Airflow is one of those aspects of roasting that’s a little bit of a mind bender to get your head around, but it’s a very important aspect of Roasting. To understand airflow and how it works we need to remember that the main ways of heating green beans are Conduction and Convection.
Conduction is heat directly from touching another source like frying in a pan. So in a classic drum roaster, it’s the heating that is applied to the beans directly from the hot steel drum.
Convection is heat which comes from fluid or air like baking in the oven. This is the main source of heat which is applied to beans during roasting. Airflow changes the air in the roasting environment and the way it heats the beans.
Airflow has several functions. Firstly, it removes smoke and chaff from the roasting environment. Too much smoke will affect the flavour of the coffee and chaff needs to be removed so that it doesn’t catch on fire.
Secondly, Airflow draws hot air through the roasting chamber and away from the beans. This means that with a low airflow beans are sitting in a hotter environment and will usually result in a faster roast time. A high airflow means that lots of heat is being drawn out and will result in a slower roast time.
A few practical thoughts
You may or may not be able to adjust your airflow (fan speed) on your Roaster. If not, because you’re roasting in a bread maker or popcorn machine, a small pedestal fan perfectly placed can move some air around to increase airflow and draw heat away.
Larger or higher density beans may benefit from lower airflows to increase heat. Whereas, smaller or lower density beans may benefit from higher airflows.
Increasing airflow can be used like brakes towards the end of the roast to slow it down enough so that the inner bean temperature can catch up to the outer bean temperature.
Gas adjustments will have a differing impact depending on your airflow. Roasts with lower airflow will need bigger gas adjustments to remove enough heat to slow down the roast as it progresses. Whereas, Roasts with higher airflow will need smaller gas adjustments to slow it down.
Roasts with lower airflow will develop faster than Roasts with higher airflow. So you may need to adjust your development time based on your airflow or vice versa. If you’re finding you have too much development, try a higher airflow. Or if you’re finding you don’t have enough development, try a lower airflow.
As roasters are all different, airflow in your machine will be slightly different so it’s best to experiment to see what works for you. If your Roaster has adjustable airflow, we recommend roasting 2 batches as you normally would, but with 2 different levels of airflow for the entire roast. This will give you a clear picture of the impact it plays on the taste of your coffee with your roaster. See which one you prefer and feel free to experiment with airflow.