Extremely good roasts.
Let me explain what is going on.
You have three modes of transferring heat to any material.
Conduction, Convection and radiation (very minimal in coffee roasting so we are going to ignore it).
Conduction is generally more efficient in transferring the heat between objects where convection is better suited to cover the atmosphere around and object.
Drum speed and power modulates the conduction.
Drum Speed, Power and fan speed modulates convection.
Power is the common dominator in the Aillio for both major modes.
Air Flow modulates the amount of cooling or heating during Convection. Less Airflow leads to higher transfer during convection and more airflow leads to the reverse where you cool the beans and in addition removes any excess humidity in the chamber (Humidity is basically a battery for heat).
Drum Speed is a unique setting, where at slow speeds you have mainly conduction transfer with less convection as you up the speed it becomes less conduction and more convection to a point where beans start to stick togather and then it becomes more conduction while you are creating a tornado effect in the drum that is still increasing convection.
The secret to a good roast is that you have a good transfer of heat on the overall bean:
1- At the start of the roast where you have a large amount of heat stored as preheat transfered as much as possible and thus your require conduction. You might say that scorching might occur but scorching occurs when the point of low water content is dried throughly during the roast. In addition, You have a minimum drum speed for that to occur. From my testing it was around d3 for 900g batchs.
2- At the start of yellowing and mallard reactions, Most of the free water has already dissipated. Now you are trying to evenly develop the roast. That occurs with a mostly convection type of heat transfer and that’s when you start to up the drum speed evenly and steadily.
3- Just prior to FC, You require to have as much stability as possible in transfering your heat. Maximum point of conduction and convection and trying to smooth out the overall power requirements to keep the beans from running away to over medium roasts. A buddy of mine actually drops power after 1min after FC and lets the beans glide to 0 ror prior to dropping them.
My routine on 900g is basically start at d4, hit d5 prior to 150’c and increase the drum speed until d9 prior to FC.
Fan speed at F2 until yellowing hit f3 and f4 prior to FC and maybe F5 to stabilise the development.
All changes after that are at the PH and Power settings depending on the type of beans.
