What is yellowing in coffee roasting?

Yellowing is the visual transition that marks the end of the drying phase and the beginning of the Maillard phase in the roasting process. As moisture evaporates from the green beans, they shift from their initial blue-green colour through to a pale straw or golden yellow - a colour change that typically coincides with an internal bean temperature of around 150-160°C.

The colour change itself is a reliable visual reference point that experienced roasters use to monitor how a roast is progressing. If yellowing happens earlier than usual in a roast session, the drum may be running hotter than intended; later than expected, and the roast may be slow to build momentum. Logging the time at which yellowing occurs - alongside charge temperature, turning point, and first crack - is part of building a reproducible roast profile.

Yellowing is sometimes also called 'colour change' or 'dry end' in roasting software and literature. The smell of the roaster also shifts at this stage - the grassy, vegetal aroma of the drying phase gives way to something sweeter and more bread-like as the Maillard reactions begin to take hold just beyond this point.