Airflow fulfils two vital functions in a roasting machine: it removes smoke and chaff from around the beans, and it enables heat transfer to the beans via convection. Convection accounts for at least 70% of the heat transfer to the beans in conventional drum roasters and close to 100% of the heat transfer in air roasters.
In drum roasters, most of the remaining heat transfer to the beans takes place via conduction. Excessive conduction can lead to bean-surface burning, resulting in roast defects such as scorching and facing. For this reason, tuning the airflow in your roaster to maximise heat transfer via convection is essential to achieving good results.
If your roaster has insufficient airflow, the coffee will pick up smoky flavours from smoke and chaff retained in the drum during roasting. Too much airflow, on the other hand, can be harder to diagnose. If your roaster has excessive airflow, you may find that the coffee tends to develop ‘roasty’ notes. These flavours can result from too much heat transfer via conduction; bean-surface burning gives the coffee a roasty flavour.
Here’s what happens: the high airflow drains too much heat from the roaster, forcing the operator to use higher gas settings to compensate. In a directly heated drum roaster, the flame is positioned directly underneath the drum, so higher gas settings lead to higher drum temperatures. The hot drum increases the amount of heat transfer to the beans via conduction, which can impart roastier, less-delicate flavours to the coffee.
The optimal amount of airflow is high enough to effectively remove smoke and chaff but low enough to allow the operator to use gas settings that don’t result in bean-surface burning.
The easiest way to tune the airflow in a drum roaster is to use the cigarette lighter trick:
- Begin roasting a test batch of beans.
- About halfway through the roast, remove the trier.
- Hold the flame of a cigarette lighter about 1 centimetre away from the trier hole.