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.
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.
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.