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.
See photos of Morten and his roaster at http://coffeenavigated.net/roasting-by-smell/
If you understand Danish you can hear more with Morten in http://smagefter.dk/podcast/afsnit-8-duften-af-kafferistning/
Made by Therese Brøndsted, CoffeeNavigated.net