Indirectly heated drum roasters, fixed-drum roasters, and fluid-bed roasters operate quite differently from one another, but all have one important feature in common: the machines exclusively use hot air as the heat source. As a result, these roasters share some common characteristics.
In all three types of air roasters, nearly all the heat transfer to the beans takes place via convection. Nonetheless, there is also a limited amount of conduction: the walls of the roasting chamber heat up and transmit energy to the beans via conduction, and heat is also transferred from bean to bean via conduction.
Working with convection requires a unique approach to coffee roasting. The airflow and heat transfer are closely linked, and the heat energy stored in the drum becomes much less important to the roast. To get insights into the technique of air roasting, we asked three roasters who work with three different styles of machines to share their expertise with us.
Talor Browne is the founder of Talormade, a specialty coffee roastery and doughnut bakery with three shops in Oslo, Norway. Talor previously used drum roasters from Probat and Diedrich, but for Talormade she chose to install a Loring S15 Falcon, a fixed-drum roaster. In 2015, Talor conducted one of the few published attempts to directly compare different roasters for specialty coffee.
Renata Henderson is the head roaster at Cxffeeblack in Memphis, Tennessee, which she cofounded with her husband Bartholomew Jones. Cxffeeblack is a social enterprise set up with the intention to reconnect the Black community with the Black history of coffee and to create more opportunities in the industry for people of colour. Renata roasts on an Artisan X-e — a small fluid-bed roaster with a partially open roasting chamber.