Scientists have discovered why some people perceive coffee as more bitter than others.

Although the caffeine found in coffee has a bitter taste, even a caffeine-free beverage can be bitter due to other compounds that contribute to this flavor. These compounds are formed during the roasting process of coffee beans. In studies published in two articles in the journal Food Chemistry, researchers from the Technical University of Munich (Germany) discovered a previously unknown class of substances that develop during coffee roasting.

To investigate this, the researchers focused on mozambioside present in Arabica beans. It is approximately ten times more bitter than caffeine and activates two out of 25 types of receptors responsible for bitter taste—TAS2R43 and TAS2R46. However, earlier studies by the same researchers indicated that the concentration of mozambioside significantly decreases during roasting, suggesting it may only have a minimal effect on the bitterness of the beverage. This led the scientists to explore whether the breakdown products of mozambioside are bitter and if they could influence the flavor of coffee.

It was found that during roasting, seven breakdown products of mozambioside are formed in varying concentrations (depending on the temperature and duration of roasting), all of which almost completely transfer into the beverage during brewing. The researchers tested their effects on cell cultures and discovered that these substances activate the same types of bitter taste receptors as mozambioside, with three of its breakdown products being even more potent than mozambioside itself.

Nevertheless, the concentration of these substances in brewed coffee remains too low to impart a noticeable flavor on their own. Only the combination of mozambioside and its breakdown products during roasting led to eight out of 11 participants in the experiment perceiving a bitter taste. One of the subjects described astringency, while the remaining two did not notice any distinct flavor characteristics.

Following this experiment, the scientists conducted a genetic test and determined that sensitivity to the bitter taste of coffee is influenced by genetic predisposition. Two volunteers possessed both defective variants of the TAS2R43 receptor gene. Seven participants had one intact and one defective variant of the receptor, while only two subjects had both copies of the gene intact.