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Roasting · Step 1

Preheating and charge

The first step of a roast – heating the empty roaster to a steady state, then charging the green coffee. What charge temperature means, why the probe reading drops to a turning point, and what studies show.

Across the chain: Science · Equipment

Source-checkedby Coffearium verification pass· Last source-checked 17 Oct 2026· Prepared with AI assistance· 14 sources (10 tier A)· 1 contested· Updated 17 Oct 2026
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Contents
  • Before the charge, the empty roaster is preheated; in one study a 5 kg drum was held at 210 ± 5 °C for at least 30 min to stabilise the drum temperature.2
  • In two studies from one research group, "charge" or "start" temperature means the air temperature when the beans enter the roaster.4,5
  • The bean probe reads a mix of bean surface and hot air; its low point after charge, the turning point, is specific to machine and probe.1,2
  • In a 2025 spouted-bed study, batch size and airflow changed the early time–temperature profiles more than inlet air temperature did.3
  • In a defect study on one drum, charge temperature alone did not set the pace; a roast charged at 230 °C reached first crack later than one charged at 210 °C.4

What happens at the start of a roast#

A roast begins before any coffee goes in. The roaster is first heated while empty. Then the green coffee is dropped in; this moment is called the charge.1 A stopwatch measures the roast time from the charge to the drop, when the beans leave the roaster.1

This is step 1 of 6 in the roast. The roasting overview describes the whole process, and the roast profile entry explains how the curve is read. Next step: drying phase.

Why roasters are preheated#

A roaster is a mass of hot metal as well as hot air. Even models of an air roaster include heat passing from the air to the metal wall, and from the metal to the beans.3 According to a roasting textbook chapter, in a drum a substantial amount of heat may pass to the beans by conduction from the hot wall.1

Preheating brings that metal to a known state. In a 2024 study, a 5 kg drum was preheated to 210 ± 5 °C for at least 30 min before each roast. The aim was “to stabilize the drum temperature”.2 Laboratory air roasters are preheated too, before the beans go in.6,9

A roasting textbook chapter names the “cold start behavior of the roaster” as one reason why the beans’ temperature can differ from batch to batch. Green-coffee moisture and the weather are others.1

Practice between batches. Practitioners describe a “between-batch protocol”: a fixed routine between roasts meant to bring the machine back to the same starting state. One practitioner quoted on a roasting-software company’s blog describes its aim as resetting “the thermal energy” of the machine.13 Practitioners also say the first roast of the day is always a little different.13 No peer-reviewed study opened for this entry measured a first-batch effect or tested a between-batch protocol.

Charge temperature#

What it means. In two studies from one research group, the charge temperature (also “start temperature”) is the air temperature when the beans enter the roaster.4,5 Roasters also log the reading of the bean probe, the temperature probe that sits in the bean mass.1 It sits where it touches both beans and hot air, so in the empty drum it reads hot air.1

Where the probe sits matters. An air temperature depends on where it is measured. In an empty laboratory air roaster, the air leaving the chamber was cooler than the air going in. At the lowest air speed tested, it read 228.5 °C against 249 °C.6 According to a roasting textbook chapter, the bean probe of a production roaster reads a mix of bean surface and hot air. Its reading is specific to the machine design.1

Machine-specific values. In eight sensory studies from one research group, most roasts on a 1 kg drum were charged at 200 °C. A 15 kg roaster was charged at 160 °C.4 With washed coffee from Sidamo, Ethiopia, in both studies, first crack came after 8:50–9:52 min on the drum and 8:25–9:25 min on the convection roaster.4 A charge temperature is therefore a setting for one machine, not a number to copy to another.1,4

The drop and the turning point#

When the green coffee goes in, the bean-probe reading falls. The probe reads a mix of bean surface and hot air, and it now sits among cool beans.1 The lowest reading is called the turning point. In one drum study, the rate of rise was highest just after it.2

The beans themselves do not cool down after the charge. They enter at room temperature, and from then on heat flows into them from the hotter surroundings.7 The dip belongs to the probe: it moves from sitting in hot air to sitting among cool beans.1 A 2025 roaster model treats the thermocouple reading and the bean temperature as two different values.3 One 2024 study describes the turning point as the moment when “the coffee beans and the roaster temperature equilibrate”.2

A · Preheated, before the charge1234B · Just after the charge56789
  1. Burner and drum wall. Preheating stores heat in the metal; one study preheated its drum to stabilise the drum temperature.2 Models include heat passing from the air to the metal and from the metal to the beans.3
  2. Hot air. Air flows through the drum and carries heat to the beans.1
  3. Bean probe, empty drum. It sits where it will touch beans and hot air; before the charge only hot air surrounds it.1
  4. Air probe. Air temperature readings depend on position: in one empty laboratory roaster, the outgoing air was cooler than the incoming air.6
  1. The charge. Green coffee at room temperature enters the hot roaster.1,7
  2. Bean probe among cool beans. Its reading, a mix of bean surface and hot air, falls to the turning point and then climbs.1,2
  3. Conduction. Beans touching the hot wall take up heat directly; in a drum this share may be substantial.1
  4. Convection. Hot air heats the bean surface. Models treat the heat flow as proportional to the gap between air and surface temperature.1,7
  5. Radiation. Hot surfaces radiate heat to the beans; how much this adds is disputed (see Contested).1,8
A drum roaster just before (A) and just after (B) the charge: where heat comes from and what the probes sit in.1,3 Schematic, not to scale.

So the turning point is a useful marker for comparing batches on one roaster, with one probe in one place. It is not a bean temperature that can be compared between roasters.1 The roast profile entry shows where it sits on the whole curve.

Batch size and charge temperature#

A bigger batch is more cool mass to heat. A roasting textbook chapter describes the air-to-bean ratio: the amount of hot air used per kilogram of green coffee. The same heat can be delivered with less, hotter air or more, cooler air.1

In a 2025 study on a 0.5 kg spouted-bed air roaster, batches of 200–500 g were roasted at different airflows.3 Batch size and airflow changed the time–temperature profiles more than inlet air temperature did, especially early in the roast.3

Practitioners adjust the charge temperature to the batch size, on the reasoning that a larger batch needs more energy.14 No peer-reviewed study opened for this entry tested a rule for matching charge temperature to batch size in a drum.

Drum and air roasters at the start#

According to a roasting textbook chapter, a substantial share of the heat in a drum may pass by conduction from the hot wall. In a fluid-bed roaster, which lifts the beans on hot air, convection predominates.1 No study opened for this entry measured how much heat the preheated drum wall adds in the first minutes. Air roasters also have metal walls that exchange heat with air and beans.3

Laboratory air roasters make the start easier to define. In one study, a batch of 100 g went into a preheated fluid bed with hot air at a set temperature.6 How the drum and fluid-bed roasters compare in other ways is covered in their own entries.

Reference values#

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

Parameter Value reported Source
Preheating, 5 kg gas drum (4 kg charged) 210 ± 5 °C for at least 30 min, to stabilise drum temperature Anokye-Bempah et al.2
First crack after the same preheat, two profiles About 8 min (high initial heat); 12 min (low initial heat) Anokye-Bempah et al.2
Charge temperature, defined Air temperature when the beans enter the roaster Giacalone et al.5; Münchow et al.4
Charge temperatures, defect study (1 kg drum) 135, 210, 220, 230 and 275 °C Giacalone et al.5; Münchow et al.4
First crack, defect study (1 kg drum) 5:41 min (275 °C); 8:01 and 9:03 min (210 °C); 9:10 min (220 °C); 10:20 min (230 °C); 17:58 min (135 °C) Münchow et al.4
Charge temperature, seven sensory studies 200 °C (1 kg drum); 160 °C (15 kg roaster) Münchow et al.4
First crack, same Ethiopian coffee 8:50–9:52 min (1 kg drum, 200 °C); 8:25–9:25 min (15 kg roaster, 160 °C) Münchow et al.4
Bean temperature at the start, single-bean model 20 °C, about 12% moisture (basis not stated); roasting temperature 200 °C Fadai et al.7
Convective heat-transfer coefficient in that model 20 and 16 J s⁻¹ m⁻² K⁻¹ (two model versions) Fadai et al.7
Air in and out of an empty laboratory fluid bed, highest setting calibrated Hot air 245–249 °C, exhaust 228.5–236.5 °C (three air speeds) Geiger et al.6
Laboratory fluid bed, HTST and LTLT 260 °C air for 170 s; 228 °C air for 720 s; same colour Geiger et al.6
Spouted-bed study 0.5 kg roaster; batches 200–500 g; inlet air 220–280 °C Al-Shemmeri et al.3
Single bean in a gas-fired drum Overall coefficient 60.20–102.55 W m⁻² °C⁻¹ against 15.91–17.43 W m⁻² °C⁻¹ convective (200–280 °C) Bustos-Vanegas et al.8

Heat transfer in the first minutes#

The driving force is the temperature gap. Single-bean models treat the heat flux into the bean as proportional to the difference between the roasting temperature and the bean surface temperature.7 At the charge, with the bean at room temperature, the gap is at its largest.7 In a 2024 drum study the rate of rise was highest just after the turning point.2

Inside the bean. A 2017 model started a bean at 20 °C and about 12% moisture (basis not stated) in air at 200 °C. In the model adapted from earlier work (Model 1), the main differences in temperature and moisture inside the bean occur only in the first minute, and after that they are nearly uniform.7 The authors’ new model (Model 2), whose drying front they consider more realistic, shows a much slower drying front.7 That model includes convection at the surface and conduction inside the bean, but no radiation.7 In extreme fast roasting, a textbook chapter warns, a steep gradient can over-roast the surface while the core stays under-roasted.1

Three paths, disputed shares. According to the textbook chapter, in any hot-air roaster convection, conduction and radiation act at the same time, and their shares vary between machines.1 The textbook chapter calls radiation usually negligible.1 A 2025 study held single beans in a gas-fired drum, touching neither wall nor beans; the fitted overall coefficient was far above the calculated convective one, and the authors concluded that radiation governs.8 In a laboratory fluid bed, the bean core never reached the set air temperature, which the authors attributed to “excessive heat transfer by radiation”, typical of small equipment with a large air-to-bean ratio.6 Production drums with a full bed and drawn air were not measured in either study.

Probe and bean. A 2025 spouted-bed model treats the thermocouple reading and the bean temperature as different values. It fits system parameters that are hard to measure, among them a “thermal response coefficient”.3 The depth and timing of the turning point therefore describe the probe in its machine as well as the beans.1,3

What studies show about charge temperature#

Charge temperature is one lever among several. In the 2024 titratable-acidity study, every roast started from the same preheat of 210 ± 5 °C.2 A “fast start” profile with high initial heat reached first crack at about 8 min; a “slow start” with low initial heat at 12 min.2

The defect study. Six roasts of one washed Kenyan coffee were charged at 135–275 °C on a 1 kg drum, with roasting time also varied.4,5 The highest charge temperature (275 °C) gave the earliest first crack, at 5:41 min, and the lowest (135 °C) the latest, at 17:58 min.4 In between the order broke: the “baked” roast, charged at 230 °C, reached first crack at 10:20 min, later than the normal roast charged at 210 °C (9:03 min).4

048121620min from charge to first crack135 °C · Underdeveloped17:584 210 °C · Light8:014 210 °C · Normal9:034 220 °C · Dark9:104 230 °C · Baked10:204 275 °C · Scorched5:414
Data table
min from charge to first crackSource
135 °C · Underdeveloped17:584
210 °C · Light8:014
210 °C · Normal9:034
220 °C · Dark9:104
230 °C · Baked10:204
275 °C · Scorched5:414
Time from charge to first crack for six roasts of one washed Kenyan coffee on a 1 kg drum, by air temperature at charge. The roasts were designed as defects, so other settings also differed.4,5

A cold charge did not remove the aroma. The “underdeveloped” roast was charged at 135 °C and roasted 8 min longer than the normal roast. The authors expected the stalled start to suppress typical coffee aromas. Instead, most aroma compounds ended up at levels comparable to the normal roast.5 Its colour was almost the same as the normal roast’s (Agtron 74.9 against 74.4).5

Held constant elsewhere. In the other seven studies of the same research group, the start temperature was kept the same within each study, so that timing could be studied.4 Their finding that colour predicts flavour better than timing therefore says nothing about charge temperature itself.4 In a multivariate study on a professional roaster, initial drum temperature was one of five variables tested; time after first crack had the largest influence.10

Fast and slow starts in research roasters. Laboratory fluid-bed studies compare high-temperature short-time (HTST) and low-temperature long-time (LTLT) roasting by setting the hot air. In one, 260 °C air for 170 s and 228 °C air for 720 s gave the same colour.6 A doctoral thesis on hot-air roasting called the temperature profile the most crucial parameter in process design.9 These studies set an air temperature for the whole roast; they do not isolate the charge.

Safety at start-up#

Roasting air carries chaff (silverskin), dust and bean fragments out of the drum; a cyclone separates them.1 In two health hazard evaluations at coffee roasting facilities, the National Institute for Occupational Safety and Health (NIOSH) described chaff build-up in roaster exhausts. At one site, an afterburner stack was smoking because of chaff residue.12 At another, NIOSH recommended exhaust cleaning “to remove chaff build up to reduce a fire hazard”.11 Neither report addresses preheating or start-up itself.11,12 Events late in the roast are covered under second crack.

Risks and defects#

  • Too cold a start. In one defect study, a roast charged at 135 °C ran 8 min longer than the normal roast; the authors describe a stalling of the curve at the start.5
  • Too hot a start. A charge at 275 °C gave the shortest roast (7:40 min) with a major change in aroma composition; trained assessors placed it with the dark roast, associated with intensity, bitterness and burnt notes.5 Practitioner names for surface marks from too much heat (scorching, tipping) are collected in the roast defects entry.
  • An unsettled machine. According to a textbook chapter, with air-temperature control the beans’ temperature path can vary from batch to batch with a cold roaster.1
  • Reading too much into the turning point. Its value depends on probe position and machine design.1

Open questions#

  • First batch and between-batch routines. Practitioners describe both13; no peer-reviewed measurement was found.
  • Charge temperature and flavour in isolation. In the sensory studies found, charge temperature changed together with other settings or was held constant.4,5
  • Batch size in a drum. The batch-size effect on early profiles was shown in a spouted bed3; no drum study opened for this entry measured it with heat.
  • Heat stored in the drum. No study opened for this entry measured how much heat a preheated drum wall gives to the first minutes of a roast.
  • Radiation at the start. Its share in a loaded production drum has not been measured in the sources found.1,8

Key studies#

Anokye-Bempah et al., 2024 – Same preheat, different starts. Roasted coffees on a 5 kg gas drum preheated to 210 ± 5 °C for at least 30 min before every batch, and sampled every minute. With the same preheat, high or low initial heat moved first crack from about 8 to 12 min.

Scientific Reports 14, 8237 · doi:10.1038/s41598-024-57256-y

Al-Shemmeri et al., 2025 – What the thermocouple sees. Models heat and mass transfer in a 0.5 kg spouted-bed roaster from logged inlet, outlet and product temperatures. It treats thermocouple reading and bean temperature as different, fits system parameters such as a thermal response coefficient, and shows batch size and airflow shaping early profiles more than inlet temperature.

Beverages 11(6), 162 · doi:10.3390/beverages11060162

Giacalone et al., 2019 – Defects made at the charge. Six roasts of one Kenyan coffee on a 1 kg drum, charged at 135–275 °C and roasted for different times, were analysed for aroma and tested with trained assessors and consumers. The cold-charged roast ended up with aroma close to the normal roast.

Food Quality and Preference 71, 463–474 · doi:10.1016/j.foodqual.2018.03.009

Fadai et al., 2017 – Heat and moisture inside one bean. Two mathematical models of a single bean starting at 20 °C in 200 °C air. In the model adapted from earlier work, gradients matter mainly in the first minute; the authors' new model, which they consider more realistic, shows a much slower drying front.

International Journal of Heat and Mass Transfer 104, 787–799 · doi:10.1016/j.ijheatmasstransfer.2016.08.083

Contested
  • Which heat path dominates at the start in a drum: a roasting textbook chapter says a substantial share may pass by conduction in a drum and calls radiation in hot-air roasters "usually very limited and negligible"1; a 2025 study of single beans held in a gas-fired drum, out of contact with wall and beans, concluded that radiation governs8; and a laboratory fluid-bed study attributes the bean core staying below the set air temperature to radiation.6
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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. AAnokye-Bempah, L.; Styczynski, T.; de Andrade Teixeira Fernandes, N.; Gervay-Hague, J.; Ristenpart, W. D.; Donis-González, I. R. (2024). The effect of roast profiles on the dynamics of titratable acidity during coffee roasting. Scientific Reports 14, 8237. doi:10.1038/s41598-024-57256-y · other copies
  3. AAl-Shemmeri, M.; Fryer, P. J.; Farr, R.; Lopez-Quiroga, E. (2025). Batch-Scale Simulation of Heat and Mass Transfer of Coffee Roasting in Spouted Bed Roasters. Beverages 11(6), 162. doi:10.3390/beverages11060162 · other copies
  4. 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
  5. 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
  6. AGeiger, R.; Perren, R.; Kuenzli, R.; Escher, F. (2005). Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans. Journal of Food Science 70(2), E124–E130. doi:10.1111/j.1365-2621.2005.tb07084.x · other copies
  7. AFadai, N. T.; Melrose, J.; Please, C. P.; Schulman, A.; Van Gorder, R. A. (2017). A heat and mass transfer study of coffee bean roasting. International Journal of Heat and Mass Transfer 104, 787–799. doi:10.1016/j.ijheatmasstransfer.2016.08.083 · other copies
  8. 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
  9. BSchenker, S. (2000). Investigations on the Hot Air Roasting of Coffee Beans (Diss. ETH No. 13620). ETH Zürich (doctoral thesis).
  10. ACatão, A. A.; Mateus, N. de S.; Garcia, C. da C.; da Silva, M. C.; Novaes, F. J. M.; Alves, S.; Marriott, P. J.; da Silva, A. I. (2022). Coffee-roasting variables associated with volatile organic profiles and sensory evaluation using multivariate analysis. Applied Food Research 2(2), 100223. doi:10.1016/j.afres.2022.100223 · other copies
  11. ANational Institute for Occupational Safety and Health (NIOSH) (2017). Evaluation of exposures and respiratory health at a coffee roasting and packaging facility (Health Hazard Evaluation Report 2015-0163-3298). US Centers for Disease Control and Prevention, NIOSH.
  12. ANational Institute for Occupational Safety and Health (NIOSH) (2017). Evaluation of exposures and respiratory health at a coffee roasting and packaging facility (Health Hazard Evaluation Report 2015-0082-3287). US Centers for Disease Control and Prevention, NIOSH.
  13. CCropster (2021). Between batch protocol for coffee roasters – the whats, whys and hows from experts. Cropster (roasting software company, blog).
  14. CMelheim, K. (2023). Balancing Efficiency and Quality: Identifying the Optimal Batch Size for Your Roast. Roast Magazine (issue 119).

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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