RoR
- SOLIDER TSAI
- Feb 15
- 4 min read
During the coffee roasting process, green beans absorb heat starting from a low temperature. The Rate of Rise (RoR)—the speed at which the temperature increases per minute—reaches its maximum value early in the roast and gradually slows down throughout the process. This is a natural physical phenomenon.
Roasters adjust heat application to influence the RoR, controlling the beans' heat absorption rate in an attempt to fine-tune the flavor profile.
A common pattern involves the bean temperature dropping immediately after the green beans are charged (dropped into the roaster) before rising again. This lowest point is typically called the Turning Point (TP). After the Turning Point, the RoR climbs rapidly and then begins a steady, continuous decline.
An excessively high RoR can cause surface scorching (tipping or facing) of the beans, while an RoR that is too low will over-extend the roasting time, potentially leading to "baked" flavors.
Deep Dive into RoR (Rate of Rise)
In coffee roasting, RoR (Rate of Rise) is the core metric for measuring the tempo of a roast. To put it simply, if Bean Temperature (BT) is your location, RoR is your speedometer: while the temperature tells you where you are, the RoR tells you how fast you are moving.
1. Core Formula and Calculation
RoR represents the change in temperature over a specific unit of time. Mathematically, it is the slope (derivative) of the bean temperature curve.
Common Units: Degrees Celsius per minute (°C/min) or Degrees Fahrenheit per minute (°F/min).
Sampling Interval: While expressed "per minute," software (like Artisan or Cropster) usually calculates this every 30 seconds or less to provide real-time feedback.
2. Standard RoR Curve Progression
In specialty coffee roasting, an ideal profile generally follows the principle of a "Steadily Declining RoR":
Turning Point (TP): The RoR transitions from negative to positive and begins to climb sharply.
Peak RoR: Usually occurs shortly after the Turning Point (at about 20-25% of the total roast time).
Maillard Phase: The RoR begins a planned descent. Roasters fine-tune the gas/heat here to prevent the beans from heating too quickly, which can lead to bitterness.
First Crack (FC): As beans release water vapor (shifting from endothermic to exothermic), the RoR can become volatile.
Drop (Finish): The final RoR is usually kept at a lower level (e.g., 4-8°C/min) to ensure the beans are cooked through without scorching the surface.
3. Common RoR Abnormalities
Mastering RoR is essential to avoiding flavor defects caused by the following:
Phenomenon | English | Description | Impact on Flavor |
失速 (Crash) | Crash | A sudden, sharp drop in RoR after the start of First Crack. | Results in "Baked" flavors; flat and lacking complexity. |
突升 (Flick) | Flick | A sudden upward surge in RoR at the very end of the roast. | Produces acrid caramel bitterness and smoky notes, masking acidity. |
燙傷 (Scorching) | Scorching | Excessive initial heat leading to an overly high RoR. | Charred spots on the bean surface; creates a spicy, harsh bitterness. |
4. Why is RoR So Important?
"Bean temperature is the result; RoR is the prediction."
Bean temperature has a lag. By the time you see the temperature is too high, it is often too late to react. By observing the trend of the RoR, a roaster can predict thermal momentum 30-60 seconds in advance, allowing them to adjust the gas or airflow early to achieve precision control.
Roasters begin by observing the maximum heat capacity and the airflow intensity of the roasting machine.
A common practice is to set the airflow first. The ideal airflow setting varies depending on the machine's design; it should be just strong enough to smoothly exhaust smoke and chaff. For instance, Probat roasters are known for their powerful airflow design. Generally, the stronger the airflow, the more heat (gas) is required, which in turn can shorten the roasting time.
Once the airflow is determined, the machine is preheated. A typical setting is 50% heat for about 10 minutes until the Bean Temperature (BT) gradually rises to the first crack temperature ($195^{\circ}\text{C}$), at which point the maximum capacity of green beans is charged.
Roasters observe and record the time at several specific temperature milestones:
Charge Temperature (Drop-in temp)
Turning Point (TP)
100℃
130℃
150℃
180℃
First Crack Start (FC Start)
Drop Temperature (Finish temp)
By recording these data points and plotting them on a graph, you create what is commonly known as a Roast Profile.
To calibrate the machine, charge the maximum bean volume with 60% heat and record the total time required to reach the peak of the first crack (usually $195^{\circ}\text{C} + 10^{\circ}\text{C}$).
If the total roast time exceeds 12 minutes, it indicates 60% heat is too low; the heat required for a full load needs to be increased.
If the total roast time is under 9 minutes, it indicates 60% heat is too high and should be reduced.
The goal is generally to adjust the total roast time to within 9–11 minutes.
Next, identify the minimum heat setting. This is the lowest heat level required to prevent the bean temperature from stalling or dropping after the start of the first crack. The specific setting for minimum heat is influenced by the airflow strength and the thickness of the drum.
The following is an example of the data recorded during a roasting process:
DP TP 100℃ 130℃ 150℃ 180℃ FC Stsrt Finish
|---------|---------|---------|---------|---------|---------|--------|
0:00 1:00 2:00 4:00 5:30 8:30 9:30 11:20
The milestones recorded above represent significant temperature points where the green beans undergo visible changes in color, shape, and physical properties after absorbing heat.
The calculation of RoR (Rate of Rise) typically begins after the Turning Point (TP). Based on the data, the RoR trends can be observed as follows:
100°C → 130°C: RoR is 30°C / 2min = 15°C/min
130°C → 150°C: RoR is 20°C / 1.5 min≈ 13.3°C/min
150°C → 180°C: RoR is 30°C / 3 min = 10°C/min
By using this method to observe the correlation between heat application and bean temperature, roasters can manipulate the RoR to fine-tune the rhythm (tempo) and flavor profile of the roast.
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