For most of it's history, coffee was roasted in the home. Factory roasting was invented at the end of the 19th century. The forerunner of the modern coffee roaster was invented by Jabez Burns in 1864 to meet the increasing demand for coffee. In the early 1900's, as gas replaced coal and heat became more easily controllable, Burns continued to modernize his invention. In 1935 he came up with the "Thermalo" process, utilizing hot air blown at high speed through the roasting cylinder.
To this day, however, proper coffee roasting is always supervised; for it is important to see the color of the bean. Each coffee variety has it's own idiosyncrasies and no two batches of coffee roast exactly alike.
Coffee roasting shrinks the bean roughly 15%...up to 20% for darkest roast in weight ...but doubles the volume, the result of beans cracking and expanding as they roast. The darker the roast, the greater the volume of coffee. Color change is caused by carmelization of the sugars.
The Caffeine component in coffee is fairly stable and does not decrease significantly in the roasting process. Coffee acidity is highest with lightest roasts.
The basic coffee roasting principals are:
Heat should be applied evenly to the bean...
Heat should be applied at the lowest temperature possible for shortest amount of time...
Coffee should be quickly cooled when roasting is finished.
Temperatures for roasting range from about 380 degrees for a light roast; through about 400 degrees for a medium roast and 425 degrees for a dark roast.
Having the temperature too high burns the coffee and having the temperature too low, or roasting the coffee for too long, bakes the bean. Both are problems to be avoided.
