Before the green coffee beans can be brewed up for your cup, they have to be roasted. Green coffee beans are hard and dense, and made up of between 8% and 14% water. That’s got to go.
The heat from the roasting process unlocks the flavour contained within the coffee beans, caramelising the carbohydrate sugars that live inside the coffee beans. It also breaks down a lot of complex flavours and balances the acidity.
During the roasting process, the coffee beans absorb the heat and start to change colour from a bluish green to a bright, vegetable green. A robust, grassy aroma is released during the process. As the beans continue to absorb heat, they begin to turn a warm, golden colour and release an aroma a bit like toasted wheat.
The water inside the coffee bean heats beyond boiling point as the colour changes to a mottled brown. The expanding liquid creates pressure within the cell walls of the coffee bean. The cellular structure of the coffee bean allows them expand, releasing the heat through a stage known as ‘Pyrolysis’, which makes a sound like the beans are popping.
As The roasting coffee beans continue to expand, they start to shed their casings – what we call ‘silverskins’. This chaff can then be removed during the roasting process.
In the final stages of roasting, the beans reach a dark brown colour and continue to expand in size. The sugars within the coffee beans are caramelised and the cellular walls of the beans become porous, releasing some of the oils contained within the beans.
During the roasting process the coffee beans lose about 15% to 18% of their weight and almost double in size. Over 800 flavour compounds are found in freshly roasted coffee, making it one of the most complex beverages available.