Heat Transfer

Endothermic - Inward heat into the green coffee, at first crack the beans begin to give off that heat. the moisture inside the bean is decreasing throughout the roast as water evaporates. The amount of water vapor being released into the roasting environment/exhaust increases as the beans heat up. You will smell a bread type scent during first crack when when the color of the bean will change

Convection - Heat transferred to the coffee by the hot air moving through the roasting drum. The hot air surrounds the beans and transfers energy into their surface as they tumble. Convection is a major source of heat during roasting and also helps carry away moisture released from the beans.

Conduction - Heat transferred through direct physical contact. In a drum roaster, this happens mainly when the beans touch the hot metal drum, and to a smaller extent when beans contact each other. Too much conductive heat—especially early in the roast—can contribute to defects such as scorching or tipping.

Convection = hot air → bean
Conduction = hot surface → bean

Radiation - Heat emitted.

Important to pay attention to why proper drum rotation, speed and fullness will effect heat transfers.