The chemistry of coffee

There are hundreds of different components in the coffee bean, some of which have yet to be identified.

The chemical composition of the two most commercially important species - Arabica and Robusta – differ both qualitatively and quantitatively.

Arabica contains more lipids than Robusta which, however, has twice the caffeine content of the former. 

The chemical composition of the raw green coffee bean is altered by torrefaction (roasting) – the thermal process that transforms the green bean into a roasted bean ready to be ground and used to make coffee.

At the beginning of the roasting process, the water content of the green bean progressively reduces until the beans reach a temperature of 160°C. At this point, part of the organic content is lost as it is transformed into carbon dioxide and expelled from the bean. At this temperature the beans begin to swell and their density is significantly reduced, as they simultaneously increase in volume and lose moisture.

By 160°C, major chemical transformations are already taking place within the bean and many hundreds of volatile compounds are created, producing the distinctive aroma of roasted coffee. It is also at this temperature that the brown pigments are produced. These are known as melanoidins, the structure of which is still only partially understood.

Above 200°C the reaction becomes exothermic, while above 240°C, the bean begins to char and produce ash.

Despite the fact that it contains numerous nutrients and many different components, coffee has been classified a ‘non-nutritive dietary component’ by the World Health Organisation’.

Undoubtedly, the most extensively studied and best known component in coffee is caffeine, a substance with a bitterish taste contained in concentrations between 0.2 and 1.8 mm/litre in a cup of coffee. It is, however, not the only compound contributing to coffee’s bitter taste, the perception of which increases with serving temperature and decreases in the elderly.

The most well known effects of coffee are attributable to caffeine, the assumption of which stimulates the nervous system and increases the threshold of concentration and lucidity.

The stimulant effects of caffeine are manifested between 15 and 45 minutes after ingestion, the period during which caffeine levels in the blood are highest. Caffeine levels begin to drop after this interval, at a rate varying significantly in relation to a number of different factors such as age, fitness, diet, smoking and the use of drugs, while the effects of caffeine will have completely disappeared in all subjects within a few hours.

Another significant characteristic of caffeine, which has made it the subject of many scientific studies, is its pharmacological action. Over 180 years since its discovery (it was isolated by the German chemist Ferdinand Runge in 1820), numerous researchers are still working on it today.

The pharmacological actions of caffeine – and its analgesic effect in particular – occur only at higher doses than those contained in a normal cup of coffee that is drunk regularly and quickly.

The most recent studies into the chemical composition of coffee identified over 600 different components.

The raw green coffee bean contains minerals – potassium in particular – proteins, amino acids, lipids and carbohydrates, which are lost during the roasting process and form aromatic elements as they combine with the amino acids.