Coffee News: from Seed to Cup

Understanding Roaster Drum Speed & Its Affect on Your Coffee

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Coffee roasting includes adjusting many variables to create your perfect roast profile. By changing factors including temperature, length of roast, and airflow, you can highlight sweetness, emphasize acidity, or create a well-balanced roast. But how does drum speed impact your results?

By adjusting drum speed, you can affect the amount of time that coffee beans are exposed to direct heat. Read on to learn more.

Lee este artículo en español Velocidad Del Tambor de La Tostadora: ¿Cómo Afecta al Café?

Coffee beans inside a drum roaster. Credit: Behmor

What Is Drum Speed? 

Drum speed is the measurement of one complete revolution of the drum of your coffee roaster. It’s measured in revolutions per minute (RPM). Coffee roasters are either fixed speed or variable and the manufacturer will tell you a machine’s drum-RPM in its literature. 

But there are some reports of drums running more quickly or slowly than advertised. Let’s explore why this matters and how you can check your machine’s RPM.

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Freshly roasted coffee spills into the cooling tray. Credit: Neil Soque

Why Is Drum Speed Important?

When we roast coffee, we expose green beans to both direct and indirect heat. The beans hit the hot walls of the drum, and they also move around in the warm air. The drum speed has an impact on how much contact the beans have with the walls of the drum, as well as the fall time of the beans (the short cooling time as the beans fall inside the drum).

At slower rotation speeds, the beans will be in contact with the direct heat of the walls for longer, meaning they’re at risk of being over-roasted.

Mark Additon is a coffee roaster and the owner of Updike Newtowne Coffee in North Kingstown, Rhode Island. He tells me that “if the drum speed is too low, the beans could be scorched. If the speed is too high, then the beans have the potential to break apart.”

Roasted coffee beans. Credit: Caleb Minear

Ida Lindhardt Kofod is Roastmaster for KONTRA Coffee in Denmark. She tells me, “I gave drum speed a thought only when I had issues with scorched beans in our old drum roaster. The problem only appeared when I did full batches, and if I wanted to keep the roast time below 13 to 14 minutes.

“If the problem [of scorching] only appears in big batches, the contact time between beans and drum is too long, meaning I toast the beans rather than roast them.” 

Coffee beans that are scorched may have an undesirable burned flavor and the lack of fall time could create a baked taste. By increasing the speed slightly, Isa was able to minimize scorching with larger batch size.

Ida says, “When roasting, you want a constant and uniform heat transferring from the roaster to your beans.

“A too high drum speed would mean that the beans never drop, like when you centrifuge clothes, but would continue to be pushed to the drum walls… With a too low drum speed, the beans would drop too soon.”

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Roasted coffee cooling down. Credit: Vladimir Proskurovskiy

Using Variable Drum Speed to Your Advantage

You may wish to adjust the drum speed during your roast to make use of the direct heat that results from a slower RPM. Just as you adjust temperature at different stages of roasting, drum speed can be used to help flavor and aroma development. By dialing down the speed, you can keep beans in the development stage for longer.

Mark says that “it is possible that it [drum speed] would have an advantage with espresso. You are looking for a different flavor set when roasting for espresso.”

But Ida cautions that experimenting with drum size can be risky. She says to “be prepared to lose beans when experimenting. If you do not have any issues with your roaster, and your profiles are working perfectly, I would be careful when starting to work with drum speed, as it changes your parameters, adds another variable, and you need to work with all your profiles again.”

Emptying a roaster in Denver, Colorado. Credit: Devon Barker

Ideal RPM & Other Factors to Consider

There is no definite rule about the ideal drum speed, but the general consensus is that the larger a drum barrel, the slower it should run. For example, Scott Rao recommends 70–80 RPM for a 1–2 kg roaster and 40–44 RPM for a 60 kg drum. But he makes note that the drum diameter is the determining factor, not the machine’s capacity.

You can check the speed of your roaster by making a mark on the edge of the drum. Wait until our machine is at working speed and then count how many times the mark passes a fixed point in one minute. This is your RPM.

Joanna Alm is a roaster at Drop Coffee Roasters in Stockholm. She tells me that the type of heat used should be a consideration when deciding whether to adjust drum speed. “Roasting is about finding your roast’s optimal settings for what taste profile you wish to achieve for your coffee. A limit of gas, air, and drum speed will all hold your abilities back,” she says.

“A higher or lower drum speed is better, depends on if you are roasting with convention or conduction heat. How hot your drum gets, the drum materials and the batch size impact how the coffee is roasting.”

Even if you choose not to vary your drum speed, it might be worth taking note of your RPM. If you notice scorched beans or baked notes that you can’t troubleshoot, check it again to make sure you don’t have a mechanical fault that has caused the roaster to slow down or speed up.

A roaster checks the roast. Credit: Battlecreek Coffee Roasters

Drum speed is just one more factor to consider when roasting coffee. When manipulated intentionally, it can be used to your advantage. But it may be easier and more accurate to change other variables, such as airflow or temperature.

Make sure that your roaster is running at a consistent drum speed by taking note of your RPM and checking it periodically. It’s one thing to purposely adjust the speed, but another to accidentally vary it. Understanding RPM can help you solve issues with subpar roasting results and contribute to you to creating great coffee.

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Written by Joe Gallagher

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