Demystifying the Fruity Aroma of Fermented Coffee

Demystifying the Fruity Aroma of Fermented Coffee

History shows that when coffee businesses around the world started sourcing high-quality specialty coffees in the 1970s, delivering freshly roasted coffees to their customers - to be used at home or in coffee houses around the world, a 3rd wave of coffee took off.  Without a doubt, these specialty coffees gained in popularity, and its market share continues to grow.

Now a fermented version could bring a fruity taste to your morning cup of joe. And while there is no consensus about what the 4th wave of Coffee could be like, the emergence of heavily fermented coffees may indeed be part of the future of coffee.

Fermented coffees exhibit very characteristic, clearly defined, and intense aromas and command a price up to 100-times higher than the standard commodity market prices for Specialty coffees

This new kind of beverage has a raspberry-like taste and aroma, but what causes these sensations has been a mystery.

Scientists report six compounds that contribute to the fermented coffee experience. The work could help increase production of the drink and make it more readily available for everyone to enjoy.

The researchers presented their results at the 2023 spring meeting of the American Chemical Society (ACS), a hybrid meeting which was held virtually and in-person earlier this year March 26–30, and features more than 10,000 presentations on a wide range of science topics.

“There are now flavors that people are creating that no one would have ever associated with coffee in the past,” noted Chahan Yeretzian, Ph.D., Professor for Analytical Chemistry, Bioanalytical Chemistry and Diagnostics at Zurich University of Applied Sciences (ZHAW), and the project’s principal investigator.

“The flavors in fermented coffee, for example, are often more akin to fruit juices,” Yeretzian added.

A Unique Flavor Experience

This unusual type of beverage provides a unique flavor experience for consumers, and the growing demand for it means that fermented coffee beans can fetch a high price, potentially benefiting farmers. And the process by which the beans are prepared requires much less water than traditional methods, making it a more environmentally friendly alternative to a standard cup of coffee.

But despite this drink’s growing popularity, the compounds that cause its distinctive flavor were unknown. And with fermented coffee becoming more popular in competitive events, some people have been concerned that the lack of knowledge about fermented coffee may make it difficult to distinguish between the genuine product and regular joe that has been illicitly adulterated.

So, Professor Yeretzian and colleagues from the Coffee Excellence Center* at Zurich University of Applied Sciences sought to identify the compounds that are responsible for these new and exciting flavors. And because flavor and smell are intimately linked, studying the beverages’ scents could help the team gain a better understanding of how fermented coffee’s complex flavor is created.

To single out the compounds unique to fermented coffee’s aromas, researchers took arabica beans and divided them into three groups.

One was prepared using a wash process, which is likely how your average afternoon pick-me-up brew is made. Here, a gelatinous substance known as mucilage is stripped from the coffee bean, which is washed with water before being dried.

The researchers prepared the second group using the pulped natural process — another common approach — in which the skin is removed from the bean, but the mucilage is left intact.

Finally, the team fermented beans in the third group using carbonic maceration, a process often used in winemaking. This method was first introduced to the specialty coffee world in 2015, when the winning contestant in the World Barista Championship used it to prepare their entry. With this process, whole coffee fruits are fermented in stainless steel tanks and infused with carbon dioxide to lower the pH of the fermentation. Unlike the other brews, the coffee made with fermented beans was described as smelling intense, like raspberries with a hint of rose.

Brewing Coffee

Next, the researchers brewed coffee using the roasted and ground coffee of each type of bean. Then they analyzed the samples using solid-phase micro extraction gas chromatography (SPME-GC) and detected by both sniffing, also called GC olfactometry (GC-O), and mass spectrometry (GC-MS). 

First, the GC instrument separated individual components in the air above each sample. Then, as the compounds left the instrument, they went to a mass spectrometer for identification, and to someone sitting at the outlet to describe what they smelled.

“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,” Professor Yeretzian explained.

This methodology can be tricky because there is a subjective element to it. “We’re using people to detect scents, and everybody perceives flavors a little differently,” added Samo Smrke, Ph.D., 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,” Smrke further noted.

The study found six compounds that are considered contributing to the characteristic raspberry flavor of the CM coffee.

Human nose

There is one major advantage to GC sniffing. The human nose can sometimes detect scents from compounds that are at such a low concentration, they’re unable to be picked up by mass spectrometry. In this case, although six compounds appeared to contribute to the intense fruity flavor and the raspberry 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, as well as judge the intensity of different flavors and scents. Additionally, the researchers would like to know more about how these unique compounds form. Potential factors include farming practices, the variety of coffee beans, the microclimate of specific farms and the microbes present during fermentation.

“There’s still quite a lot of unknowns surrounding this process,” Smrke said.

"A better understanding of the sources of these compounds could help the team standardize production methods, making it easier to produce fermented coffee at larger scales and allowing even more people to enjoy this distinctive flavor," Smrke concluded.

The researchers acknowledge support and funding from Project Origin Australia and Zurich University of Applied Sciences.

The Coffee Excellence Center was founded in 2008 Chahan Yeretzian, Ph.D. and has ever since expanded its scope and expertise to become the leading public science, technology and innovation center on coffee. Professor Yeretzian believes that high-quality coffee can only be obtained with a profound and holistic understanding of the coffee value chain. Therefore, the Center is dedicated to state of the art research on coffee along the whole value chain. 


[1] Exploring unique coffee flavours of fermented high-end specialty coffee: Towards the fourth wave coffee [Presented during the spring meeting of the American Chemical Society, held March 26 - 30, 2023, Indianapolis and virtual]

Featured image: Identifying the compounds that give fermented coffee, pictured brewing here, its unique flavor and aroma could allow more people to enjoy it. Courtesy:  Samo Smrke. Used with permission.

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