How to roast with airflow speed variable

June 7, 2023

In the process of roasting coffee, airflow is naturally formed inside the roasting machine.

Airflow in the roasting machine changes with power of burner, exhaust, size of batch, etc.

Everything a roaster does during the entire roasting process, including preheating and draining, is related to changes in airflow.

So it is very important to understand the effects of airflow during coffee roasting.

We control the air flow rate during roasting by adjusting the opening and closing of the gate in the exhaust pipe or by adjusting the fan speed of the blower.

However, the fundamental factor that forms airflow is increasing the temperature of the air around the heater from gas burner or electricity supply.

It is necessary to know the basic nature of the gas for the flow of air.

The average kinetic energy of a gas molecule is affected by the temperature of the gas. As the temperature of the gas increases, the pressure of the gas increases, the volume expands, and the speed of movement of the gas molecules increases.

This can be seen from the gas equation.

This shows that the flow of exhaust that affects the roasting machine is not the only important thing.

What we need to consider is the flow of air formed by the firepower used during roasting.

Therefore, roasting is necessary to adjust the proper balance by adjusting the exhaust in proportion to the firepower.

This reduces the probability of underdevelopment and bake, which are frequently occurring defects.

Roast with fast airflow

Figure 1

The way to speed up the airflow is to set high firepower throughout the roasting process.

More specifically, the roasting machine is set to high firepower and fast exhaust.

When roasting begins, add coffee to low temperature to proceed with sufficient roasting time.

Figure 1 shows an example of roasting with fast air flow.

Roast with slow airflow

Figure 2

Set the roasting machine to a lower firepower, as opposed to the above.

And also set the exhaust speed of the roasting machine to slow down.

Figure 2 shows an example of roasting with slow airflow.

Figure 3 shows the comparison of roasting profiles (blue curves) that formed fast air flow in Figure 1 using a fire scope and roasting profiles (red curves) that formed slow air flow in Figure 2.

Figure 3

The speed of the air flow that matches the green beans depends on the production area, variety, processing method, roasting discharge point and batch size.

If you sample green beans that lack information, I recommend that you proceed with roasting in the above two ways.

It would be helpful to choose a more positive outcome of the two methods and refer to the next roasting profile design.

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