What is airflow and how is it controlled?

Airflow refers to the flow of air inside the coffee bean roasting machine. But it will actually be affected by the motion of the roaster system. And let’s also take a look at how air affects everything that’s going on inside the roaster system. Being able to figure this out will open up a coffee world full of surprises for us.

There will be some level of the draft when the roaster is in operation, mainly because the heat will cause the air to move. Of course, we can also manually ventilate during roasting.

We can control the temperature of the air, the velocity of the air, and the pressure of the air, so why should we change the airflow? Because airflow is a critical component in the roasting process.


The relationship between airflow and heat

Heat can be transferred by conduction, convection, and radiation.

Heat conduction is the transfer of thermal energy from one solid to another. During the roasting process, after the drum containing the coffee beans is heated, the heat energy is used to heat the coffee beans through conduction. Convection is heat transfer caused by airflow.

Airflow is one of the key elements in getting the coffee beans through the air during the roasting process to create taste, aroma, and color, so air plays a vital role not only in heat transfer but also in the combustion of gases. It is also important for the development of coffee flavor.

Of course, airflow isn’t just about heat. It also removes the hulls and small particles from the coffee beans during the roasting process.

Controlling airflow is a critical step in controlling the roasting process. Changing the ratio between conduction and convection, the two modes of heat transfer, allows our baristas to change the final roast result.

Coffee is a very complex and organic product, and one of the reasons it’s exciting is that we can roast coffee a million different ways and get millions of different coffee flavors.

best 12kg coffee roaster machine

best 12kg coffee roaster machine

How does airflow affect coffee flavor?

Roasting requires heat transfer by conduction and convection. Their roles are complementary to each other. Especially in terms of presenting coffee flavors.

Generally speaking, conductive heat transfer can help us lock in the flavor and sweetness of coffee, while convective heat transfer can help us find and reflect the clearer flavor characteristics of coffee. The two are not contradictory.

Judgment of airflow problems

There is no single “right way” in coffee roasting: not a set temperature, not a roast time, and certainly not the right airflow. It all depends on the type of coffee bean, the performance of the roaster, and the roaster’s idea of what flavor profile he wants the coffee to exhibit.

In other words, we cannot say “There must be airflow between process X and process Y”. However, there are methods that can help roasters achieve specific roast results with specific roasters for specific coffees when airflow is too low or too high.

1. What will be the impact of insufficient airflow?

When the airflow is too low, convective heat transfer is not sufficient. Insufficient convective heat transfer is manifested by lower roasting temperatures. But it is precisely this low temperature of roasting that causes the beans to be over-roasted. The reason is that coffee beans have heat-absorbing characteristics similar to the dough, and the low-temperature state will cause coffee beans to absorb excessive heat for a long time in the roaster, resulting in slow roasting. This can affect the flavor development and the consistency of the roast. A coffee bean drum roaster can help to avoid this problem by providing a more even and stable heat distribution.

Too little airflow can be the result of a poorly maintained roaster or an exhaust fan that isn’t powerful enough to produce the necessary airflow per minute. A measure of the rate at which air flows into or out of the roaster is the acidity of the coffee, which can give a coffee a “mild” acidity, or a coffee with no character or a flat flavor profile.

In addition, flavor is not only related to the degree of development of coffee beans, but when there is no good air flow during roasting, it is impossible to remove unwanted impurities, because these impurities may burn into fine ash and adhere to the roaster drum. On the inner wall, the coffee beans inside will eventually absorb a dusty smell, although the roasted coffee beans look beautiful.

Without enough airflow, it is difficult to have a clear flavor in the roasted coffee beans. This is because the carbon-containing waste gas generated during the roasting process cannot be discharged.

Hot Sale 3KG coffee roaster
Hot Sale 3KG coffee roaster

2. What is the impact of excessive airflow?

On the other hand, too much airflow can cause the coffee beans to heat up too quickly, causing uneven heating and burning of the edges of the beans. And because the temperature is too high, the surface of the coffee beans will be charred before the internal flavors have developed.

Even if we can avoid uneven heat, we still get that pungent taste when cupping, and in general, excess airflow can make the coffee beans appear sour due to the shortened roasting time.

The temperature, velocity, and pressure of the airflow, what is the difference?

Heat transfer during roasting is affected by the temperature, velocity, and pressure of the air.

While air velocity is important, the judgments we make may rely more on measurements of air pressure. It’s hard to test air velocity because the smoke and oil can hinder the accuracy of these devices, whether it’s an airflow meter or something similar.

Pressure and speed are interrelated and affect each other. It may be better to test with a pressure gauge such as a Magnehelic. That is to say, we can control the speed according to the reading of the pressure gauge. The greater the negative pressure in the drum, the higher the speed of the airflow.

So what effect does pressure have on air velocity? In fact, air velocity and air pressure are inextricably linked, because we can’t really control pressure like we can control air velocity and temperature.

These questions may seem complicated at first, but over time, it becomes simple. We can observe the setting value of the pressure gauge on the roasting software and the temperature and roasting time set on the roasting machine to adjust the pressure setting node.

How to control airflow?

Knowing what air velocity is and how it affects it is only one aspect. How do we control it? We recommend adjusting by shock absorbers or variable speed fans at critical points in roasting. Some roasters may prefer to use full airflow and adjust other variables, such as gas pressure or heat.

If so, we recommend that batches should be kept at 50-60% of the roaster drum capacity. Because if you have good airflow inside the drum during roasting, don’t expect all coffee beans to have equal airflow throughout the roast.


Change the airflow through the roaster

Let’s take a look at each step in the roasting process:

In the initial stage of roasting, if there is too much heat conduction (airflow), it will quickly take away the moisture in the coffee beans. We want to dry the beans, but we don’t need to make the beans too dry, otherwise, many flavor characteristics will be lost.

And in the initial stage of roasting, we also need to reduce the airflow, which can allow the inside of the coffee beans to undergo an endothermic reaction to absorb enough heat (energy is absorbed in the form of heat), because we need to let the coffee beans reach a certain amount of heat After accumulation, the Maillard reaction is carried out, and the coffee beans are pyrolyzed during the whole process to develop its flavor characteristics.

When browning begins, we need to adjust the airflow to help the beans drain moisture. Because the coffee beans are in a state of dehydration at this time. At this point, we open the air intake valve, which will help remove free water molecules in the coffee.

Airflow also affects the end temperature of the roasting process. Compared with the roaster that lacks airflow control, the roaster with multi-stage airflow control can allow coffee beans to be roasted at a lower terminal temperature, and ultimately achieve the purpose of reducing the weight loss of coffee beans.

Example of changing airflow

Controlling airflow can’t just follow a set pattern. We can adjust the firepower, even the drum speed, but for different varieties of coffee, such as Pacamara, Marago Ripe (Elephant Bean), Peaberry Natural Process, and low-density coffee beans, observe the airflow. Change is equally important.

So, in this case, how should we adjust the airflow during the roasting process? Take natural-processed coffee beans as an example: we prefer to reduce the airflow during the drying stage, and gradually increase the airflow after entering the first crack.

We mentioned earlier that heat conduction increases the sweetness of the coffee beans and heat convection or airflow increases the cleanliness of the mouthfeel (natural-processed coffee beans are known for their sweetness and fruity flavors, and washed coffees for their Famous for dry mouth). So better heat transfer can enhance the fruit flavor and sugar in natural coffee beans.

However, we must also control the airflow at the end of the roasting, otherwise, the sun-dried beans may be prone to odor due to poor air circulation in the roaster drum.

Nevertheless, we should continue to practice, there is no “one-size-fits-all” rule, the right one is the best because coffee can always bring us surprises.


Airflow is the key to coffee roasting. High airflow means more heat convection, which can make the coffee beans show a clearer flavor. This may seem simple, but really wanting to master this is as important as mastering other elements of coffee roasting. challenge.

How much airflow should we use to roast high-altitude natural beans with great sweetness? What airflow rate should be used during the drying stage of washed beans in Columbia? The answers to these questions can only be understood and judged the more we practice coffee roasting.

Therefore, to control the airflow during the roasting process, not only try different roasting plans with the same coffee, but also try the same roasting plan with different coffees, take notes and evaluate them, and discover the law of airflow control. Most importantly, find the fun in comparison.

Leave a Comment

Your email address will not be published. Required fields are marked *

+86 17836969360