ROASTING
We have discussed where the coffee bean comes from; but how does the bean become the coffee we know and love to drink?
First, the coffee cherry needs to be processed and dried out so the bean can be removed. Then the bean needs to be sun-dried. This leaves a green coffee bean ready for roasting.
The roasting process takes place in a heated steel drum. The beans are continuously rotated and heated until they gradually become darker in colour. The darker they are, the stronger the roast.
As the temperature of the drum increases, a number of chemical reactions occur. These lead to the breakdown of sugars into ‘aromatic compounds’ – special ring-shaped molecules that gives the bean its colour and flavour. The time the bean spends roasting in the drum determines how strong these flavours are.
It is all about time and temperature. The first chemical reaction, known as the ‘Mailard Reaction’, occurs at 148 °C. This is the initial conversion of sugars and proteins into different groups of molecules known as ‘furans’, which provide a burnt flavour, and ‘hydroxymethylfurfurals’ (HMF), which create the sweet aromas.
The next reaction, called the ‘first crack’, occurs at 195 °C. It is an endothermic reaction, absorbing energy. This causes chlorophyll to break down. As moisture is turned into steam, pressure builds within the bean that eventually ruptures it, resulting in a ‘popping’ sound.
The ‘second crack’ occurs at 225-230 °C. This reaction is both endothermic and exothermic – releasing energy. Pressure increases with heat, breaking the structure of the bean. Compounds adding additional flavour are also formed, such as carbon dioxide and nitrogen oxide.