Roasting coffee often requires adjusting many variables to create the perfect roast profile.
By changing certain factors, such as temperature, the length of the roast, as well as airflow, roasters can showcase characteristics within the coffee. This includes highlighting sweetness, emphasising acidity, or creating a well-balanced roast.
One critical factor in creating the ideal roast profile is drum speed, also commonly referred to as drum revolutions per minute (RPM). A rotating drum is the crux of many roasting systems, and many machines allow roasters to adjust its speed.
By adjusting drum RPM, roasters can alter the amount of time coffee beans are exposed to direct heat, helping to unlock unique flavour profiles within the beans.
To learn more about how roasters can use drum RPM to their advantage, I spoke with the senior roaster at The Gentlemen Baristas, Alexandru-Catalin Marin.
What is drum RPM?
A traditional drum roaster will use a rotating drum with an energy source, heating both the drum surface and the air inside.
Therefore, the drum speed can be sped up or slowed down to change the transfer of energy.
“Drum RPM or drum speed, is the measurement of one complete revolution per minute,” says Alexandru, who is also an experienced barista. “Based on the drum’s capacity and size, a roaster will adjust the speed per minute.”
Coffee expert and author, Scott Rao recommends an RPM of between 70 and 80 for a 1 to 2kg roaster, and an RPM of between 40 and 44 for a larger 60kg drum.
When it comes to roasters manipulating drum speed to their advantage, Alexandru says a good starting point would be Scott Rao’s recommendations regarding roasters nominal capacity and RPM.
“While this is an effective starting point,” Alexandru says, “Roasters should also experiment themselves in order to gain a better understanding of their machine.
“Learning to understand the sweet spot of a roaster is not easy, but those who invest the time will benefit greatly,” he adds.
Some of the benefits include allowing roasters to buy higher quality coffees and profile them within 1 to 2 batches, and fewer blown batches.
To understand drum RPM when roasting coffees that flick or crash, roasters can reduce the drum speed to counteract flicks after the first crack. This can be effective for coffees that are high in energy after cracking, due to their processing method or high moisture content.
Reducing the drum RPM may help with a smooth development that will not accelerate too fast and has a high rate of rise (RoR).
For coffees that tend to lose energy after first crack, roasters may benefit from speeding up the drum RPM to increase the convective energy.
This ensures the drum reaches the end temperature in an efficient time without causing the beans to stall.
How does drum RPM affect the roast process?
“The importance of RPM comes in when a roaster is setting up the machine for the first time and they are trying to ‘dial in’ new batch sizes and coffees,” Alexandru explains.
Drum walls provide conductive heat transfer during the roast, and the beans are mixed as the drum spins.
“The higher the drum speed, the more convective heat a roaster has. Whereas, the lower the drum speed, the more conductive heat the roaster will have,” Alexandru says.
That said, too high a drum RPM, and the coffee will be centrifugal, which results in the beans becoming stuck to the walls of the drum.
“This is something we actively try to avoid,” Alexandru adds. “As this often results in roasting defects, such as scorching and tipping.”
Both scorching and tipping are roasting defects that occur from too much conductive heat transfer.
Conversely, the lower the drum RPM, the more conductive heat will be transferred. “The mass of beans will have more contact time with the drum walls and will spend less time floating in the hot air. Therefore, there is less convective heat,” Alexandru explains.
That said, an RPM that is too low will result in the beans being almost stationary, giving them too much contact with the drum walls. Often, this will also result in the same roast defects as having an RPM that is too high.
Alexandru adds that if the drum speed is too low for a higher batch, around 75% to 90% of the nominal capacity of the machine, the total roast time will be significantly longer. “This could result in underdeveloped coffee, or cause the RoR to crash, as there is not enough energy pulled into the beans from the hot air.”
While working on a Geisen w15, for example, Alexandru found a small batch size required a slower drum RPM. On the other hand, a larger batch requires a higher RPM to be able to mix the beans within the drum.
“This obviously varies from machine to machine. For instance, some may have drums that are quite narrow, and may require a faster drum RPM than others.”
Expanding on this, Alexandru believes drum diameter is an essential factor to consider. “A wider drum requires less speed to keep the beans afloat. A narrow drum will need more speed in order to get enough energy from the air.”
The benefits of understanding drum RPM
As with any industry, roasters can benefit from understanding how each component of their machinery works.
That said, some manufacturers may not inform roasters of the machine’s RPM. Instead, some may refer to RPM as “hertz”.
“For example, I’m currently working with a Probat 12 and it indicates the percentage of speed per minute,” Alexandru says.
“In my experience, the roaster would need to count the machines RPM themselves to be sure. The only way to familiarise yourself with the roaster is through trial and error.”
Alexandru believes the most important thing a roaster can do is take notes and cup every batch they roast. Additionally, he suggests roasters only adjust one variable at a time to understand exactly what makes the difference in the cup.
“This can help ensure the roast does not take too long, but also will not happen too quickly,” Alexandru explains. “In my experience, a faster RPM requires a shorter development time. Whereas a lower RPM needs a longer development time in order to get a particular colour for the coffee.”
Drum speed is an important factor to have control over, as it works in harmony with airflow in a roast to control energy transfer and reduce roasting defects
By finding the correct drum speed and understanding your roasters RPM, you can profile coffees quicker and improve the quality of roasts, preventing crashes or spikes in your ROR and loss of energy.
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