There are countless blends and ways to brew coffee. Indeed, some of the world’s best baristas have come up with very interesting ways to serve coffee through constant experimentation. One of the more out-there kinds of coffee which is increasingly making its way into specialty shops is fermented coffee.
Rather than bitter, fermented coffee has a fruity taste that has been often been described as raspberry-like with a hint of rose. However, what exactly is responsible for this striking flavor and aroma has been a mystery — until now.
In a new study, scientists at the Coffee Excellence Center at Zurich University of Applied Sciences singled out the chemical compounds that are responsible for these exciting new flavors using a combination of state-of-the-art analytical tools and the good ol’ fashioned human nose.
Demystifying the secrets behind the flavor of fermented coffee
Fermented coffee is made using a natural fermentation process that occurs when coffee cherries are left to ferment in their own juices for several days. All coffee beans are at least a bit fermented, but unlike traditional coffee processing methods that involve washing and drying the cherries, fermented coffee is left to ferment in a controlled anaerobic environment.
During the fermentation process, yeast and bacteria present on the coffee cherry’s skin consume the sugars and amino acids in the cherry, breaking them down into chemical compounds that give this special brew its distinct flavor. The resulting beverage is a complex and aromatic coffee with a unique taste profile that can range from fruity and floral to earthy and spicy.
“It is quite surprising how this coffee can taste completely different from what most drinkers are used to. Those coffees, prepared as filter brew, espresso, or with milk, perhaps would not be recognizable as a coffee in a blind tasting to the typical coffee drinker. Most aroma compounds that we see in coffees are very similar, however, a very special aroma with a very strong odor can completely change the flavor of a coffee. It is like a symphony of odors that work together and create a complex tasting experience,” Samo Smrke, a research associate in the lab at the Coffee Excellence Center, told ZME Science, adding that:
“High-quality fermented coffee is like expensive wine – it is not for everyday drinking and gives us a unique sensory experience. There is one big difference though – wine gets better as it ages, but coffee should be consumed fresh, somewhere around between 2 and 6 weeks after roasting.”
The rapid rise in popularity of fermented coffee in craft shops and cafés inspired Smrke and colleagues to investigate the strong fruity flavors of this specialty brew in greater detail than ever before.
Working closely together with an important coffee producer, the researchers fermented Gesha coffee beans in three different ways.
One was prepared using a standard wash process that strips away a gelatinous substance known as mucilage, which your typical local corner store probably coffee underwent before brewing.
The second group was treated with a pulped natural process, which is yet another common approach, in which the skin of the bean is removed but the mucilage is left intact.
Finally, the third type of beans was fermented using carbonic maceration, a process that is also used in winemaking. This is the same method as that used by the winning contestant of the 2015 World Barista Championship.
In order to make sure that they were comparing apples to apples, the researchers went to great lengths to source their beans from the same lot on the coffee farm. These are not your typical coffee beans, as they can cost hundreds of dollars per pound.
Each sample of brewed coffee was analyzed using gas chromatography, which separated the volatile compounds into separate individual units. These separate compounds were then funneled through a mass spectrometer that identifies the nature of each compound.
But they didn’t stop there. The researchers also had human subjects sniff each brewed sample — and their contribution proved quite substantial and important. This step was deemed necessary because gas chromatography and mass spectrometry, despite their objective measurements, cannot really tell you what something smells like. In that regard, the nose is still king.
“We have found that some of the coffees are incredibly strong smelling with our nose being able to detect it in very low concentrations. It was extremely surprising to see the odor description of the panelists who sniffed the samples from the gas chromatograph. We saw descriptors, such as “extremely sweet”, “crazy like strawberry”, and “artificially sweet”. However, when looking at the mass spectrometry data, we were not able to detect any compounds that could be attributed to those flavors. We found that despite that technological advancements, we still end up being limited with the instruments that are outperformed by our noses,” Smrke said.
Although six compounds appeared to be responsible for the intense fruity aroma of fermented coffee, the researchers could only identify three of them in their samples: 2-methylpropanal, 3-methylbutanal and ethyl 3-methylbutanoate.
There are many questions that remain unanswered, though. The researchers hope to identify the rest of the flavor-inducing compounds, as well as how these compounds form during the fermentation process in the first place. They suspect factors like farming practices, the variety of coffee beans, the climate in which the beans are grown, and the microbes involved in the fermentation process all play key roles.
There are also practical implications to the findings. Farmers can take note and modify coffee flavors through fermentation and thus generate significantly more added value as these products fetch a much higher price than regular coffee.
“The coffee farm that we worked with is a well-known farm among the high-end coffee producers and our study demonstrates that post-harvest fermentation generates extremely fruity flavors. This brings the industry to a better view as the allegations around the coffees “infused” with fruit have not been positive to the industry,” Smrke said.
The findings were presented at this week’s ACS Spring 2023 meeting. You can follow Samo Smrke on Instagram, where he regularly shares coffee science content.
