The researchers identified two compounds associated with the bitterness of coffee.

Adding some milk and sugar to a steaming cup of coffee is a sign that many people start a new day. Although this is done to reconcile the bitterness of the drink, the reason why coffee makes us frown has puzzled scientists for decades. Now, researchers have narrowed the search by identifying two compounds associated with the bitterness of coffee, whether mild breakfast coffee or Italian espresso. A study presented at the annual meeting of the American Chemical Society in Boston showed that it was the roasting process-not the coffee beans themselves-that produced these compounds, a discovery that opened the door to improving the processing of coffee beans.

A cup of coffee is a complex drink made up of more than 30 chemicals that determine the taste, aroma and acidity of the coffee. Since the 1930s, scientists have isolated and identified a variety of chemicals related to the sensory composition of coffee, but no one can figure out what makes coffee so bitter.

To solve the mystery, Thomas Hofmann, a food chemist at the Technical University of Munich, Germany, and his colleagues continuously filtered the brewed coffee. They found that a small portion of the filter products contained the lowest molecular weight and the most bitter compounds, which undoubtedly provided a goal for the research team to conduct in-depth analysis. Using mass spectrometry, the researchers determined that one of the compounds was a decomposition product of chlorogenic acid lactone, chlorogenic acid, which is found in almost all plants. The team then baked a range of drinks from regular coffee to bitter coffee and measured the amount of chlorogenic lactone in each coffee.

The researchers found that roasting coffee beans will produce a continuous reaction, first, the chlorogenic acid in coffee beans will be converted to chlorogenic acid lactone, and then, if the processing continues, the latter will be broken down into phenyl dihydrogenated indene. Hofmann pointed out that chlorogenic acid lactone can produce moderate bitterness in lightly or moderately roasted coffee, while the subsequent secondary decomposition products are the root cause of the "bitterness" of espresso.

Peter Martin, a professor of psychiatry at Vanderbilt University in Nashville, Tennessee, said the identification of secondary decomposition products provides new material for studying the potential health effects of coffee. Martin is currently the director of the Coffee Research Institute of the university. "just as we know very little about chlorogenic acid, we don't know much about other products related to it," he said. " "this study broadens our horizons to better understand the health effects of coffee, and for the coffee industry, it will help improve the taste of coffee," Martin said.

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