Scorching

In the context of coffee roasting, what is scorching? It is a common roasting defect that occurs when the surface temperature of the roasting drum is too high at the start of the process, or when the drum speed is insufficient to keep the beans in constant motion. When green coffee beans make prolonged, static contact with the hot metal of the drum, the outer layers of the bean burn before the internal structure has had a chance to develop. This results in visible, charred, or blackened patches on the flat sides of the beans.

Scorching is primarily a result of conductive heat transfer. While conduction is a necessary component of drum roasting, it must be balanced with convection and radiation. When the charge temperature—the temperature of the roaster when the green coffee is added—is set too high, the initial contact between the cool, dense green beans and the hot metal drum creates a thermal shock. If the drum speed is too slow, the beans do not tumble effectively, allowing them to sit against the hot metal for too long, leading to these localized burn marks.

For a roaster, identifying scorching is a matter of visual inspection during the cooling phase. While a few scorched beans may not ruin an entire batch, a high percentage of scorched beans indicates a failure in roast profile management. The defect is often associated with a lack of consistency in the roast batch size or improper management of the roaster's airflow and drum speed settings.

In practice, scorching negatively impacts the cup quality by introducing harsh, acrid, or ashy notes that mask the inherent characteristics of the coffee. It is a sign of an underdeveloped roast, as the surface has been subjected to extreme heat while the core of the bean remains under-roasted. To avoid this, roasters focus on managing their charge weight and ensuring that the drum speed is optimized to keep the beans moving, preventing the static contact that leads to this defect.