The bitter fate of coffee beans
Q
:What chemical changes occur with dark-roasting coffee beans? Are they what make coffee bitter?
A
The hundreds of chemicals that are extracted when coffee is brewed interact with one another, and with a person’s taste buds in ways that scientists are still sorting out. But recent studies by Thomas Hofmann of the Technical University of Munich and others point to some substances produced by roasting coffee beans as the key culprits.
An important chemical change that occurs with roasting is the breakdown of chemicals called chlorogenic acids. The darker the roast, the more of these acids are changed into other chemicals, notably antioxidants called chlorogenic acid lactones and phenylindanes.
Higher levels of lactones are the main suspects in light-roast to medium-roast coffee, and the phenylindanes, which result from the further breakdown of the lactones, are believed to be involved in the bitterness of very dark coffee, like espresso.
Espresso coffee is also made with the highest temperature of water
at the highest pressure, extracting even more of the bitter compounds.
The variety of bean also has an effect on perceived bitterness, with both caffeine and chlorogenic acids being more prominent in the robusta variety than in Arabica beans.
Adding a pinch of salt to a cup of coffee can counteract bitterness, the American Chemical Society suggests.
Dissolved, it releases sodium ions that can block the bitter chemicals. Another edible acid, like citric acid, or sugar would have a similar effect.