First crack marks a critical thermal transition in coffee roasting where internal pressure causes cellular fracture. This event is driven by the rapid expansion of gases produced during pyrolysis and the breakdown of complex carbohydrates.
During this phase, bound water vaporizes and internal pressure exceeds the strength of the cell walls. The cracking sound is a physical release of energy rather than a chemical reaction itself. However, it coincides with accelerated Maillard reactions and sugar degradation.
Roasters must manage energy carefully around first crack. Excessive heat input can cause surface scorching, while insufficient energy leads to stalled development. Understanding first crack as a thermophysical milestone improves roast consistency.