What is heat transfer in coffee roasting?

Heat transfer in coffee roasting refers to how thermal energy moves from the heat source into the coffee beans. There are three mechanisms, and most roasting machines use a combination of all three:

Conduction is direct heat transfer through physical contact - beans touching the hot drum surface absorb heat directly. It's most significant early in the roast when beans are cool and the drum is hot, and in slower, lower-airflow roast profiles.

Convection is heat transfer through moving air - hot air flowing through the bean mass carries energy into the beans. It's the dominant mechanism in fluid bed roasters and is controlled in drum roasters through fan speed. Higher fan speeds increase convective heat transfer.

Radiation is heat transfer through infrared energy emitted by the hot drum walls and surfaces, without requiring direct contact or airflow. It contributes throughout the roast but is less controllable than the other two mechanisms.

Understanding the balance between these three mechanisms helps explain why adjusting fan speed, drum speed, or power produces different effects on the roast curve, and why different machine types - drum vs fluid bed - produce different flavour results even from the same green coffee and profile.