How do you decide when to stop the roast ? Once, I asked roaster Morten Riiskjær this question. He said by smell. The smell of onion has to be over before stopping.
He has been roasting coffee for 7 years. I visited him in the roastery to try to pick up what he could smell during a roast. He also looks at the bean surface.
Hear Morten talk about it in episode 2 in the podcast Coffee Roasting Navigated http://coffeenavigated.net/podcast/2-roasting-by-smell/