Before the coffee bean can be used to make coffee, it has to be roasted.
The roasting process changes the colour of the green coffee beans. First they turn yellow and then brown. The longer they roast, the darker they become. The strongest roasts result in black beans.
At the same time, the heat causes changes in the taste and smell of the beans. Aromatic oils are released that bring out the flavour of coffee.
In the nineteenth century, people roasted their coffee at home on their stoves or over open fires. Nowadays, the commercial coffee roasters use enormous ovens to roast the coffee.
Temperature and time are carefully controlled, sometimes by computers, because just a few seconds can dramatically change the final flavour of the coffee. The roasting can be done in just a minute.
In general, a light roast gives a mild taste, a medium roast produces a well–rounded, rich flavour and aroma and a dark roast gives a strong, distinctive flavour.