In our previous articles, we explored the history and evolution of coffee roasting. Now we are going to look at one of the most important differences between traditional drum roasters and hot-air roasters:
How do they transfer heat, and how can that affect Rate of Rise?
This first article focuses on heat transfer in traditional drum roasters.
How Does a Traditional Drum Roaster Work?
Whether you are using a direct-fire, semi-direct-fire, or indirectly heated drum roaster, these machines all have one obvious feature in common:
They have a rotating drum.
During roasting, the drum keeps the coffee beans moving while heat is transferred to them. However, it is important to understand that drum roasting does not rely on only one form of heat transfer.
Coffee beans inside a drum roaster can receive heat through:
- Convection: Heat transferred by the hot air moving through and around the beans
- Conduction: Heat transferred when beans touch the heated drum or one another
- Radiation: Heat emitted by hot surfaces inside the roasting system
The balance between these mechanisms depends on the machine’s design, drum material, airflow, burner configuration, drum speed, and batch size.
In many modern drum roasters, convection from hot air provides a substantial part of the heat. Direct contact with the drum still matters, but it should not be described as the only—or always the dominant—source of energy.
Why Does the Drum Store Heat?
As the roaster heats up, thermal energy builds within the metal drum and other parts of the roasting system.
This stored energy is often described as thermal inertia.
The amount of energy stored depends partly on the mass and material of the drum. A thick, heavy drum generally stores more heat than a thinner, lighter roasting chamber.
When green coffee enters the machine, energy is transferred from the hot air, drum, and surrounding components into the cooler beans. As roasting continues, the entire system keeps absorbing, storing, and releasing energy.
That means the burner setting is only one part of what determines how the coffee develops.
Why Doesn’t RoR Respond Immediately?
Rate of Rise, or RoR, describes how quickly the measured bean temperature is increasing over time.
When you adjust the burner, the RoR curve may not respond immediately. Several factors can contribute to this delay:
- The drum and roasting system already contain stored heat.
- The beans have their own thermal mass.
- Hot air already inside the system continues transferring energy.
- Changes in burner output take time to affect the roasting environment.
- Temperature probes and software may introduce some measurement delay.
Turning down—or even temporarily turning off—the burner does not mean that all heat transfer stops. The hot drum, roasting air, beans, and surrounding metal components can continue releasing stored energy.
This is why experienced drum-roaster operators often make heat adjustments before the desired change appears on the RoR curve.
The exact response time will vary between machines, so there is no single adjustment rule that works for every drum roaster.
Think of a Cast-Iron Pan
A simple comparison is cooking a steak in a cast-iron pan.
A cast-iron pan stores a large amount of heat. Even after you turn down the stove, the pan remains hot and continues transferring energy to the steak.
If you wait until the steak looks almost finished before reducing the heat, the energy already stored in the pan may continue cooking it beyond the result you wanted.
A traditional drum roaster behaves in a similar way.
The comparison is not perfect—coffee roasting also involves airflow, radiation, moisture loss, and changing bean structure—but it illustrates the central idea:
Reducing the burner does not instantly remove the heat already stored inside the roasting system.
Why Anticipation Matters in Drum Roasting
Managing a traditional drum roaster therefore involves anticipation.
The roaster must consider not only the current burner setting, but also:
- How much energy is already stored in the machine
- How quickly the beans are absorbing heat
- How airflow is affecting heat transfer
- How the measured RoR is developing
- How long the machine takes to respond to an adjustment
Roast data can help record and compare these changes, but successful control still requires an understanding of the specific machine.
This thermal inertia is one of the main differences we will use when comparing drum roasters with hot-air fluid-bed roasters.
A hot-air roaster typically depends more heavily on high-velocity convective airflow and has less heat stored in a heavy metal drum. As a result, changes in heat or airflow can behave differently—but that is the subject of the next article.
