Coffee roasters have found that when the first pot of coffee beans are roasted each day, the beans will have a lower temperature recovery point than normal, and the roasting curve will be slower. In the following roasting, the temperature recovery point of each pot of beans will be higher and higher, the roasting time will be gradually shortened, and the roasting rhythm will be faster and faster.
Roasting rhythm will affect the final flavor expression of coffee beans. In order to make the roast more stable, many roasters usually increase the temperature of the beans in the first roast, and gradually reduce the temperature of the beans in the subsequent roasts, so that the roasting rhythm tends to be consistent. If the baker is experienced and operates properly, this adjustment method is effective, and the baking results can be accepted by most people. However, the disadvantages of this method are also very obvious. On the one hand, it requires the roaster to adjust the temperature of the beans for each pot of beans. During the roasting process, the firepower also needs to be adjusted in real time according to the temperature development trend, which complicates the roasting work. On the other hand, this can also lead to a lack of consistency in coffee roast results, and savvy cuppers can easily tell the difference between them.
The root cause of this situation is that the concept of input temperature is wrongly used, and the roaster is not properly preheated.
01 Bean input temperature for coffee roasting
The roaster is usually equipped with at least two temperature probes, one is installed at the hot air outlet to monitor the air temperature, which is called ET (Environment Temperature), which is the ambient temperature. The other is installed near the bottom of the boiler and is in contact with the bean pile, which is called BT (Bean Temperature), which means bean temperature.
When we pour green coffee beans into the roaster, the reading of the bean temperature probe will drop immediately. We call the reading before the temperature drops the bean entry temperature, but this definition fails to reflect the physical meaning of the bean entry temperature.
When the roaster is heated with no load, the reading of the bean temperature probe will rise rapidly when the bean temperature probe is in contact with hot air. During this process, the temperature change rate of the boiler inner wall, insulation layer, and fuselage of the roaster is much slower than that of the airflow temperature. We might as well call the overall temperature of the roaster as the thermal state of the roaster, and the rapidly changing temperature cannot accurately reflect the thermal state of the roaster. The thermal state of the roaster represents the latent heat stored in the roaster, which will have an obvious impact on the subsequent roasting process.
When choosing an input temperature, we mean to choose a certain thermal state of the roaster. If there is no correlation between the two, the input temperature is meaningless. So how do you connect the two? A concept of thermal equilibrium state needs to be introduced here.
02 Heat balance of coffee roaster
The heat balance state of the coffee roaster means that at a certain constant firepower value, the heat absorbed by the roaster as a whole is equal to the heat dissipated. At this time, the temperature of the roaster does not rise or fall, and the temperature curve BT is a horizontal straight line (the absolute values of ΔBT and ΔET are close to 0), and the roaster reaches a state of thermal equilibrium. Note that there is more than one heat balance state for a roaster. As long as the heating time is long enough, any constant firepower value corresponds to the heat balance state of the roaster at a certain temperature. Think about why this is?
By bringing the roaster to a state of thermal equilibrium, we established a correspondence between the heat value when the roaster was empty, the temperature probe readings, and the thermal state of the roaster. Only when it is in a state of thermal equilibrium, can the input bean temperature have a corresponding relationship with the thermal state of the coffee roaster, and then the input bean temperature has a clear physical meaning.
Now we know why some bakers are baking faster and faster. Because a roaster that has not been properly preheated is not in thermal equilibrium, even though it appears to be choosing the same input temperature, the latent heat of the roaster is actually getting higher and higher.
In turn, we can use this to verify that the roaster is properly preheated. For the same beans, if the same temperature of the beans is selected, the same return temperature point and time appear, which proves that the roaster is indeed in a state of thermal equilibrium when the beans are placed. On the contrary, it proves that the thermal state of the roaster is not the same when the beans are inserted twice.
Knowing the heat balance state of the roaster, preheating the roaster becomes very simple. No matter what approach is taken, as long as the roaster reaches thermal equilibrium at the input temperature as soon as possible, preheating is successful.
03 Wave preheating method
In the actual roasting, it is not difficult to find that after roasting 4 or 5 batches, the roaster only needs a very low firepower value under the no-load state, so that the roaster can operate at the bean input temperature we selected. maintain thermal equilibrium. The firepower value is the firepower value corresponding to the temperature of the beans when the roaster is in a state of thermal equilibrium. Theoretically, use this firepower value directly to preheat the machine. As long as the time is long enough, the machine will slowly heat up to the thermal equilibrium state and reach the bean input temperature.
In order to save time and fuel in actual production, we let the machine heat up quickly to the overheating temperature. Although the reading of the temperature probe has exceeded the temperature of the beans at this time, the latent heat stored in the body is still relatively small. In layman’s terms, the fuselage is still cool at this time, and it is not completely heated. Then we lowered the firepower in stages. After each firepower reduction, the reading of the temperature probe first dropped and then rose, and the temperature curve would form a trough. When the temperature rises to the superheated temperature again, lower the firepower value again… Repeat this operation process many times, the firepower value will gradually approach the firepower value of the beans, and the shape of the temperature curve seems to have gone through several waves, and finally subsides gradually, as shown in Figure 2 Shown, so I call this method the wave type preheating method.
In the final stage of preheating, directly adjust the firepower value to the bean-input firepower and keep it for a few minutes. The roaster will gradually reach a state of thermal equilibrium, and the temperature will slowly drop to the bean-input temperature and remain in a straight line. At this point, the preheating is complete, waiting for the beans to enter.
04 Operation example of wave preheating method
Next, we use a case to demonstrate the actual operation process of the wave preheating method.
We selected a bean input temperature of 185°C on a certain roaster. According to previous experience, when the roaster has a firepower value near 0.3kPa (here refers to the gas pressure value), the machine will maintain a heat balance state at 185°C. The default overheat protection temperature of this roaster is 240°C. During the preheating process, we choose 200°C as the overheat protection temperature. This temperature is slightly higher than the bean input temperature, which can help us to preheat quickly and is also more friendly to the machine. The following is a detailed explanation of the operation steps:
● After starting up, start to preheat with 100% firepower and a small damper. ● When the temperature is close to 200℃, reduce to 75% firepower value, at this time the temperature reading will drop first, the temperature curve will show a trough, and then the temperature will continue to rise. ● When the temperature is close to 200℃ again, reduce to 50% firepower value. ● Repeat the above steps, the firepower value will decrease continuously. The waves of the temperature curve gradually flattened. ● If the firepower value is set to 0.3kPa for entering beans, and the damper is adjusted to bean entering damper, the temperature of the roaster will slowly drop to around 185°C. ● Finely adjust the firepower value again to keep the temperature curve at 185°C and form a horizontal straight line. At this time, the absolute values of ΔBT and ΔET are close to 0, and the roaster is in a state of thermal equilibrium.
At this point, the preheating is complete, waiting for the beans to enter.
05 Preheating procedure between adjacent batches
During the roasting process, in addition to the machine needing to be preheated before the first pot of beans, the roaster between adjacent roasting batches also needs to be preheated. The principle of preheating between batches of the roaster is the same as that of the first preheating, which is to make the roaster reach a thermal equilibrium state at the temperature of the beans. The difference is that, as opposed to being under-latent when first preheating, the roaster tends to be overheated at the end of a batch.
In order to make the roaster quickly approach the hot state we need when we enter the beans from the overheated state, I usually turn off the heat and cool down immediately after the last batch of roasting. Preheat the roaster with the bean-input firepower value to the bean-input temperature, and fine-tune the firepower value again to keep the temperature curve at the bean-input temperature in a horizontal line.
The warm-up procedure between batches is much simpler than the first warm-up. Compared with the wave-type preheating method in the previous section, it can be found that after the subcooling and cooling, the last two operation steps of the wave-type preheating method are directly adopted.
This article focuses on the application of the firepower value in the preheating of the roaster. In addition, factors such as ambient temperature and air flow (damper management) also affect the preheating process of the roaster. Therefore, it is necessary to make corresponding adjustments according to the actual situation.
It should be noted that the preheating principle of the roaster is to make the machine reach a state of thermal equilibrium before entering the beans, regardless of the path (method) to achieve this state. In addition to the wave preheating method introduced in this article, the preheating and inter-pot operation protocol introduced by Scott Rao, the PID temperature control preheating method, and the exclusive methods of different bakers are common. No matter which method is used, as long as the roaster can reach thermal equilibrium before the beans are loaded, preheating is successful.
Different roasters have vastly different thermal properties, and there is no one-size-fits-all approach. According to the correlation between the temperature of the bean input and the heat value of the bean input and the thermal state of the roaster, you can easily formulate a preheating program suitable for your machine. As long as the roaster is in thermal equilibrium at your chosen input temperature, your preheat is successful.