Coffee Roasting Stages

Stage One – green beans are dried by gently vaporizing away free water molecules – while using the water’s conductivity to pass heat throughout the bean. In this stage the beans turn bluish-green to yellow-orange and it is easy to singe their surfaces, which imparts a bitter aftertaste to the coffee. However, too slow of a bake will make the coffee flat and lifeless.

Stage Two – this is where the actual roasting begins about half way through the process. As the beans turn from yellow to light brown, going past 320º F, they begin the lumped capacitance model of cooking from within; escaping steam and carbon dioxide begin building pressure on the beans cell walls. When the bean temperature passes 380º F, the surface becomes increasingly brittle, and begins to expand and crack open along the center line, emitting crackling sounds. This moment is referred to as ‘the first crack’ that announces proudly – I am now roasted. The roast is now at the first of several stations where it can be stopped; it is up to the roaster to decide which one, as the beans cruise at an ever increasing speed from the hues of caramelized brown to carbonized glistening black.

Stage Three – The final stage is the roast termination. With the bean core temperature at 420º F or higher, it is now critical to stop the beans from over roasting as quickly as possible. The traditional method is to deposit the freshly roasted coffee beans into a perforated tray and stirring them while drawing the cool surrounding air through the beans with a fan below and in many cases the atomization of fine water mist helps to prevent the beans from further increasing in roast color. by cooling them down rapidly and dropping their core temperature faster. This results in a consistently precise roast profile.