During
the roasting process, green coffee slowly turns yellow and then brown
shortly before it undergoes a major expansion that causes distinct
popcorn-like sounds. At this point, know as First Crack, flavor oils in
the coffee are released and begin to blend inside the beans. A roast
stopped near the end of first crack is referred to as a City Roast.
After first
crack and a short period of quiescence, the coffee expands further
causing fissures that allow the oils to emerge and caramelize on the
surface of the beans. Second Crack sounds a lot like crinkling paper. A
roast stopped at the very cusp of this stage is known as a Full City
Roast. Ending the roast during this expansion leads to darker roasts,
such as Vienna, French, and Italian.
Coffee beans
themselves are exothermic, meaning that once they reach a high enough
temperature they begin generating their own heat. Even when removed
from an external heating source, coffee would continue to roast itself.
In order to stop coffee from roasting further, the beans must be
actively and rapidly cooled with air.
