Roast makes a huge difference in the caffeine content, as well as the flavor. Beans that are identical before the roasting process can produce very different flavors, depending on how long they were roasted. Contrary to what you might guess, the darker a coffee's roast, the less caffeine it will likely have. This is because prolonged exposure to heat breaks down the caffeine. In terms of flavor, Sinnott says roasting trends come and go. Darks were recently popular but now, he says, light roasts are the rage.