Roasting unlocks and frames a coffee’s flavor potential; it is a science and an art that requires loads of experience, concentration and split second timing. Roasting is also the means by which the roaster imprints his/her signature. How bright or muted will the coffee be? How much depth or ‘chiaroscuro’ should the roaster impart to the coffee’s intrinsic flavor? Should there be a touch of bitterness at the end?
The Three Stages of a Roast
Stage One – During the first stage the green beans are dried by gently vaporizing away stored free water molecules – while using the water’s conductivity to pass heat throughout the bean and begin effective roasting in the second stage, a very fine line! The beans turn from bluish-green to yellow-orange in this first stage and it is easy to singe their surfaces, imparting a bitter aftertaste to the coffee brew, or to bake the coffee, resulting in a flat, lifeless cup.
Stage Two – The second stage of actual roasting begins about half way through the roast. As the beans turn from yellow to light brown, going past 320 F, they begin cooking from within; escaping steam and carbon dioxide begin building pressure on the beans’ cell walls. When the beans’ temperature passes 380F, their surfaces, increasingly brittle, begin to expand and crack open along their center lines, emitting popping sounds. Roasters call this moment ‘the first pop’. The roast is now nearing the first of several stations where it can be stopped; it is up to the roaster to decide which one, as the beans go from darkening tones of caramelized browns and beyond – to carbonized glistening black.
Stage Three – The third stage is terminating the roast. With bean temperature at 420 F or higher, it is now critical to stop the beans from cooking as quickly as possible. The traditional way is to release them into a perforated tray that is stirred while drawing the cool surrounding air through the beans with a fan.