Bitter taste can ruin a cup of conventional or organic coffee. German and U.S. scientists announced last week that they’ve identified the chemicals responsible for bitterness—a finding that could ultimately lead to a better-tasting brew.  

Over the years, research has identified roughly 25–30 compounds that could contribute to coffee’s perceived bitterness. The main cause, however, has remained largely unexplored.  

“Everybody thinks that caffeine is the main bitter compound in coffee, but that’s definitely not the case,” says study leader Thomas Hofmann, PhD, a professor of food chemistry and molecular sensory science at the Technical University of Munich in Germany. Only 15% of coffee’s perceived bitterness comes from caffeine, he estimates, noting that caffeinated and decaffeinated coffee have similar bitterness qualities.  

“Roasting is the key factor driving bitter taste in coffee beans,” Dr. Hofmann says, “so the stronger you roast the coffee, the more harsh it tends to get.” Prolonged roasting triggers a cascade of chemical reactions that lead to the formation of the most intense bitter compounds.  

Using advanced scientific techniques and trained human taste testers, Dr. Hofmann and his associates linked coffee bitterness to two classes of compounds: chlorogenic acid lactones and phenylindanes (antioxidants found in roasted coffee beans). The compounds are not present in green (raw) beans, the researchers note.  

How you brew your coffee also matters. Espresso-style coffee, made using high pressure combined with high temperatures, tends to produce the highest levels of bitter compounds. While home-brewed coffee and standard coffee-shop brews are relatively similar in their preparation methods, their perceived bitterness can vary considerably, depending on the roasting degree of the beans, the amount of coffee used and the variety of beans used.  

Some instant coffees are actually less bitter than regular coffee, Dr. Hofmann says, because the way they’re prepared (pressure extraction) degrades some of the bitter compounds.  

“Now that we’ve clarified how the bitter compounds are formed, we’re trying to find ways to reduce them,” he says. He and his associates are exploring ways to process raw beans after harvesting to reduce their bitterness potential. They’re also experimenting with different bean varieties to improve taste.  

The researchers believe a better cup of coffee is just around the corner. This makes Dr. Hofmann particularly happy: He’s an avid coffee drinker with a passion for dark-roasted varieties.