New insights into the perception of coffee taste -- genetic predisposition plays a role

Why does coffee taste more bitter to some people than it does to others? Researchers at the Leibniz Institute for Food Systems Biology at the Technical University of Munich have now come closer to answering this question

Why does coffee taste more bitter to some people than it does to others? Researchers at the Leibniz Institute for Food Systems Biology at the Technical University of Munich have now come closer to answering this question. They have identified a new group of bitter compounds in roasted Arabica coffee and have investigated how they influence its bitter taste. In addition, they demonstrated for the first time that individual genetic predisposition also plays a role in determining how bitter these roasting substances taste.

Although caffeine has been known as a bitter tasting substance for a long time, even decaffeinated coffee tastes bitter. This shows that other substances also contribute to the bitter taste of roasted coffee. Coline Bichlmaier, a doctoral student at the Leibniz Institute, explains: "Indeed, previous studies have identified various compound classes that are formed during roasting and contribute to bitterness. During my doctoral thesis, I have now identified and thoroughly analyzed another class of previously unknown roasting substances."

The starting point of her research was the mozambioside contained in Arabica beans. It tastes about ten times more bitter than caffeine and activates two of the approximately 25 bitter taste receptor types found in the human body, namely the TAS2R43 and TAS2R46 receptors. "However, our investigations showed that the concentration of mozambioside decreases significantly during roasting, so that it only makes a small contribution to the bitterness of coffee," says principal investigator Roman Lang, and continues: "This prompted us to test whether roasting produces breakdown products of mozambioside are also bitter and could affect coffee's taste."

Combination effect and genetic predisposition are important

As the research team shows, seven different degradation products of mozambioside are formed during roasting. These compounds are found in roasted coffee in varying concentrations, depending on roasting temperature and duration, and almost completely pass into the beverage during brewing.

Investigations in a cellular test system established at the institute show that these roasting substances activate the same bitter taste receptor types as mozambioside. Three of the roasting products even had a stronger effect on the receptors than the original compound. However, the researchers found that the concentrations of these roasting products measured in brewed coffee were too low to induce a noticeable taste on their own. Only the combination of mozambioside and its roasting products in a sample led eight out of eleven test subjects to perceive a bitter taste. One person found the taste astringent and two did not perceive any particular taste.

A genetic test showed that taste sensitivity depended on the genetic predisposition of the test subjects: two people had both copies of the TAS2R43 gene variant defective. Seven had one intact and one defective variant of the receptor and only two people had both copies of the gene intact.

What do the results mean for the future?

"The new findings deepen our understanding of how the roasting process influences the flavor of coffee and open up new possibilities for developing coffee varieties with coordinated flavor profiles. They are also an important milestone in flavor research, but also in health research," says Roman Lang, explaining: "Bitter substances and their receptors have further physiological functions in the body, most of which are still unknown." According to Lang, there is still a lot of work to be done, since for many bitter substances in coffee alone, it is not yet known which bitter taste receptors they activate, even though millions of people worldwide drink coffee every day.

More information

Mozambioside

Arabica coffee contains bitter-tasting mozambioside (11-O-β-D-glucosyl-cafestol-2-one). It is a hydrophilic derivative of cafestol that is particularly abundant in naturally caffeine-free varieties. Its perception threshold in humans is 60 ± 10 micromolar (Lang et al., 2015; Lang et al., 2020). Its concentration in raw Arabica coffee is about 0.4-1.2 micromol/g. Roasting breaks down the substance to below the taste threshold in the end product.

Other bitter-tasting roasting substances in coffee

The most important bitter tasting substances that are reportedly formed during roasting include caffeoylquinides, which are formed from chlorogenic acids, diketopiperazines, which originate from coffee proteins, and oligomers of 4-vinylcatechols, which are formed from caffeic acids. These compound classes do impart a bitter taste, but it is not yet known which bitter taste receptor types mediate their perception.

Bitter taste receptors

In humans, about 25 different taste receptor types are responsible for the perception of bitter substances. These bitter taste receptors are not only found in the mouth, but also on cells of other organs and tissues. The many functions they perform there are the subject of numerous studies, including those conducted at the Leibniz Institute for Food Systems Biology at the Technical University of Munich. Various studies already indicate that bitter taste receptors in the respiratory tract help to ward off pathogens and accelerate the movement of cilia. They also suggest that endogenous bitter receptors in the intestines and blood cells support defense mechanisms or are involved in the regulation of metabolism.