Second Crack: This is a stage in the coffee roasting process and usually occurs a few minutes after the first crack. More specifically, when the coffee reaches about 224 °C (435 °F), it will emit a “second explosion”, which indicates that the structure of the coffee begins to break. If roasting continues further, coffee will soon turn into coal and eventually burn.
Roasted coffee beans when they reach the Second Crack stage will be slightly shiny as oils begin to increase on the surface. This is seen as a sign of the full development of the nut flavor with a heavier body than light roast or medium roast with the smell of smoke, and spicy prevailing.