It was probably an Ethiopian some time in the 13th century who first thought of roasting coffee. For nearly 700 years the system has hardly changed. It was common to roast at home or in the coffee house immediately before drinking.
In the mid 19th century the cylinder roaster was developed, a perforated cylinder which turned over fire and in later years over gas or electricity. By the 20th century the coffee merchants had developed modern industrial equipment for roasting. First there were huge cylinders, then they developed systems for delivering hot air at high speed across the beans. Next came the continuous roasters with beans pouring through in a never ending system, being heated to 240 degree celsius for periods of eight to twelve minutes.
In the roasting process there is a change in the chemical composition of the bean; various elements - mainly sucrose are decomposed and carbon dioxide along with carbon monoxide are released. The result is an astonishing loss of weight - anything up to 20 percent. Depending on the darkness of the roast, the bulk can increase by 80 percent.
There was a need to suit a variety of palates so different degrees of roasts were developed. These days there are four main categories:
Light Roast - for delicate, mild beans. Good for breakfast coffee, a brew that you might drink with milk and sugar.
Medium Roast - normal coffee the way the French, Americans and Australians like it.
Full Roast - Very dark in colour, a bitter powerful brew, and the way they like it in Southern Italy or Vienna
Double Roast - Almost black, not good for the subtleties of the original flavour, but very strong and bitter. This is good for espresso coffee or the thick syrupy coffee that is the passion in Latin America and Turkey.