Why Is Coffee Bitter?

Some pour a bit of milk. Others add sugar - which is quite unhealthy - to 'improve' the taste of their coffee. But now researchers have determined why dark-roasted coffee is so bitter, this being a first step towards producing a natural milder type of coffee. The research team made chemical analyses and follow-up tests with the help of some people trained to determine coffee bitterness, the chemicals that make coffee bitter and how they form.

A cup of coffee contains over 30 chemicals that confer its taste, aroma, and acidity. "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 % of coffee's bitter taste comes from caffeine," said Hofmann.

Hofmann's team has encountered two types of chemicals that give coffee most of its bitterness. Both types are antioxidants encountered in roasted coffee beans, not in the green (raw) beans.

One group, named chlorogenic acid lactones, is found 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. 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. Prolonged roasting leads to the formation of the most intense bitter compounds found in dark roasts. ", Hofmann said.

The way the beans are brewed has an impact on bitterness. The high pressures and temperatures required for brewing espresso-type coffees lead to the synthesis of the highest amounts of bitter organic chemicals. "Now that we've clarified how the bitter compounds are formed, we're trying to find ways to reduce them," said Hofmann.