Caffeine is an intensely bitter compound, and coffee beverages can contain caffeine at concentrations far above the human threshold for perceiving bitterness. They first compared the bitterness of liquid caffeine in decaffeinated coffee with liquid caffeine in water. The coffee masked caffeine’s taste so effectively that it took roughly 10 times the amount of caffeine found in a standard brew before the panel perceived it. The researchers propose that caffeine interacts with high-molecular-weight melanoidins, leaving less free caffeine available to activate bitter taste receptors. “A plethora of bitter stimuli, generated during the roasting process, culminate in the unique, bitter taste of coffee beverages,” said Gigl.