The most obvious change when coffee is roasted is in the color. The bean moves along a scale from green to varying shades of brown to near black. Underneath the surface a number of other changes are occurring. One of which is Pyrolysis, a process were the acids and sugars in coffee begin to break down. Another is the process of reconstituting the sugars and acids called the Maillard Reactions.
In the early phases of the roast process the bean primarily looses moisture and gains heat. As the roast progresses, the water deep inside the cellular structure of the coffee is forced out. The bean expands to nearly twice its original size. Like a corn kernel popping, this process usually coincides with an audible sound. This sound is an important marker in the roast and is called First Crack. Once this occurs, the coffee is officially considered roasted. The process continues and the degree of roast gets darker and darker until, once again a crack marks the move into the next stage of roast. After this point, if the bean continues to be roasted, it will eventually turn black, and charred. All of the oils will eventually come to the surface with much of the flavor. Nearly all of Victrola’s roasts are stopped before second crack.
If you are interested in more information check out this informative blog post or the visual guide and video on Sweet Maria's website.
