Why is pure coffee bitter?

Why is coffee bitter? This problem has plagued scientists for decades.

Now, scientists have narrowed down their research and targeted two chemical molecules. At the annual meeting of the American Chemical Society, scientists reported that the two bitter chemical molecules were produced during the processing of coffee, a discovery that could revolutionize the coffee processing industry.

Thomas Hoffman of the University of Technology in Munich, Germany, and his colleagues once again separated the brewed coffee. After analysis by mass spectrometer, Hoffman identified one of the molecules, chlorogenic acid lactone, which is a decomposition product of chlorogenic acid (found in almost all plants). Hoffman's team then prepared a series of different coffees to determine the content of chlorogenic acid lactone. Although lactone will only produce mild bitterness in mild and moderate roasted coffee, the secondary decomposition products of lactone will produce strong bitterness if the coffee beans are roasted for a long time.