Context
One of the practical questions that comes up when roasting multiple batches on the Gene Café CBR-301 is simple:
How should the second, third and fourth roasts be adjusted when the machine is already warm?
The Gene Café manual recommends starting from a cold machine. In practice, when roasting multiple batches this is rarely realistic. The drum retains significant heat and the machine often begins the next roast at 60–70 °C or higher.
This experiment was designed to observe the effect of drum thermal state on roast progression and determine whether a simple, repeatable adjustment could stabilise roast outcomes across consecutive batches.
The coffee used for this test was Brazil Santos (Wanted), roasted at 250 g batch size with a 250 °C set temperature.
Experiment Design
Four roasts were performed sequentially.
Only one variable was allowed to change:
Drum start temperature
Everything else remained constant:
- Coffee: Brazil Santos
- Batch size: 250 g
- Set temperature: 250 °C
- Ventilation: normal setup
- Roast termination method: Total Roast Time (TRT)
The aim was to see whether adjusting TRT could compensate for the extra energy stored in the drum.
Roast Data Summary
| Roast | Start Temp | 200 °C | 230 °C | Cool Trigger | Output | Weight Loss |
|---|---|---|---|---|---|---|
| Roast 42 | 20 °C | 04:34 | 07:29 | 13:18 | 209.9 g | 16.0% |
| Roast 43 | 71 °C | 03:15 | 06:06 | 12:38 | 209.4 g | 16.2% |
| Roast 44 | 72 °C | 03:12 | 06:06 | 12:38 | 208.3 g | 16.7% |
| Roast 45 | 69 °C | ~03:20 | ~06:10 | 12:28 | ~209 g | ~16.2% |
Machine behaviour was normal across all roasts:
- Standard fan step to level 3 occurred around the usual drying transition.
- No thermal protection events occurred.
Observations
1. Warm Starts Accelerate Early Roast Progression
The impact of a warm drum is immediately visible.
Cold start (Roast 42):
- 200 °C reached at 04:34
Warm starts (Roasts 43–45):
- 200 °C reached around 03:15
This represents an acceleration of approximately:
~1 minute 20 seconds
This behaviour is consistent with the drum already containing stored heat from the previous roast.
2. The Roast Structure Remains Stable
Despite the earlier start, the internal roast phases remained very consistent.
Example:
| Phase | Cold Roast | Warm Roast |
|---|---|---|
| 200 °C → 230 °C | 2:55 | 2:51–2:54 |
This suggests that while the roast begins earlier, the internal chemistry progresses at nearly the same pace once drying is complete.
3. A TRT Adjustment Compensates for Drum Heat
The key result of the experiment was that a simple TRT reduction of ~40 seconds compensated almost perfectly for the warm drum.
Cold start:
TRT ≈ 13:18 → 16.0% WL
Warm start:
TRT ≈ 12:38 → 16.2% WL
This produced nearly identical roast levels.
4. Repeatability
Roast 44 repeated the same warm-start TRT.
Result:
16.7% WL
This shows that while the machine is very consistent, minor variation still occurs due to factors such as:
- internal drum heat distribution
- heater cycling
- small variations in bean moisture
Reducing TRT slightly in Roast 45 returned the roast back to approximately the original target.
The Practical Between-Batch Protocol
For 250 g batches at 250 °C, the following pattern appears reliable on this machine.
First Roast (Cold Machine)
Start temperature:
~18–22 °C
Target:
TRT ≈ 13:15–13:20
Expected result:
~16% weight loss
Subsequent Roasts (Warm Machine)
Start temperature typically:
65–75 °C
Adjustment:
Reduce TRT by ~40 seconds
Example:
Cold roast TRT:
13:18
Warm roast TRT:
≈12:38
Expected result:
~16% weight loss
Why This Works
The Gene Café behaves as a convection-dominant roasting system.
Once the machine reaches approximately 200 °C, airflow increases and the roast progression becomes very stable.
At that point, roast level is largely determined by how long the beans remain in the system, which is why TRT correlates strongly with weight loss on this machine.
A warm drum simply shifts the timeline earlier.
Reducing TRT compensates for the additional stored heat.
A Simple Calculation Others Can Test
Based on this experiment, the adjustment for this machine can be approximated as:
Warm Start TRT ≈ Cold Start TRT – 40 seconds
For example:
Cold roast TRT: 13:18
Warm roast TRT: ≈12:38
Other Gene Café users may find the adjustment slightly different depending on:
- room temperature
- ventilation
- batch size
- bean density
However, the principle should remain similar.
Why This Matters
When roasting multiple batches, the Gene Café can appear inconsistent if each roast is treated the same.
In reality the machine is behaving predictably — the drum simply contains more energy.
Once this is accounted for, consecutive roasts can produce very similar outcomes, even across several batches.
Final Thought
This experiment reinforced something that has become increasingly clear after many roasts on the CBR-301:
The machine itself is very consistent.
Most variation comes from how the roaster is used, not from instability in the system.
Understanding the thermal behaviour of the drum is therefore one of the most useful things a home roaster can learn.
If you roast on a Gene Café, try running two batches of the same coffee back-to-back and adjust TRT by around 40 seconds for the second roast.
I’d be very interested to hear whether the same pattern appears on other machines.