It is very funny that you ask this question when I am currently finishing up the first batch of roasted coffee in my Gene cafe roaster!
The answer is the temperature that the beans get to when they are roasted. The hotter they get too the more oil comes out and the darker the bean gets. you can also roast them to the point that pieces shatter off--makes little pits in the beans
Off to roast two more batches!
Addicted to roast.
Love our roaster. 5 batches last week. Should last several weeks before doing again. Where do you get your green beans?
