shimbun

(Read the article in Japanese.)