What is BT (bean temperature) in coffee roasting?

BT stands for bean temperature - the temperature reading from a probe positioned in contact with, or in the middle of, the coffee bean mass inside the roasting drum. It is the primary measurement used to track and control a roast and is the line most roasters focus on when reading a roast curve.

Bean temperature tells you the actual heat absorbed by the coffee itself, as opposed to the environmental or air temperature (ET) surrounding it. In the early stages of a roast, BT drops sharply as cold green beans absorb heat from the drum - this is the turning point. From there, BT rises steadily through the drying phase, yellowing, Maillard phase, first crack, development phase, and finally to the drop point.

The shape of the BT curve - how steeply it rises, whether the rate of rise is declining steadily or crashing - is the primary diagnostic tool for understanding what's happening in the roaster. A smoothly declining rate of rise through the Maillard phase and into development is associated with well-developed, consistent results. A sudden RoR crash produces baked coffee; a spike can push the roast too fast. Most roasting software plots BT in real time alongside ET, giving roasters an immediate picture of how the batch is progressing.