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Roasting · Step 2

Drying phase

The first stage of a roast, from charge to the visible yellowing of the beans – water heats up and evaporates, the bean starts to swell, and much of the heat goes into warming and evaporating that water.

Across the chain: Science

Source-checkedby Coffearium verification pass· Last source-checked 17 Oct 2026· Prepared with AI assistance· 20 sources (13 tier A)· 4 contested· Updated 17 Oct 2026
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Contents
  • In roasting practice and in one drum study, the drying phase runs from charge until the beans visibly change colour, a point roasters call yellowing.6,19
  • The early roast is mainly endothermic because the water in the bean must be heated, which demands a high energy input.7
  • Water does not stop leaving at yellowing; in a fluid-bed study, water formed by reactions made up 36–41% of all water evaporated.2
  • One single-bean model predicts a drying front moving inward from the surface, with a ridge of high water-vapour pressure near it.3
  • No study opened for this entry tested drying-phase length alone for flavour; at constant colour, time to first crack mattered less than development time.7

What the drying phase is#

The drying phase is the first stage of a roast. It begins at the charge, when green coffee enters the preheated roaster.1,6 In roasting practice it ends when the beans begin to change colour, usually described as “yellowing”.19 A 2024 drum study used the same marker and called the phase “pre-colour change”.6

Researchers divide the roast into drying, flavour and colour development, and cooling.3 One modelling study calls drying generally the longest phase of the roast.3 In the 2024 study, it lasted 4.5–8.5 min of 16 min roasts, depending on the profile.6

Previous step: preheating and charge. Next step: browning phase. It is step 2 of 6 in Coffearium’s roast steps; the curve as a whole is explained under roast profiles.

Heat in, water out#

Green coffee still holds water when it enters the roaster. Studies cited here used green coffee at 8.3% and 10.5% moisture (wet basis, wb), and at 10% (basis not stated).2,6,8 Moisture content and water activity are explained in their own entry.

Heat reaches the bean from hot air (convection) and from hot surfaces such as the drum wall (conduction).1 At first, much of that heat goes into warming the water in the bean. The early roast is therefore mainly endothermic: it takes in a lot of energy.7 Evaporation also cools the bean, and a 2025 single-bean model places the start of that cooling at 100 °C.4

Inside the bean, water turns to vapour as the temperature rises. According to a Specialty Coffee Association (SCA) handbook, the vapour builds up pressure inside the dense green coffee.12 A 2017 model predicts a drying front: a boundary between a drying outer layer and a moist core, moving inward.3 Near this front, the model shows a ridge of high water-vapour pressure.3

A · Early in the roastB · Later125346789

A · Early in the roast

  1. Hot air. Convection carries heat from the air to the bean surface.1
  2. Drum wall. Conduction carries heat from hot walls to the beans; in drum roasters this share can be substantial.1
  3. Moist core. The inner bean still holds most of its water.3
  4. Dry outer layer. The layer outside the front has already lost much of its water.3
  5. Drying front. Evaporation is concentrated here, with a ridge of high vapour pressure nearby, in one model.3
  6. Water vapour. Vapour leaves through the dry layer; its pressure starts the expansion of the bean.4,12

B · Later

  1. Smaller moist core. The front has moved towards the centre.3
  2. Drying front, further in. In the model the authors consider more realistic, this movement is slow.3
  3. Reaction water. Chemical reactions form new water, which also evaporates; in one study it became measurable at a bean core temperature of 180–200 °C.2
Heat and water in a bean cut across, early and later in the drying phase. Dot density stands for moisture, with no measured values behind it. Schematic, not to scale.

As the water leaves, the bean changes. Vapour release begins the expansion of the inner tissue.4 Above 100 °C, browning reactions start.9 The colour moves from greenish grey-blue towards yellow.1

When drying ends#

Yellowing is a visual marker, not a physical boundary. A practitioner course notes that it has no clear definition and is somewhat subjective.19 The same course says beans release moisture steadily until shortly before first crack.19 Measurements agree with that second point.

In a laboratory fluid-bed study, beans lost water throughout the roast, not only at the start.2 The green coffee’s own water left first. In the slow roast, its evaporation had become almost negligible after about 300 s of 720 s.2 After that, most of the water leaving came from chemical reactions.2 Over the whole roast, reaction water made up 41% of the water evaporated in a fast roast and 36% in a slow one.2

Scientists also define the end of drying in different ways. A chemistry chapter describes a drying phase up to about 100 °C.11 A single-bean model defines it as the bean being below 160 °C.5 The chart shows these and other early-roast events on one temperature scale.

020406080100120140160180200220°C as reported (measuring point differs)Evaporative cooling starts(single-bean model)1004 Browning begins (above)1009 Gas release, swellingbegin(around)1509 Reaction heat assumed(above, one model)1504 Drying stage ends(one model)1605 Reaction water and CO₂start(bean core)180–2002 Bean cracking reported180–2009
Data table
°C as reported (measuring point differs)Source
Reaction water and CO₂ start (bean core)180–2002
Bean cracking reported180–2009
Evaporative cooling starts (single-bean model)1004
Browning begins (above)1009
Gas release, swelling begin (around)1509
Reaction heat assumed (above, one model)1504
Drying stage ends (one model)1605
Temperatures reported for early-roast events, from models,4,5 a fluid-bed study2 and a colour study.9 The sources do not all say where they measured, so values are not strictly comparable.

A roaster’s bean probe shows yet another number. According to a roasting textbook chapter, it reads a mix of bean surface and hot air, specific to each machine.1 The chapter therefore calls comparing such temperatures between roasting systems problematic.1

What shapes the drying phase#

The heat applied. According to a roasting textbook chapter, faster heating makes dehydration proceed faster.1 In the fluid-bed study, the peak evaporation rate was higher in the fast roast, while more water evaporated in total in the slow roast.2 The same chapter says the profile decides how much water leaves in the first stages.1

Airflow. More air per kilogram of coffee raises the air speed at the bean surface. According to the same chapter, this speeds up the evaporation of water.1 An airflow change acts on convection almost at once, whereas a flame change takes longer.1

Drum speed. The chapter describes faster drum speeds as tossing the beans more, so they spend less time on the hot wall.1 Too fast a drum presses beans against the wall and can scorch their flat sides.1 No study opened for this entry measured drum speed and drying directly.

Batch size. A 2025 study on a small spouted-bed roaster varied batches from 200 to 500 g and the airflow.13 This gave more varied time–temperature curves than changing the inlet air temperature, especially early in the roast.13

The green coffee. Wetter beans lose more water in the first phase and end at a similar final moisture, according to the textbook chapter.1 Practitioners also adjust the early roast for bean density; this is roasting practice, not a tested finding (see Deep dive).20

Reference values#

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

Parameter Value reported Source
Green coffee moisture before roasting 10% (basis not stated); 10.5 ± 0.5% (wb); 8.3 g/100 g (wb); 10–11 g/100 g (wb) Alstrup et al.8, Anokye-Bempah et al.6, Geiger et al.2, Schenker15
Green coffee density, one study 880 g/L (10% moisture, basis not stated; screen 17; washed Colombian arabica) Alstrup et al.8
Moisture after roasting 1–2.5% (basis not stated); 2–4% (single beans, 200–280 °C; basis not stated); 2.68 (fast, 260 °C) and 2.15 g/100 g (slow, 220 °C), wb Münchow et al.7, Bustos-Vanegas et al.4, Schenker15
Moisture nearly constant until About 100 °C (single beans in a gas-fired drum, 45 rpm) Bustos-Vanegas et al.4
Evaporative cooling Starts at 100 °C and decreases to the end of the process (model) Bustos-Vanegas et al.4
Drying stage, one model’s definition Bean below 160 °C Alonso-Torres et al.5
Bean core vs surface, single bean in hot air (230 °C, 2 m/s), one simulation at 180 s Difference of 16 K Alonso-Torres et al.5
Water evaporated during roasting, fluid bed (100 g batch, 8.3 g water in) 10.2 g (fast, 170 s, 260 °C air); 11.4 g (slow, 720 s, 228 °C air) per 100 g green coffee Geiger et al.2
Share of evaporated water formed by reactions 41% (fast); 36% (slow) Geiger et al.2
Green coffee’s own water almost gone After about 300 s (slow roast of 720 s) Geiger et al.2
Reaction water and CO₂ start Bean core 180–200 °C Geiger et al.2
Browning begins; gas release and swelling begin; bean cracking Above 100 °C; around 150 °C; 180–200 °C Bicho et al.9
Exothermic reaction heat included in one single-bean model Above 150 °C Bustos-Vanegas et al.4
Mass-loss rate, 350 g gas drum Constant for each setting; 0.6–3.0% per min; roast time 24 to 9 min to 16% mass loss Vargas-Elías et al.14
Water content at which most aroma compounds form fastest 7% to 2% (wb) Schenker & Rothgeb1, Schenker et al.16
Glass transition of stored green coffee (calorimetry), by water activity Tg 48.76 °C at aw 0.155; 34.89 °C at aw 0.512 Iaccheri et al.10
Drying (pre-colour-change) phase, 5 kg drum, 4 kg batch, 16 min roasts 4.5–8.5 min across seven profiles Anokye-Bempah et al.6
Time to first crack, seven sensory studies 7:21–10:07 min Münchow et al.7
“Underdeveloped” defect roast (stalled, slow start) Air at charge 135 °C (normal 210 °C); 20:20 min total (normal 11:25); development 2:30 min (normal 2:40) Giacalone et al.17
Latent heat of evaporation used in one model 2.3 × 10⁶ J/kg Fadai et al.3

Water inside the bean#

Drying front. A 2017 study modelled one bean as a 4 mm sphere roasted at 200 °C, with an initial moisture of 12%.3 Its new first-principles model (Model 2) shows a drying front that moves to the centre, with a ridge of high water-vapour pressure near it.3 A model adapted from earlier work (Model 1) predicts temperature and moisture nearly uniform after the first minute. The authors judge their new model’s drying front more realistic, because evaporation is a local process; in its simplified form, the front is much slower than in Model 1.3 The model was fitted to published moisture data; the authors name the lack of physical data as its main drawback.3

Gradients. In a 2013 computational model of a single bean in hot air, validated by experiment, the core was 16 K cooler than the surface after 180 s in one simulation with air at 230 °C.5 That model assumes water diffuses to the surface and evaporates only there.5 The two models therefore place evaporation differently: at an internal front or at the surface. Neither has been checked by measuring moisture inside a roasting bean.

Pressure and expansion. In the 2017 model, water evaporating inside the cells produces high gas pressure, which makes the cells expand.3 A 2025 study describes the same: vapour release during drying starts the expansion of the inner matrix.4 An SCA handbook adds that this vapour pressure is only released once the temperature is high enough.12 Later, carbon dioxide from reactions adds to the gas pressure.1,12

Energy. Evaporating water absorbs heat: one model uses a latent heat of 2.3 × 10⁶ J/kg.3 In a 2025 lumped single-bean model, leaving out evaporation made the model overestimate bean temperature during the exothermic reaction period, and evaporation had a greater effect on the estimate than heat released by reactions.4 This is why the early roast needs a high energy input.7

0246810121416min from chargeFast start5.0 min6 Medium5.5 min6 Production5.0 min6 Exaggerated flick5.0 min6 Negative rate of rise4.5 min6 Extended Maillard4.5 min6 Slow start8.5 min6
Data table
min from chargeSource
Fast start5.0 min6
Medium5.5 min6
Production5.0 min6
Exaggerated flick5.0 min6
Negative rate of rise4.5 min6
Extended Maillard4.5 min6
Slow start8.5 min6
Length of the drying phase, defined as charge to visible colour change, in seven profiles of one washed African coffee roasted for 16 min (5 kg drum, 4 kg batch, green coffee at 10.5% wb).6

How much water leaves, and when#

Two kinds of water. The water evaporated in a roast is partly the green coffee’s own and partly water formed by reactions.1,2 A 2005 fluid-bed study separated them by roasting pre-dried beans (1.1 g water per 100 g, wb) next to normal ones (8.3 g/100 g, wb).2 Early in both roasts, only the beans’ own water was released.2 The reactions that release water and carbon dioxide in relevant amounts started at a bean core temperature of 180–200 °C.2 The authors note that most of the 1.1 g of residual water in the pre-dried beans had to leave before reaction water could be detected.2

Shape of the curve. In that study, moisture fell almost linearly in the fast roast and levelled off only at the end; in the slow roast, the curves were exponential.2 A roasting textbook chapter places the highest rate of roast loss in the early stages, mainly from dehydration.1 In a 350 g gas drum, however, mass was lost at a constant rate for each roasting setting, from 0.6 to 3.0% per min.14 In single beans, moisture stayed nearly constant until about 100 °C and then decayed exponentially.4 The shape depends on the roaster and the heat applied, so no single curve describes “drying”.

What the water does chemically. According to a roasting textbook chapter, the speed of key flavour reactions depends on water availability, and some slow down below a critical moisture.1 According to the same chapter, the first roasting stage does not produce large quantities of aroma but may form precursors; most aroma compounds formed fastest at water contents of 7% to 2% (wb).1,16 Keeping colour and time constant, a different early profile can evaporate more or less water and create new combinations of temperature and water activity.1 Loss of water also contributes to the brittle texture of the roasted bean.18

Glass transition and swelling#

A glass transition is the change of an amorphous material from rigid (“glassy”) to soft (“rubbery”). In green coffee, more water lowered the temperature at which it happens: the transition temperature fell from 48.76 °C to 34.89 °C as water activity rose from 0.155 to 0.512.10 These measurements are of stored coffee, not of beans during a roast.

For the roast itself, the evidence is a textbook account. The cell-wall polysaccharides are described as glassy or rubbery depending on moisture and temperature; in principle the bean passes from glassy to rubbery and back to glassy during roasting.1 The bean expands while rubbery, so the dehydration kinetics – and thus the profile – help decide how much it swells.1 An SCA handbook places the glass transition around first crack instead.12 No measurement of the transition inside a roasting bean was found (see Contested).

Green coffee moisture and density#

Moisture. A doctoral thesis on hot-air roasting adjusted green coffee to between 3.2 and 18.2 g/100 g (wb) to study its effect.15 According to a roasting textbook chapter, wetter beans lose more water in the first phase and reach a similar final moisture, so roast loss is higher.1 Initial moisture is described as influencing aroma mainly in light roasts; in dark roasts most differences level off.11 The authors of a 2025 single-bean model recommend a low initial moisture to make the roast easier to control.4 Roast loss as a guide to roast degree is affected too, because green moisture varies from batch to batch.1

Density. The textbook chapter lists green density, moisture, screen size and shape among factors that shape the roast plan.1 In a trade magazine, practitioners describe giving denser, high-grown coffees a higher charge temperature and more aggressive early heat; one expects denser seeds to hold more water.20 No peer-reviewed study opened for this entry tested how density changes the drying phase, so these remain practitioner views.

Drying-phase length: practice and evidence#

What roasters do. According to a roasting textbook chapter, roasters log the time and probe temperature as the colour changes from green to yellow.1 Courses treat the phase from charge to yellowing as a named stage of the profile.19 No peer-reviewed study opened for this entry set a target length or share of the roast for this phase, so none is given here.

What studies measured. In a 2024 study on a 5 kg drum, the drying phase lasted 4.5–5.5 min in six of seven profiles and 8.5 min in the slow-start profile.6 That study measured titratable acidity, not flavour.6

What sensory studies found. No study opened for this entry varied drying alone and tested flavour. The closest evidence splits the roast at first crack:

  • Time to first crack. Across seven sensory studies at constant colour, time to first crack – drying and browning together – significantly affected five of nine attributes; development time affected all nine.7 Longer time to first crack went with less acidity, more bitterness and a less clean cup.7
  • Total time. In the one study that also varied total roast time, total time alone was not correlated with any flavour attribute.7
  • A slow, cool start. In a 2019 defect study, the roast called “Underdeveloped” was charged into air at 135 °C instead of 210 °C. It lasted 20:20 instead of 11:25 min, with nearly the same development time (2:30 against 2:40 min).17 The sensory profile did not separate this roast well from the normal roast, though it scored significantly lower in two attributes, caramel and dark berries.17

The studies opened for this entry therefore give no basis for any particular drying time for flavour. Roast colour remained the stronger predictor of flavour than timing.7

Risks and defects#

  • Scorching. According to a roasting textbook chapter, too high a drum speed can press beans against the hot wall and scorch the flat sides.1
  • Steep gradients. The same chapter describes a possible problem in extremely fast roasting: high heat transfer at the surface may leave the outside over-roasted while the core stays under-roasted, which could give “burnt” and “greenish” notes together.1
  • Batch-to-batch drift. With fixed air-temperature control, green-coffee moisture, weather and a cold roaster all change how the beans heat.1 Varying green moisture also makes roast loss a less reliable guide to roast degree.1
  • Stalling. A practitioner quoted in a trade magazine warns that the extra water in denser coffee can easily create a stall as the roast approaches first crack.20 No peer-reviewed study of this was found.

Open questions#

  • Measuring moisture inside the bean. Models disagree on where evaporation happens, and the main model names the lack of physical data as its weakness.3,5
  • Glass transition during roasting. Measured only on stored beans; its timing in a roast rests on textbook accounts that differ.1,10,12
  • Drying length and flavour. No sensory study separated the drying phase from the browning phase.7
  • Density. Its effect on drying is described by practitioners but was not tested in the studies found.20
  • What yellowing marks. Studies use the visible colour change as a boundary6, but no study opened for this entry linked it to a moisture content or a temperature inside the bean.

Key studies#

Geiger et al., 2005 – Water and carbon dioxide during roasting. Measures water vapour and CO₂ in the exhaust of a laboratory fluid bed for a fast and a slow roast to the same colour, and roasts pre-dried beans to separate the green coffee’s own water from reaction water. The beans' own water left first; reaction water and CO₂ started at a bean core temperature of 180–200 °C and made up 36–41% of all water evaporated.

Journal of Food Science 70(2), E124–E130 · doi:10.1111/j.1365-2621.2005.tb07084.x

Fadai et al., 2017 – Heat and mass transfer in one bean. Builds a new first-principles model of a roasting bean with liquid water and water vapour in its pores and compares it with a model adapted from earlier work. The new model, whose drying front the authors consider more realistic, shows a slow drying front moving to the centre, with a ridge of high vapour pressure near it.

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

Bustos-Vanegas et al., 2025 – Evaporation and heat generation. Models the temperature of single beans roasted at 200–280 °C in a gas-fired drum. Moisture stayed nearly constant until about 100 °C; evaporation had a greater effect on the modelled bean temperature than heat from reactions.

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

Anokye-Bempah et al., 2024 – Roast phases in a 5 kg drum. Roasts three coffees with up to seven profiles and times each phase, including "pre-colour change", the drying phase. Its length ranged from 4.5 to 8.5 min; the study followed titratable acidity, not flavour.

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

Münchow et al., 2020 – Colour versus timing. Pools eight descriptive sensory studies of washed arabica. At constant colour, time to first crack – which includes drying – affected flavour less than development time, and colour was the strongest predictor overall.

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

Contested
  • Where drying ends: a chemistry chapter describes a drying phase up to about 100 °C11; a single-bean model defines the drying stage as the bean below 160 °C5; practitioners and one drum study use the visible colour change6,19; and a fluid-bed study measured the green coffee's own water still leaving until about 300 s of a 720 s roast, with reaction water after that.2
  • Drying front or even moisture: in a 2017 heat and mass transfer study, a model adapted from earlier work (Model 1) predicts temperature and moisture nearly uniform inside the bean after the first minute, while the authors' new first-principles model (Model 2), whose drying front they consider more realistic, shows a much slower front in its simplified form3; a 2013 single-bean model assumes water diffuses to the surface and evaporates only there.5
  • When the bean softens: a roasting textbook chapter says the cell walls pass from glassy to rubbery and back to glassy during the roast, depending on moisture and temperature1; an SCA handbook places the glass transition around first crack.12 Neither is a measurement inside a roasting bean, and no such measurement was found.
  • Shape of the moisture-loss curve: in a fluid bed, moisture fell almost linearly in a fast roast and exponentially in a slow one2; a textbook chapter puts the highest rate of roast loss in the early stages1; in a 350 g drum, mass was lost at a constant rate for each setting.14
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Sources

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  3. 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
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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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