What is drum speed in coffee roasting?

Drum speed refers to the rotation speed of the roasting drum - the rate at which it turns during the roast. On most home and commercial drum roasters, drum speed is adjustable, typically expressed as a numbered setting or in RPM. On the Aillio Bullet, for example, settings run from 1 (slowest) to 9 (fastest).

Drum speed affects two things: the agitation of the bean mass and heat transfer. Faster drum rotation keeps beans tumbling more actively, ensuring more even exposure to heat and reducing the risk of facing - where beans sit too long against the hot drum wall and burn on one side. Slower rotation allows beans more prolonged contact with the drum surface, which can increase conductive heat transfer but also increases the risk of uneven development.

For most specialty roasting on drum machines, a moderate drum speed that keeps beans freely tumbling without flying around is the target. Very small batch sizes may require a lower drum speed to maintain enough bean contact for even development; larger batches may need faster rotation to prevent the mass from compacting against the drum wall. Drum speed is one of the less frequently adjusted variables in home roasting but worth understanding when diagnosing facing defects or uneven colour development.