The first parameter to tune on your new roaster is the drum speed, which is measured in revolutions per minute (rpm). The optimal drum speed depends on the size of your roaster: smaller machines require a higher rpm, while larger machines operate best at a lower rpm.
| Roaster size | Recommended rpm |
| 1 kg | 70–75 |
| 5 kg | 60–70 |
| 12 kg | 52–56 |
| 15 kg | 50–54 |
| 30 kg | 46–50 |
| 60 kg | 40–44 |
| Note that the optimal rpm varies according to the drum size, not the batch size. | |
The optimal rpm for your roaster depends on the diameter of the drum. We explain the importance of drum diameter in more detail in our Roasting Science course.
The roaster sizes in the chart above, given in kilograms, refer to the drum size, not the batch size. If you are roasting a 10-kg batch in a 15-kg roaster, for example, you should still aim for the drum speed recommended for a 15-kg roaster.
Different manufacturers use drums with different proportions, so drums of the same capacity can have different diameters. For this reason, roasters of the same capacity might need slightly different drum speeds. For example, a long, narrow drum such as a 15-kg Joper will need a higher drum speed than a shallow drum with a larger diameter, such as a Probat UG-15.
Drum Speed Too Low
If your drum speed is set too low, the beans will remain in a pile at the bottom of the drum during roasting. The rotation of the drum isn’t fast enough to fully mix the bean pile, resulting in uneven roasting. Some of the beans at the bottom of the pile will spend too much time touching the hot drum wall,