Coffee in its raw state is light yellow to light blue-green in color, depending upon processing and growing region, but is most often a light green in appearance. It has a smell something like hay or split peas. Roasting changes not only coffee's color and smell but also its size. During roasting coffee beans change from green, to yellow, to brown. Between yellow and brown the beans pop like popcorn ("first crack") and double in size. From this point on the coffee becomes palatable. If you go beyond a light roast coffee will begin to pop again ("second crack") this is the point when the "roast" flavor will begin to develop and can, if let go long enough or not cooled fast enough, take over the flavor of the bean. At "second crack" the woody structure of the bean begins to break down and burn and the oils within the bean begin to surface quickly.
Coffee flavor has two sources (if you don't count the water!), the flavor of the bean and the flavor of the roast. The taste difference between different origin coffees is most apparent at a light or medium roast. At darker roasts the flavor of the roast begins to dominate, acidity is lost, body increases and flavor is generally stronger. Very dark roasts are even stronger and tend to have a caramel-like stickiness.
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