Roasting coffee transforms the chemical and physical properties of green coffee beans.
When roasted, the green coffee bean expands to almost double its initial size, with the color and density being affected as well. As the bean takes in heat, its color changes to yellow, a light brown, and then to a heavier looking brown color. During the roasting process, oils begin to appear on the surface of the bean, and the lighter the roast, the less you will see visible shine on the surface of the bean. The beans will continue to darken until they are removed from the heat source.
The most economical and used by the majority of coffee roasters due to its simple design and capacity range. Its comprised basically of a rotating cylindrical drum with direct heat applied under the drum or through the center through a conduit. The heat source can be either electric or gas flames with or without a profile controller.
During drum roasting, heat is transferred mostly through convection and a bit through conduction. Temperature is reported by gauges on the system. The first one measures the flame temperature and the second measures ambient bean temperature inside the drum. If too much heat is applied to the roaster, or if the speed of the drum is on too high of a setting, centrifugal force will push the beans against the barrel, transferring too much heat to sections of the beans. Drum roasters have various capacity loads, from nano sized 200-500 grams, to 120 lb. to over 5,000 lb. in a single batch.
Keeps the beans aloft by roasting them on 680 degree hot air vortex (known as a fluid bed), inside the roasting chamber. The heat and furious spinning allows perfectly even roasting. As the beans crack during air roasting, the chaff is blown into a separate chamber to prevent burning and smoking, which would impart their own strong flavors, hiding the true flavor profile of the bean. As the right roast level is reached, digital controls stop the heat, and begin the cooling process. Air roasters are typically not used to roast commercially due to its smaller capacity batch size.