The first stage of roasting is commonly known as the “drying phase,” although beans lose moisture at similar rates throughout most of the roasting process. During the first few minutes of roasting, degradation of chlorophyll causes beans to change color from green to yellow. As roasting progresses, the beans change from yellow to tan to light brown, primarily due to Maillard reactions. Late in a roast, as the beans approach first crack [the point at which enough water has evaporated from the beans that they emit a cracking sound], the brown color deepens due to caramelization. In a dark roast, carbonization may turn beans black.
Classic Definitions of Roast Degree:
Light roasts offer acidic, floral, and fruity flavors, more delicate aroma, and less body than dark roasts. Dark roasts develop smoky, pungent, bitter, and carbonized flavors. If one takes roasting to an extreme, burnt flavors dominate and body declines.
Despite what almost everyone has heard, darker roasting does not decrease the caffeine content of coffee beans. Caffeine levels are virtually unchanged by roasting, as caffeine is stable at typical roasting temperatures. Given that beans lose mass during roasting, their proportion of caffeine by weight increases during roasting. Therefore, assuming one brews coffee of all roast degrees with a particular ratio of water-to-ground-coffee mass, rather than volume, darker roasts will yield brewed coffee with higher caffeine content