Spotlight on Drum Roasting: Drumming Up Slow, Dark Roasts

By Karl Schmidt, PROBAT Inc.

Have you ever wondered why there are so many types of coffee roasters? What are the differences between them? Why does one coffee roasting shop swear by a particular roasting technology, when another shop uses a completely different type of roaster?

Welcome to the first in a series of articles developed to help you answer these questions! Throughout the series, we’ll explore each of the main coffee roasting technologies — drum, continuous, centrifugal and tangential — and the various advantages of each. Because Probat Burns offers all four roasting technologies, we have an exceptional understanding of the differences between them, and now we want to share this knowledge with you!

We’ll start by taking a look at the drum roaster, America’s most commonly used roasting technology.

Karl Schmidt

Drum roastings technology

Drum roasters employ simple technology that hasn’t really changed a whole lot over the years. In a basic sense, the typical drum roaster is just a cylinder rotating on a horizontal axis. Flites, vanes or paddles mix the beans, and heated air moves through the drum and removes the roasting byproduct.

There are two types of drum roasters: solid drum and perforated drum roasters. The perforated design allows for more convective heat transfer — or heat transfer through air — than the solid drum.

Why drum roasters are so prevalent

While the tangential is the most commonly used roasting technology in Europe, drum roasters are used to roast 70% of the coffee in North America. There are many reasons why drum roasters are so prevalent in the United States. First of all, these types of roasters come in smaller capacities, so they are more suitable for small and midsize businesses, or for larger ones that roast many different types of coffee throughout the day or even at the same time on multiple drum roasters. Drum roasters also tend to be more affordable — though they don’t necessarily operate as fuel efficiently as the other roaster types.

Drum roasters are especially popular in the specialty coffee industry because of the manual control options. Roastmasters can manipulate the dampers and eyeball samples mid-roast to achieve their optimal coffee profile, much in the same way a painter dabbles with pigments to create the ideal hue. If you enjoy the artistic side of the roasting process, the drum roaster is for you.

Finally, drum roasters are ideal for dark roasts, which are in high demand in North America, particularly in the United States. You can’t roast fast and dark at the same time without risking an exothermic reaction — you have to slow down the entire process to fully develop the flavor of the beans. Drum roasters allow you to do just that, with a single batch taking anywhere from eight to 20 minutes. That’s because drum roasting uses less convective heat transfer (only about 85%) than other types of roasters. The remaining 15% of the heat transfer in drum roasting happens through conduction, or transfer of heat from bean to bean. The beans stay in constant contact as the drum turns, allowing flavor compounds, aroma and acidity to transfer between the beans for flavor development. Roastmasters can manipulate these compounds to develop their intended flavor profile through a truly artistic process.

If you’d like to learn more about drum roasters, I’d be happy to talk to you! Please feel free to contact us anytime.

Coffee roasters at a glance

Drum roasters
  • Solid drum
  • Perforated drum

Both are designed especially for slow, dark roasting.

Centrifugal / bowl roasters

High air velocity speeds the roasting time.

Continuous roasters

Beans enter and exit the machine continually, rather than in batches.

Tangential roasters

Agitator moves coffee within a closed chamber.

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