Two compounds related to the bitterness of coffee.

Adding milk and sugar to a steaming cup of coffee is a sign that many people start their day.

Although this is done to temper the bitter taste of the drink, what makes coffee frown has puzzled scientists for decades. Now, researchers have narrowed that quest by identifying two compounds linked to the bitter taste of coffee, whether mild breakfast coffee or Italian espresso. A study presented at the annual meeting of the American Chemical Society in Boston shows that it is the roasting process-not the coffee beans themselves-that produces these compounds, a finding that opens the door to improving the process of processing coffee beans.

A cup of coffee is a complex beverage made up of more than 30 chemicals that determine its taste, aroma and acidity. Since the 1930s, scientists have isolated and identified a number of chemicals involved in coffee's sensory makeup, but no one has been able to figure out what makes coffee so bitter.

To solve this mystery, Thomas Hofmann, a food chemist at the Technical University of Munich in Germany, and colleagues performed a series of filters on brewed coffee. They found that a small fraction of the filtered product contained the smallest and most bitter compounds, which undoubtedly provided the team with a target for further analysis. Using mass spectrometry, the researchers identified one of the compounds as chlorogenic acid lactone, a breakdown product of chlorogenic acid, which is found in almost all plants. The team then roasted a range of drinks from regular to bitter coffee and measured the amount of chlorogenic lactone in each coffee.

The researchers found that roasting coffee beans produces a continuous reaction in which chlorogenic acid in the beans is first converted to chlorogenic acid lactone, which then breaks down to phenylindane if processing continues. Hofmann points out that chlorogenic acid lactone can produce moderate bitterness in light or medium roast coffee, and the secondary decomposition products are the root cause of strong coffee "bitter".

Peter Martin, a professor of psychiatry at Vanderbilt University in Nashville, Tennessee, said the identification of secondary breakdown products provides new material for studying coffee's potential health effects. Martin is currently Director of the Coffee Institute at the university. "Just as little is known about chlorogenic acid, we don't know much about other products related to it," he said. Martin said: "This research broadens our horizons to better understand the health effects of coffee, which for the coffee industry will help improve the taste of coffee."