Using chemical analyses and follow-up taste tests by humans trained to detect coffee bitterness, chemists in Germany and the United States have discovered the compounds that make coffee bitter and also how they form.
'Everybody thinks that caffeine is the main bitter compound in coffee, but that's definitely not the case,' said study leader Thomas Hofmann, a professor of food chemistry and molecular sensory science at the Technical University of Munich in Germany.
Just 15 percent of coffee's bitter taste comes from caffeine, said Hofmann, who presented his findings at a meeting of the American Chemical Society in Boston.
Using advanced chromatography techniques and a human sensory panel trained to detect coffee bitterness, Hofmann and his associates found that coffee bitterness is due to two main classes of compounds: chlorogenic acid lactones and phenylindanes, both of which are antioxidants found in roasted coffee beans. The compounds are not present in green (raw) beans, the researchers note.
Chlorogenic acid lactones, is present at high levels in light- to medium-roast brews. Dark roasts, such as espresso, showed high levels of phenylindanes, which form when the chlorogenic acid lactones break down and give a more lingering, harsh taste than their precursors, Hofmann said.
'We've known for some time that the chlorogenic acid lactones are present in coffee, but their role as a source of bitterness was not known until now,' Hofmann says.
'Roasting is the key factor driving bitter taste in coffee beans. So the stronger you roast the coffee, the more harsh it tends to get,' Hofmann added.
He further said that prolonged roasting leads to the formation of the most intense bitter compounds found in dark roasts.
How the beans are brewed also affects bitterness, the scientists found. The high pressures and temperatures used for brewing espresso-type coffees produce the highest levels of bitter compounds.
'Now that we've clarified how the bitter compounds are formed, we're trying to find ways to reduce them,' Hofmann said.
Hofmann and his associates are currently exploring ways to specially process the raw beans after harvesting to reduce their potential for producing bitterness. They are also experimenting with different bean varieties in an effort to improve taste.