Why is Coffee Bitter? – The Chemistry of Coffee

Mention coffee, and caffeine is the chemical compound name that immediately springs to mind. However, whilst caffeine’s effects on the brain are well documented – it binds to adenosine receptors in the brain – it has relatively little impact when it comes to the taste of coffee. Coffee, as it turns out, is a cornucopia of chemical compounds that influence its taste; whilst some of these compounds are poorly characterised, one group of compounds about which plenty is known are the chlorogenic acids.

These compounds account for up to 8% of the composition of unroasted coffee beans. Deceptively enough, despite the name their structure doesn’t contain any chlorine atoms – rather, it refers to the light green colour produced when these acids are oxidised. When coffee beans are roasted, these chlorogenic acids react to form a variety of different products, which can all affect the taste of the coffee.

In medium to light coffee brews, the main source of bitterness is from chlorogenic acid lactones; the two dominant members of this family of compounds in coffee are shown below.

In dark roasted coffees, the breakdown products of these chlorogenic acid lactones have an increasing effect on the bitterness of the flavour. These products are called phenylindanes, and their bitterness is harsher than that of the chlorogenic acid lactones – explaining, for example, the bitterness of espresso coffee.

A final class of compounds, melanoidins, are also formed as byproducts of the roasting of coffee beans. They are formed during the Maillard reaction, a chemical reaction between proteins and sugars responsible for flavouring in many types of cooked foods. The melanoidins are very poorly characterised due to their complexity, and their chemical structures remain largely unknown, despite it being estimated that roasted coffee bean composition may contain up to 30% of these compounds. Very little is known about this class of compounds, but it is suspected that they could also have an impact upon the flavour of coffee.

To download the high resolution PDF of this graphic, click here. To read more about the chemical composition of coffee, you can follow any of the links below, all of which contain further information on the compounds discussed in this article.

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References & Further Reading

• ‘Phenolic Compounds in Coffee’ – A Farah & C M Donangelo, Brazilian Journal of Plant Physiology 2006.
• ‘Coffee Brew Melanoidins’ – E K Bekedam, Wageningen University 2008.
• ‘Chlorogenic Acid’ – CoffeeChemistry.com.