The mystery of fermented coffee

As specialty coffees gain traction around the world, a fermented version could bring a fruity-tasting coffee to the market. This beverage has a raspberry-like taste and aroma, but the cause of this has been a mystery. Scientists have reported six compounds that contribute to this, which may help increase production of the drink and make it more readily available.

The researchers presented their results at the spring meeting of the American Chemical Society (ACS). ACS Spring 2023 is a hybrid meeting being held virtually and in-person from 26–30 March in the US, and featuring more than 10,000 presentations on a wide range of science topics.

Chahan Yeretzian, the project’s principal investigator, said that the flavours coming from fermented coffee are some that are not traditionally associated with coffee.

This beverage can provide a unique flavour experience and potentially benefit farmers as it grows in demand. It can also be a more environmentally friendly option because the process by which the beans are prepared requires less water than traditional methods.

Yeretzian and colleagues from the Coffee Excellence Centre at Zurich University of Applied Sciences sought to identify the compounds that are responsible for these flavours.

To single out the compounds unique to fermented coffee’s aromas, researchers divided Arabica beans into three groups. One was prepared using a common wash process, where a gelatinous substance known as mucilage is stripped from the coffee bean, which is washed with water before being dried. The second group was prepared using the pulped natural process — another common approach — in which the skin is removed from the bean, but the mucilage is left intact. The third group of beans was fermented using carbonic maceration, a process often used in winemaking. 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.

The researchers then brewed coffee using each type of bean and analysed the samples with gas chromatography (GC) sniffing, also called GC olfactometry. The GC instrument separated individual components in the air above each sample. 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 like.

Yeretzian said the researchers have to rely on the human nose to detect the scent of each compound because the chemical signature doesn’t detect it.

This methodology can be tricky because of its subjective element, but the panel was consistent in the smells they described for this research, according to Samo Smrke, a research associate in the lab.

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 flavour 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 and judge the intensity of different flavours and scents. The researchers would also like to know more about how these compounds form. Some 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 how the compounds form could help the team standardise production methods and make it easier to produce fermented coffee at larger scales.

Image credit: iStock.com/EdwardGiovanniLuna