I don’t have the fine skills of an experienced roaster, but with every roast, I’m getting one more step closer.

Coffee beans at different stages of roasting
So let me tell you what I know about what a roasted coffee bean looks like. The picture shows the coffee bean at different stages of being roasted.
The bean to the far left is unroasted. Unroasted beans don’t even smell like coffee. They actually stink, smelling raw.
The second bean from the left right has begun to roast, but has not even gotten to the “first crack”. At this point you should smell a slightly sweet aroma – kind of like sugar starting to caramelize.
The third bean has reached the “first crack”. The “first crack” sounds like popcorn popping, occurring around 4.5 to 5 minutes in my popcorn popper. This is where the water in the bean has finally evaporated and the bean begins to really roast. Roasting caramelizes the sugars and releases the oils found in the beans. The roasted sugars and the oils produce the wonderful aroma and flavor of the coffee.
Coffee roasting requires a sense of smell, hearing, and sight. I’m not able to monitor the temperature of the beans – so I rely on those three senses. Once you hear the “first crack” now it’s time to pay close attention (this is usually when one of my kids ask me to do something for them). Trust your nose and the color of the beans that you see. The beans start to smell like coffee after the “first crack”.
For a medium roast, I typically stop the roasting at about 8 minutes. The beans have roasted beyond the “first crack” but not to the “second crack”. The fourth bean is a sample of this. As you can see in the picture, a medium roasted bean is nicely browned, but not yet shiny – like you see in dark roasted coffee. Medium roasted coffee typically gives you the inherent taste of the beans.
If I let the beans go to about 9 minutes, you will begin the hear the start of the “second crack”. When I pour these beans out they usually continue to crack on the cookie sheet until they cool down. These beans are a medium-dark roast. I like this best for my blend of Kenya and Brazil or Sumatra.
Most coffee drinkers that I know like dark roasted coffee. They like the strong attack of their taste buds. For a dark roast I’ll let my popcorn popper go all the way up to 12 minutes. Most of the beans have reached the “second crack” by this time. The fifth bean in the picture is a dark roasted bean. It begins to shine since the oils have made its way to the surface of the bean. Starbucks likes to roast their beans pretty dark. In fact, they have a carbon taste to me. I like dark roasts, but I think I like to stop just before the carbon taste kicks in.
So that’s it. One key to roasting is knowing what the beans should look like. The difference between a medium roast and a dark roast in the popper is about 4 minutes. My 12 minute mark for a dark roast is a basic guidline. It’s still best to watch the beans, making sure they look like beans at the level of roast you want.