Battling Bitter Coffee -- Chemists Vs. Main Source of Coffee Bitterness

Newswise — Bitter taste can ruin a cup of coffee. Now, chemists in Germany and the United States say they have identified the chemicals that appear to be largely responsible for java's bitterness, a finding that could one day lead to a better tasting brew. Their study, one of the most detailed chemical analyses of coffee bitterness to date, was presented today at the 234th national meeting of the American Chemical Society.

Research by others over the past few years has identified an estimated 25 to 30 compounds that could contribute to the perceived bitterness of coffee. But the main cause of coffee bitterness has remained largely unexplored until now, the researchers say.

"Everybody thinks that caffeine is the main bitter compound in coffee, but that's definitely not the case," says study leader Thomas Hofmann, Ph.D., a professor of food chemistry and molecular sensory science at the Technical University of Munich in Germany. Only 15 percent of java's perceived bitterness is due to caffeine, he estimates, noting that caffeinated and decaffeinated coffee both have similar bitterness qualities.

"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," Hofmann says, adding that prolonged roasting triggers a cascade of chemical reactions that lead to the formation of the most intense bitter compounds.

Using advanced chromatography techniques and a human sensory panel trained to detect coffee bitterness, Hofmann and his associates found that coffee bitterness is due to two main classes of compounds: chlorogenic acid lactones and phenylindanes, both of which are antioxidants found in roasted coffee beans. The compounds are not present in green (raw) beans, the researchers note.

"We've known for some time that the chlorogenic acid lactones are present in coffee, but their role as a source of bitterness was not known until now," Hofmann says. Ironically, the lactones as well as the phenylindanes are derived from chlorogenic acid, which is not itself bitter.

Chlorogenic acid lactones, which include about 10 different chemicals in coffee, are the dominant source of bitterness in light to medium roast brews. Phenylindanes, which are the chemical breakdown products of chlorogenic acid lactones, are found at higher levels in dark roasted coffee, including espresso. These chemicals exhibit a more lingering, harsh taste than their precursors, which helps explain why dark-roasted coffees are generally more bitter, Hofmann says.

The type of brewing method used can also influence the perception of bitterness. Espresso-type coffee, which is made using high pressure combined with high temperatures, tends to produce the highest levels of bitter compounds. While home-brewed coffee and standard coffee shop brews are relatively similar in their preparation methods, their perceived bitterness can vary considerably depending on the roasting degree of the beans, the amount of coffee used, and the variety of beans used.

Some instant coffees are actually less bitter than regular coffee, Hofmann says. This is because their method of preparation, namely pressure extraction, degrades some of the bitter compounds. In some cases, as much as 30 to 40 percent fewer chlorogenic acid lactones are produced, leading to a reduced perception of bitterness, he says.

"Now that we've clarified how the bitter compounds are formed, we're trying to find ways to reduce them," Hofmann says. He and his associates are currently exploring ways to specially process the raw beans after harvesting to reduce their potential for producing bitterness. They are also experimenting with different bean varieties in an effort to improve taste. But so far, none of these approaches — details of which are being kept confidential by the researchers — is ready for commercialization, he notes.

But the researchers are optimistic that a better cup of Joe is just around the corner. Perhaps no one could be happier about the news than Hofmann, who admits that he is an avid coffee-drinker with a passion for the dark-roasted varieties.

Funding for the study was provided by the Technical University of Munich, the University of Münster, and The Procter and Gamble Company.

The American Chemical Society — the world's largest scientific society — is a nonprofit organization chartered by the U.S. Congress and a global leader in providing access to chemistry-related research through its multiple databases, peer-reviewed journals and scientific conferences. Its main offices are in Washington, D.C., and Columbus, Ohio.

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The paper on this research, AGFD 104, will be presented at 1:30 PM, Tuesday, 21 August 2007, during the symposium, "Thermal Generation of Flavors and Off-flavors."

AGFD 104

What makes coffee taste that bitter? Program Selection: Division of Agricultural & Food ChemistryTopic Selection: Thermal Generation of Flavors and Off-flavors: Overview and Perspectives

Lead Presenter's Email: [email protected]Thomas Hofmann1, Oliver Frank2, Simone Blumberg2, Christoph Kunert2, and Gerhard Zehentbauer3. (1) Food Chemistry and Molecular Sensory Science, Technical University of Munich, Germany, Lise-Meitner-Str. 34, Freising 85350, Germany, Fax: 498161714216, [email protected], (2) Institute for Food Chemistry, University of Muenster, Münster 48149, Germany, (3) Miami Valley Innovation Center, The Procter & Gamble Company, Cincinnati, OH 45252

Abstract

In addition to its stimulatory effect, the wide popularity of a freshly brewed roast coffee beverage is mainly based on its pleasant, attractive overall flavor including a balanced bitter taste. Compared to the aroma-active volatiles, the information available on these bitter tasting non-volatiles is yet very fragmentary. Although, caffeine, chlorogenic acid degradation products, and a series of heterocycles such as 2,5-diketopiperazines are known as bitter compounds in roasted coffee, it is still unclear as to which of these compounds are coffee key tastants or which additional yet unknown compounds are responsible for the typical coffee bitter taste. By application of a molecular sensory science approach to roasted coffee, combining analytical natural product chemistry and human psychophysical tools, multiple bitter tasting caffeoylquinides, feruloyl quinides, and dicaffeoylquinides have been identified in medium roasted coffee on the basis of LC-MS and 1D/2D-NMR spectroscopy as well as synthetic experiments. Quantitative analysis revealed that quinides are formed with increasing roasting time up to a medium roasting degree. Thereafter, the quinides were found to be degraded again and converted into previously not reported bitter tasting vinylcatechol oligomers. Differing from the pleasant bitter taste profile of the quinides, these compounds exhibited an intense harsh bitter taste quality with human threshold concentrations between 0.023 and 0.178 mmol/kg (water). Preliminary sensory experiments revealed that a synergisitic interplay between the bitter quinides and the harsh bitter vinylcatechol oligomers is contributing to the bitter taste of roasted coffee.

Researcher Provided Non-Technical Summary

Briefly explain in lay language what you have done, why it is significant and what are its implications (particularly to the general public)

By combining analytical natural product chemistry and human psychophysical tools, we successfully identified for the first tim the key compounds responsible for the typical bitter taste of coffee. We could demonstrate that not caffeine, but two major classes of thermal transformation products generated from polyphenol precursors in the coffee bean are the key players for the bitter taste of roasted coffee.

Based on these study, the key bitter compounds can be used as analytical marker substances to objectify the taste quality of coffee and to improve coffee taste by means of science-driven optimization of processing parameters. Furthermore, such these compounds are now available as test compounds for the deorphanization of bitter taste receptors which is the prerequisite for the development of bitter inhibitors.

How new is this work and how does it differ from that of others who may be doing similar research?

The identification of these compounds and the evaluation of their sensory impact has not been reported by any other group.