What is drum temperature in coffee roasting?

Drum temperature is the temperature of the drum surface itself, as distinct from the bean temperature (BT) or the environmental air temperature (ET) inside the roasting chamber. It's the physical temperature of the metal that beans are in contact with during the roast.

In traditional roasting, drum temperature is inferred from air temperature readings and experience rather than measured directly. The Aillio Bullet introduced an infrared bean temperature sensor (IBTS) that can measure drum surface temperature directly during preheat, giving roasters a precise and consistent reference point for charge temperature. Before this, roasters relied on bean probe readings and feel to judge whether the drum was ready.

Drum temperature matters most at preheat and charge. If the drum surface is hotter than intended when beans are loaded, early conductive heat transfer can be excessive - leading to scorching or tipping. If it's cooler than intended, the roast may struggle to build momentum in the early stages. For home roasters without a direct drum temperature sensor, consistent preheat duration and charge temperature logging is the practical equivalent - establishing a repeatable thermal starting point before every batch.