Eventually steam, still smelling like grass or burlap, will begin rising from the beans. Gradually this steam will darken and take on a coffee-like odor. On the heels of the appearance of the darker, coffee-smelling roasting smoke you will hear a subdued crackling sound, confirming the onset of pyrolysis , or the inner transformation of the bean. From this moment forward you must depend mainly on sight. How soon after the onset of pyrolysis you need to start your visual inspection of the beans depends partly on how dark you want to roast.
Roasts develop rather quickly in hot-air corn poppers, later in gas oven roasting.
Regardless of whether you are pursuing a lighter or darker roast you will need to visually observe the beans as they approach the critical moment when they match the color of your sample. When they are about the same or a little lighter than your sample you must stop the roast and begin the cooling process, usually by dumping the roasting beans into a colander or bowl. Never abandon roasting beans. This is a particularly important caution with hot-air popper methods. You should not even leave the room until the heat is off and the session over.