What is preheat in coffee roasting?

Preheat is the process of bringing a roasting machine up to a stable target temperature before loading any green coffee. Just as you preheat an oven before baking, preheating a coffee roaster ensures the drum and machine components have absorbed enough thermal energy to deliver consistent heat transfer from the moment the beans are charged.

Without adequate preheat, a roaster's thermal behaviour will be unstable at the start of the roast - the machine is still warming up, the drum is heating unevenly, and the roast will behave differently to a session where preheat was consistent. This variability is one of the most common causes of batch-to-batch inconsistency in home roasting.

Target preheat temperatures vary by machine, batch size, and roast profile. On the Aillio Bullet, the IBTS (infrared bean temperature sensor) measures drum surface temperature directly during preheat, allowing precise and repeatable charge temperature targets. On machines without drum temperature sensors, a stable preheat duration - running the machine at a consistent power setting for a fixed time - is the practical equivalent. The key principle: every batch should start from the same thermal baseline.