What do exothermic and endothermic mean in coffee roasting?

Endothermic and exothermic describe the direction of heat flow during the chemical reactions happening inside the roasting bean - concepts that explain one of the most important events in the roast: first crack.

An endothermic reaction absorbs heat from its surroundings. The majority of the roasting process up to first crack is endothermic - the bean is continuously absorbing energy from the roasting environment, which is why bean temperature (BT) rises steadily as heat is applied. The roaster must keep supplying energy to drive the process forward.

At first crack, the beans transition through a brief exothermic phase - the reactions occurring at this point release heat rather than absorb it. This is why the Rate of Rise (RoR) often increases or "flicks" upward at first crack even without any change in applied heat: the beans are momentarily generating their own heat. After first crack, as the exothermic burst subsides and development phase chemistry takes over, the roast returns to a broadly endothermic character.

For roasters, understanding the endo/exo shift explains why heat management at first crack is critical. If you're still applying maximum heat as the exothermic flick arrives, the combination can push the rate of rise sharply upward and drive the roast faster than intended. Many experienced roasters reduce heat input slightly just before anticipated first crack to manage this transition and maintain a controlled, declining RoR through development.