The fruity flavors of brewed coffee dispel the mystery

Specialty coffees are gaining traction in coffee shops around the world – and now a brewed version can add a fruity note to your morning cup of joe. This new type of drink has a raspberry-like taste and aroma, but the reason for these sensations has been a mystery. Today, scientists report six compounds that contribute to the brewed coffee experience. The work could help increase production of the drink and make it more readily available for everyone to enjoy.

The researchers will present their findings at the American Chemical Society (ACS) spring meeting. ACS Spring 2023 is a hybrid meeting that takes place virtually and in person from March 26-30.

“There are now flavors that people make that no one would associate with coffee in the past,” says Chahan Yeretzian, PhD, the project’s principal investigator. “The flavors in brewed coffee, for example, are often more akin to fruit juices.”

This unusual type of beverage offers a unique flavor experience to consumers, and the increased demand for it means that brewed coffee beans can fetch a high price, which may benefit farmers. And the process of brewing the beans requires less water than traditional methods, making it a more environmentally friendly alternative to a regular cup of coffee.

But despite the beverage’s growing popularity, the compounds causing its distinctive flavor were unknown. And with brewed coffee becoming more and more popular at competitive events, some people were concerned that a lack of knowledge of brewed coffee might make it difficult to distinguish the original product from the usual brewed coffee that had been illegally brewed. Therefore, Yeritzian and colleagues from the Center of Coffee Excellence at Zurich University of Applied Sciences sought to identify the compounds responsible for these new and exciting flavours. And because flavor and aroma are so closely linked, studying the beverage’s aromas could help the team gain a better understanding of how coffee’s complex flavor is created.

To characterize the unique flavor compounds of brewed coffee, the researchers took arabica beans and divided them into three groups. One was brewed using the washing process, which is likely the way the regular afternoon brew is brewed. Here, a gelatinous substance known as mucilage is stripped from the coffee beans, which are washed with water before being dried.

The researchers prepared the second group using a natural pulping process — another popular method — in which the skin is removed from the bean, but the gum remains intact.

Finally, the team fermented the beans in the third batch using carbon maceration, a process often used in winemaking. The method was first introduced to the world of specialty coffee in 2015, when the winning contestant of the World Barista Championship used it to prepare his share. With this process, whole coffee berries are fermented in stainless steel tanks and infused with carbon dioxide to lower the pH of the brewing process. Unlike other drinks, coffee made from brewed beans has been described as strong in aroma, like raspberries with a touch of rose.

Next, the researchers brewed coffee using each type of bean and analyzed the samples using gas chromatography (GC), also called olfaction. First, the GC instrument separated the individual components into the air above each sample. Then, when the vehicles left the device, they went to a mass spectrometer to identify them, and to a person sitting at the outlet to describe the odor they smelled.

“Because a chemical signature doesn’t tell us what a compound smells like, we have to rely on the human nose to detect the odor as each compound comes out of the chromatography instrument individually,” says Eritzian. This methodology can be tricky because it has a subjective component to it. “We use people to detect smells, and everyone perceives flavors a little differently,” says Samo Smrke, PhD, a research associate in the lab presenting the findings. “But in this case, the painting was very consistent in the scents they described. So what is traditionally considered a challenge wasn’t actually a problem because the scents were so pronounced.”

There is one major advantage to GC sniffing. The human nose can sometimes detect odors from compounds at a lower concentration, which cannot be picked up by a mass spectrometer. In this case, although six compounds appear to contribute to the intense fruity flavor and aroma of berries in brewed coffee, the team was only able to identify three: 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 aromas. In addition, the researchers want to know more about how these unique compounds form. Possible factors include agricultural practices, the variety of coffee beans, the microclimate of particular farms, and the microbes present during brewing.

“There are still a lot of unknowns surrounding this process,” says Smrke. A better understanding of the sources of these compounds could help the team standardize production methods, making it easier to produce coffee brewed at greater levels and allowing more people to enjoy this distinctive flavour.