Chemists have figured out the compounds behind fermented coffee’s fruity flavour 

Far from your average cup of joe, fermented coffee, also known as cultured coffee, takes its flavour profile to a whole new level.

First introduced by Australian barista and coffee roaster, Sasa Sestic, during the 2015 World Barista Championships, fermented coffee has since taken the specialty coffee world by storm.

“There are now flavours that people are creating that no one would have ever associated with coffee in the past. The flavours in fermented coffee, for example, are often more akin to fruit juice,” says coffee scientist Dr Chahan Yeretzian, principal investigator of a project investigating the cultured coffee experience.

Despite its growing popularity, the compounds that contribute to fermented coffee’s flavours and aromas have remained unknown until now. But with it becoming more popular in competitive events, there have been concerns that this lack of knowledge may make it difficult to distinguish between the genuine product and ones that have simply been infused with flavour.

In new research Yeretzian and colleagues discovered that six compounds are responsible and identified three of them.

Fermented coffee is made through a process called carbonic maceration, which is often used in winemaking. During this process, whole coffee fruits are fermented in stainless steel tanks and infused with carbon dioxide to lower the pH of the fermentation.

The researchers took arabica beans and divided them into three groups: two prepared using standard processes and the third with carbonic maceration.

After brewing coffee using each type of bean, they analysed the samples using gas chromatography (GC) sniffing, also called GC olfactometry.

During this process the GC instrument separates out the components in the air above each coffee sample. Then, as they leave the instrument, the compounds are identified by a mass spectrometer, and then by a human sitting at the outlet describing what they smell.

“Because the chemical signature doesn’t tell us how a compound smells, we have to rely on the human nose to detect the scent as each compound comes out of the chromatography instrument individually,” says Yeretzian.

This methodology can be tricky because there is a subjective element to it, but the researchers say it worked incredibly well in this case.

Espresso being extracted from fermented coffee beans
Identifying the compounds that give fermented coffee, pictured brewing here, its unique flavor and aroma could allow more people to enjoy it. Credit: Samo Smrke

“We’re using people to detect scents, and everybody perceives flavours a little differently,” says Dr Samo Smrke, a research associate in the lab who is presenting the results.

“But in this case, the panel was very consistent in the smells they described. So, what is traditionally considered a challenge was actually not an issue because the aromas were so clear.”

Unlike the other brews, the coffee made with fermented beans was described as: “smelling intense, like raspberries with a hint of rose.”

There’s just one draw-back to this technique; the human nose can sometimes detect scents from compounds that can’t be picked up by mass spectrometry.

So, while six compounds appeared to contribute to the scent of the fermented coffee, the team was only able to identify three of them: 2-methylpropanal, 3-methylbutanal and ethyl 3-methylbutanoate.

In the future, the researchers hope to identify the remaining compounds and understand more about how they form, to potentially help standardise production methods and allow fermented coffee to be produced at larger scales.

This research was presented at the Spring 2023 meeting of the American Chemical Society.

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