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Roasting · Roaster

Drum roasters

The most widespread batch coffee roaster – a heated, rotating drum through which hot air is drawn. How it is built, how heat reaches the beans, what the burner, airflow and drum speed controls do, and how batch size changes bean motion.

Across the chain: Science · Equipment

Source-checkedby Coffearium verification pass· Last source-checked 14 Oct 2026· Prepared with AI assistance· 10 sources (5 tier A)· 1 contested· Updated 14 Oct 2026
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Contents
  • The drum roaster is the most widespread batch roaster design; the beans tumble in a horizontal rotating drum while hot air flows through it.1
  • Heat reaches the beans by conduction from the hot drum wall, by convection from the hot air and by radiation; the shares depend on the machine and are disputed.1,2,3
  • With direct heating, the burner sits under the drum and wall conduction becomes substantial; with indirect heating, the drum is insulated and hot air carries more heat.1
  • On small drum roasters the burner is the main control; airflow and drum speed are adjusted less often. Airflow changes convection almost at once, a flame change more slowly.1
  • In a 2023 particle-tracking study, batch size and drum speed changed how beans moved between the dense bed and free flight, which the authors link to conduction versus convection.2

What a drum roaster is#

A drum roaster holds one batch of coffee in a horizontal drum that turns while it is heated. It is the most widespread batch roaster design.1 Batch roasters dominate industrial roasting because they are more flexible and easier to control than continuous systems.1

The drum does two jobs. It keeps the beans moving so that they heat evenly, and it lets hot air flow through the batch.1 Vanes on the inside wall (also called baffles) lift and mix the beans.1,2 Other designs move the beans differently: a fluid-bed roaster keeps them moving with fast air instead of moving parts.1 What happens inside the bean during a roast is described in the roasting overview.

The parts and the air path#

The figure follows the coffee and the air through a typical small machine. Large machines add air recirculation and off-gas cleaning (see the Deep dive).

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  1. Charge. The green coffee enters the hot roaster; this moment is called the charge.1
  2. Burners. In direct heating, the furnace sits directly under the drum.1 Gas burners or electric induction heaters heat the drum wall.2 On gas machines, burner sets can be used singly or together.1
  3. Drum and vanes. The drum turns, and its vanes keep the beans moving and mixed with the hot air.1 A drum-speed control changes how the beans tumble.1,2
  4. Hot air in. Hot air enters at the back of the drum through a screen and flows through it.1
  5. Bean bed and beans in flight. Many beans lie in a dense bed; others are lifted by the vanes and fall through the drum.2
  6. Bean probe. A temperature probe sits where it touches both beans and hot air, so it reads a mix of the two.1
  7. Trier. A sampling device on the drum face lets the roaster pull out beans to check colour, size, texture and aroma.1
  8. Exhaust. The air leaves at the front through an expansion chamber.1 An airflow control restricts or increases the air speed through the drum.1
  9. Cyclone. Silverskin (chaff), dust and small bean fragments are carried off with the air and separated in a cyclone.1
  10. Discharge and cooling. A door at the front opens, and the turning drum and vanes empty the batch into the cooler.1 Air flows through the stirred beans; larger machines can first spray a measured amount of water (water quench).1
Cutaway of a batch drum roaster, front plate removed: coffee runs from hopper to cooling tray, hot air from burner to cyclone.1,2 Schematic, not to scale.

Cooling. Small drum roasters drop the beans into a cooling tray.1 Industrial machines most often use a round-bed cooler with a slowly turning stirrer; the cooling air passes through the bed from below or from above.1 Most medium- to large-scale machines also have a water quench. It is optional, helps keep the roast degree the same from batch to batch, and raises bean moisture if overdone.1 Its effects on flavour, moisture and staling are covered in the roasting overview.

How heat reaches the beans#

Heat reaches the beans in three ways at once.1 Their shares differ from one roaster to another, and they are hard to measure.1

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  1. Burner. In a directly heated drum, the flame heats the drum wall from below.1,2
  2. Conduction from the wall. Beans touching the hot wall take up heat directly; with direct heating this share becomes substantial.1 Wall-to-bean conduction matters more in drums than in spouted beds.2
  3. Bean bed. The dense bed is carried up the rising side of the drum. In this region, conduction within the bean dominates.2
  4. Beans in flight. The vanes lift beans, which then fall through the drum.2
  5. Convection. Hot air flowing through the drum heats the bean surfaces.1 For beans in flight, convection dominates.2
  6. Radiation. Hot surfaces also radiate heat to the beans. A textbook calls this share usually negligible; a 2025 single-bean study in a gas-fired drum concluded that it governs (see Contested).1,3
Section through a directly heated drum: conduction from wall to bed, convection from hot air to beans in flight, radiation from the wall.1,2,3 Schematic, not to scale.
  • Conduction. Heat passes from the hot drum wall to the beans that touch it.1 Inside the dense bean bed, conduction within the bean dominates.2
  • Convection. Hot air carries heat to the bean surface.1 Beans thrown into the air by the vanes are heated mainly this way.2
  • Radiation. Hot walls radiate heat to the beans without touching them. How much this adds in a drum is disputed (see Contested).1,3

A drum usually works with relatively little air per kilogram of coffee (a low air-to-bean ratio). The upper limit is the air speed at which beans would be blown out of the drum.1 A fluid bed relies mainly on convection; a drum may pass a substantial share of the heat by conduction.1

Direct and indirect heating#

The balance of conduction and convection depends largely on how the drum is heated.1

  • Direct heating. The furnace is directly under the drum. The wall gets relatively hot, and beans touching it gain a substantial amount of heat by conduction.1
  • Indirect heating. The drum is insulated, and the hot air does not heat its bottom directly. The air is led to the back of the drum, for more convection inside.1

In one multi-study sensory paper, the coffees were roasted on a “flame-on-drum” roaster (1 kg) and, in one study, a “convection-based” roaster (15 kg).5 The authors warn that their results may not carry over to other roasters.5

Single-wall and double-wall drums. Roaster makers also describe drums by their wall. Practitioners describe a double-wall drum as two layers of metal with an insulating air layer between them.10 One manufacturer writes that the air layer reduces direct contact heat and the risk of scorching but can slow heat transfer, and puts the emphasis on the quality of the steel.10 No study opened for this entry compared single-wall and double-wall drums.

The controls#

On small drum roasters, the roaster works mainly with the burner and the trier; drum speed and airflow are adjusted less often.1

  • Burner (gas or power). It sets the energy going into the drum. Stepless gas controls with a pressure gauge let a roaster note a setting and repeat it.1 A flame change takes time to change the roasting environment.1 In eight sensory studies, mostly on a 1 kg drum, the roasts started with high energy input and only ever reduced it.5
  • Airflow. It restricts or increases the air speed through the drum and so changes convection almost at once.1
  • Drum speed. A faster drum tosses the beans up, giving less wall contact and more convection. Too fast a drum presses the beans against the wall instead, which can scorch the flat sides.1

Larger machines often run automatically. Conventional control holds a set hot-air temperature by adjusting the burner; more advanced systems follow a stored bean-temperature curve.1 How these choices show up on the roast curve is covered in the roast profile entry.

Batch size and drum speed#

How full the drum is changes how the beans move. In a 2023 study, beans were tracked inside a pilot-scale drum (160 mm across) loaded with 300, 600 or 900 g.2

  • Larger batches spread the bean bed over a larger area but packed it less densely.2
  • At a fixed drum speed, batch size hardly changed the median bean speed, but larger batches made the motion more uniform.2
  • At high drum speed (78 rpm), small batches threw beans higher, towards a “cataracting” motion in which beans are flung across the drum.2

Conduction dominates in the bed and convection in flight. The authors therefore name batch size and drum speed as two levers for shifting the balance between them.2 The study measured bean motion without heating; it did not measure heat transfer or flavour.2

A short history#

Two mid-19th-century US patents describe small rotating cylinders for roasting coffee over a stove. A US patent of 1864 shows a hand-turned cylinder over a stove plate. It is perforated, has two inclined vanes that move the beans from end to end, and is emptied by tilting it.8 A US patent of 1865 describes a cylinder of wire cloth turned by a clockwork motor over a stove.9 Both were small machines for the stove top.

Reference values#

Values reported in the sources. They describe particular machines and conditions; they are not targets.

Parameter Value reported Source
Roasting chamber temperature, roasters in general Typically 200–300 °C, depending on design and capacity; drums at the lower end Al-Shemmeri et al.2
Pilot drum used for particle tracking 160 mm diameter, 295 mm deep, three internal vanes Al-Shemmeri et al.2
Batch sizes tested in that drum 300, 600 and 900 g Al-Shemmeri et al.2
Drum speeds tested 42, 56 and 78 rpm Al-Shemmeri et al.2
Single-bean drum study Direct gas-fired drum, four burners, 45 rpm; roasting at 200–280 °C Bustos-Vanegas et al.3
Convective coefficient, calculated (hc) 15.91–17.43 W m⁻² °C⁻¹ (200–280 °C) Bustos-Vanegas et al.3
Overall coefficient, fitted (h) 60.20–102.55 W m⁻² °C⁻¹ (200–280 °C) Bustos-Vanegas et al.3
Bean-centre temperature, 3D model vs thermocouples (rotating drum prototype) RMSE 5.97 °C Fabbri et al.4
Batch size of a sample-scale drum used in sensory research 1 kg Münchow et al.5, Giacalone et al.6
Roast times on that drum and a 15 kg convection roaster (eight studies) 7:40–20:20 min total Münchow et al.5
Air temperature at charge, defect study (1 kg drum) 210 °C normal roast; 135 °C for an underdeveloped roast, 8 min longer Giacalone et al.6

No source opened for this entry gives a sourced range of drum sizes from sample roasters to industrial machines, so none is given here.

Heat transfer in a turning drum#

Two regions. Particle tracking divides the drum into a dense bean bed, with high occupancy and low speed, and a dispersed region of beans in flight.2 In flight, convection dominates; in the bed, conduction within the bean dominates.2 Beans next to the heated wall also take up heat from the metal, and the authors expect wall-to-bean conduction to matter more in drums than in spouted beds, where the wall is heated only by the air.2

Motion regimes. Granular flow in a rotating drum is described as rolling, cascading or cataracting.2 At 78 rpm, small batches of green and part-roasted coffee moved towards cataracting; for part-roasted coffee, it was also observed in large batches (900 g).2 At 42–56 rpm, batch size did not significantly change how far beans travelled vertically.2 As beans lose density during the roast, the authors suggest that changing drum speed could compensate.2

Radiation. In a 2025 study, single arabica beans were held on a thermocouple at the centre of a gas-fired drum, touching neither wall nor other beans, so conduction was excluded by design.3 The calculated convective coefficient was low, 15.91–17.43 W m⁻² °C⁻¹, which the authors attribute to the low air speed created only by the drum’s rotation.3 The overall coefficient fitted to the measured bean temperatures was 60.20–102.55 W m⁻² °C⁻¹, and the authors concluded that radiation is the dominant mechanism.3 A roasting textbook calls radiation in hot-air roasters usually very limited and negligible.1 The two settings differ: a lone bean in a drum without forced air flow, and production drums with hot air drawn through a bed of beans.

200220240260280020406080100120Roasting temperature (°C)Coefficient (W m⁻² °C⁻¹)Overall, fitted (h)3 Convective, calculated (hc)3
Data table
SeriesRoasting temperature (°C)Coefficient (W m⁻² °C⁻¹)Source
Overall, fitted (h)20060.23
Overall, fitted (h)22072.363
Overall, fitted (h)240683
Overall, fitted (h)26081.623
Overall, fitted (h)280102.553
Convective, calculated (hc)20015.913
Convective, calculated (hc)22016.273
Convective, calculated (hc)24016.653
Convective, calculated (hc)26017.043
Convective, calculated (hc)28017.433
Overall heat-transfer coefficient fitted to measured bean temperatures, and the calculated convective coefficient, for a single bean in a gas-fired drum at five roasting temperatures. The gap is what the authors attribute to radiation.3

Models. Heat-transfer models of drum roasting simplify the machine in different ways. A 2011 three-dimensional model of a single bean assumed a rotating cylinder roaster under natural convection.4 Validated against beans with inserted thermocouples in a drum prototype, it predicted bean-centre temperature with an RMSE of 5.97 °C.4 The 2025 single-bean model treats the bean as one temperature and includes evaporative cooling and heat released by reactions.3 Neither model follows a full bed of beans in contact with the wall.

Same bean curve, different machine#

When a similar temperature profile was applied on a traditional drum and on a laboratory fluid bed, the coffees had similar physical properties and aroma formation.7 In production, the probe reading is a machine-specific mix of bean and air temperature, so the bean curve is not directly visible.1 Research drums are sometimes fitted with extra temperature sensors to follow bean temperature.6

Process air and off-gas#

Smaller roasters usually draw in process air at one end and release the off-gas at the other: an open system.1 Because the off-gas is still hot, open systems are not energy efficient.1 Most large-scale operations therefore recirculate a major part of the off-gas to the heating unit; the rest leaves the system and may pass an off-gas cleaning step.1 Design standards and norms for industrial roasting systems are set out in VDI guideline 3892 of the Association of German Engineers, according to the roasting textbook.1

Risks and defects#

  • Scorching. Too high a drum speed can press beans against the hot wall, over-develop their outside and, at worst, scorch the flat sides.1 Practitioner names and causes of surface marks are collected in the roast defects entry.
  • Bean breakage. Any bean movement causes some mechanical stress; a good design keeps breakage low.1
  • Opening the trier. Each pull of the trier lets cool air into the drum and can change the final stages of the roast.1
  • Over-quenching. Too much quench water is absorbed, raises the final moisture and speeds up staling.1
  • Carrying air limits. Above a certain exit air speed, beans are carried away with the air; this caps the air a drum can use.1

Open questions#

  • Share of each heat path in a loaded drum. No study opened for this entry measured conduction, convection and radiation separately in a full production batch; the radiation finding comes from a single suspended bean.1,3
  • Single-wall versus double-wall drums. Claims come from manufacturers only.10
  • Batch size, drum speed and flavour. The particle-tracking study measured motion without heat; how batch size and drum speed change heat transfer and the cup is untested in the sources found.2
  • Drum and convection roasters. The main sensory evidence comes from one flame-on-drum and one convection-based roaster.5

Key studies#

Al-Shemmeri et al., 2023 – Bean motion in a rotating drum. Tracks single beans by positron emission particle tracking in a pilot-scale drum roaster, unheated, at three batch sizes and three drum speeds. Separates a dense bed from beans in flight and shows that batch size and drum speed change the motion regime, which the authors link to the balance of conduction and convection.

Food Research International 163, 112253 · doi:10.1016/j.foodres.2022.112253

Bustos-Vanegas et al., 2025 – Single-bean heating in a gas-fired drum. Holds single arabica beans at the centre of a direct gas-fired drum at 200–280 °C and fits a lumped heat-transfer model with evaporation and reaction heat. The fitted overall coefficient far exceeds the calculated convective one, from which the authors conclude that radiation dominates.

Frontiers in Food Science and Technology 5, 1603783 · doi:10.3389/frfst.2025.1603783

Fabbri et al., 2011 – A 3D model of the bean in a rotating cylinder. Builds a finite-element model of heat and moisture transfer in a bean, with a simplified and a 3D-scanned bean shape, for a rotating cylinder roaster under natural convection. Bean-centre temperatures agreed with thermocouple measurements in a drum prototype.

Journal of Food Engineering 105(2), 264–269 · doi:10.1016/j.jfoodeng.2011.02.030

Münchow et al., 2020 – Roasting conditions and flavour. Pools eight sensory studies roasted on a 1 kg flame-on-drum roaster and, once, a 15 kg convection-based roaster. Colour predicted flavour better than timing; the authors caution that results may not transfer to other roasters.

Beverages 6(2), 29 · doi:10.3390/beverages6020029

Contested
  • Which heat path dominates in a drum: a roasting textbook chapter says a substantial amount of heat may pass by conduction in a drum, and calls radiation in hot-air roasters "usually very limited and negligible"1; a 2023 particle-tracking study describes convection as dominant for beans in flight and conduction for the bean bed2; and a 2025 study of single beans held in a gas-fired drum, out of contact with wall and beans, inferred that radiation governs, from a fitted overall heat-transfer coefficient (60.20–102.55 W m⁻² °C⁻¹) well above the calculated convective one (15.91–17.43 W m⁻² °C⁻¹).3
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Sources

  1. BSchenker, S.; Rothgeb, T. (2017). The Roast – Creating the Beans' Signature. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 11, pp. 245–271. doi:10.1016/B978-0-12-803520-7.00011-6 · other copies
  2. AAl-Shemmeri, M.; Windows-Yule, K.; Lopez-Quiroga, E.; Fryer, P. J. (2023). Coffee bean particle motion in a rotating drum measured using Positron Emission Particle Tracking (PEPT). Food Research International 163, 112253. doi:10.1016/j.foodres.2022.112253 · other copies
  3. ABustos-Vanegas, J. D.; Martins, M. A.; Corrêa, P. C.; Baptestini, F. M.; de Oliveira, G. H. H. (2025). Modeling and simulation of coffee bean heating during roasting: effect of heat generation. Frontiers in Food Science and Technology 5, 1603783. doi:10.3389/frfst.2025.1603783 · other copies
  4. AFabbri, A.; Cevoli, C.; Alessandrini, L.; Romani, S. (2011). Numerical modeling of heat and mass transfer during coffee roasting process. Journal of Food Engineering 105(2), 264–269. doi:10.1016/j.jfoodeng.2011.02.030 · other copies
  5. AMünchow, M.; Alstrup, J.; Steen, I.; Giacalone, D. (2020). Roasting Conditions and Coffee Flavor: A Multi-Study Empirical Investigation. Beverages 6(2), 29. doi:10.3390/beverages6020029 · other copies
  6. AGiacalone, D.; Degn, T. K.; Yang, N.; Liu, C.; Fisk, I.; Münchow, M. (2019). Common roasting defects in coffee: Aroma composition, sensory characterization and consumer perception. Food Quality and Preference 71, 463–474. doi:10.1016/j.foodqual.2018.03.009 · other copies
  7. BPoisson, L.; Blank, I.; Dunkel, A.; Hofmann, T. (2017). The Chemistry of Roasting – Decoding Flavor Formation. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 12, pp. 273–309. doi:10.1016/B978-0-12-803520-7.00012-8 · other copies
  8. A/BHyde, E. J. (1864). Improvement in coffee-roasters (US Patent 41,436). United States Patent Office.
  9. A/BMills, C. A. (1865). Coffee-roaster (US Patent 50,878). United States Patent Office.
  10. CThe San Franciscan Roaster Co. (2024). The Coffee Roaster Drum Debate – Single Wall vs Double Wall. The San Franciscan Roaster Co. (roaster manufacturer, blog).

Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.

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