The actual roast is a balancing act. Depending on the condition of the beans and the desired degree of roast, the beans are roasted at temperatures between about 200-240°C/392-464°F, give or take about 20°C/68°F on either side. Many roasters have devices for air ventilation, which, like fan-assisted ovens, cause the beans to cook faster. As the roast goes on (in total the process will take between about 8 and 14 minutes in a conventional-heat roaster), the beans retain more heat, and start to turn first yellow-green, then gold and into shades of brown. (All but the smallest drum roasters have devices allowing samples to be manually taken and returned to the drum.) The critical decision-making time is when the beans start to make popping sounds, as within seconds of this they turn dark very quickly. The drier the beans, the sooner the "popping" of pyrolysis begins; therefore, beans which are less green when un-roasted, such as Robusta, roast much quicker. Moisture-laden high-grown Arabica will take a bit longer to reach the same degree of roast.

The problem is deciding at what degree of roast each coffee tastes best, and the same coffee, roasted to different colors, will taste different. Even more complicated is the fact that the same coffee roasted to the same color at a higher temperature for a shorter time, will taste different if roasted at a lower temperature for a longer time. With extremes of either time or temperature, the same color roast can produce coffee that tastes under-cooked internally - cerealy or even green, or over-cooked - dried out, brittle when ground, or even burnt.

The last maddening fact of roasting is that the beans really need to be removed from the roaster immediately before the optimum color is obtained, as they will darken in spite of cooling devices, such as revolving air-cooled trays outside the front of a drum roaster, or even the quick burst of water (quenching) of larger commercial machines, designed to stop the beans' continued cooking after they have left the drum. In later stages of roasting, beans release oils. Sometimes, even if a darker "oily" roast is not intended, the beans, if left in the roaster a fraction too long, will develop oil on their surfaces while in the cooling tray.