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Roasting · Science

Porosity and cell structure

How roasting turns the dense, thick-walled cells of green coffee into a swollen, porous structure – and why those pores decide how a bean holds gas, sweats oil, grinds and extracts.

Across the chain: Science

Source-checkedby Coffearium verification pass· Last source-checked 20 Oct 2026· Prepared with AI assistance· 14 sources (9 tier A)· 2 contested· Updated 20 Oct 2026
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Contents
  • Green coffee is a compact tissue of cells with unusually thick polysaccharide walls and no spaces between them; the cells measure about 25–50 µm.1,4,6
  • Roasting swells the bean to about 1.5–2 times its green volume; each cell empties into a large gas-filled void while its walls stretch.1,8,9
  • A 2018 degassing study, citing earlier work, gives a porosity of about 0.5 for dark roasts and a bean density roughly half the green value.2
  • At the same roast degree, fast high-temperature roasting gave greater bean volume, more pore volume and larger micropores in the cell walls than slow roasting.3
  • The pore structure affects how gas and oil leave the bean and how brittle it is in the grinder, and may affect how easily water reaches the solubles.1,5,13

What porosity means in a coffee bean#

Porosity is the share of a material’s volume taken up by empty space. A green coffee seed has almost none: it is a dense, hard tissue.1,11 A roasted bean is full of holes.

Roasting changes the dense green structure into a very porous one.1 A 2018 study, citing earlier work, gives a porosity of about 0.5 for dark roasts. That means about half of the bean’s volume is pore space.2

The pores matter after the roast. They hold the carbon dioxide formed in the roast and let oil and gas move out of the bean.1,5 Together with brittleness, they make the bean easy to grind.13

The green seed: a dense block of cells#

The part of the seed that is roasted is the endosperm, a storage tissue. It is built of cells, each enclosed by a wall.4 The cells are about 25–50 µm across.6

The cell walls of coffee are unusually thick compared with those of other plants.1 They are made mainly of polysaccharides – long chains of sugars – and carry reinforcement rings.1 There are no spaces between the cells.4,11 This is why green coffee is so hard.11

Inside each cell is the cytoplasm. According to a 2000 thesis review, it holds mainly lipids, proteins, carbohydrates and caffeine.4 The oil sits in tiny oil bodies of about 0.2–2.5 µm.12 A roasting textbook chapter places them along the cell walls.1 Fine channels called plasmodesmata cross the walls in places and connect neighbouring cells.4

What roasting does to the cells#

During roasting, water turns to steam and chemical reactions form carbon dioxide inside the cells. The gas pressure pushes the cells to expand.1,14 The thick walls hold against it.1 The roast gases and CO₂ entry covers where the gas comes from.

The walls are not always stiff. Depending on moisture and temperature, their polysaccharides are glassy (rigid) or rubbery (soft).1 According to the textbook chapter, the bean swells while the walls are rubbery.1 The glass–rubber transition entry explains this change of state.

Inside, the cells are hollowed out. The shrinking cytoplasm is pushed against the walls, and a large gas-filled void opens in the centre of each cell.1 A thin layer of cytoplasm with small bubbles stays along the walls.1 The walls themselves become porous, with pores far too small to see in images.5

A · Green coffee1234B · Roasted coffee56798

A · Green coffee

  1. Thick cell walls. Polysaccharide walls, unusually thick, with no spaces between the cells.1,4,11
  2. Cytoplasm. Fills the cell; mainly lipids, proteins, carbohydrates and caffeine.4
  3. Oil bodies. About 0.2–2.5 µm, along the cell walls.1,12
  4. Plasmodesmata. Fine channels that cross the walls in places.4

B · Roasted coffee

  1. Stretched walls. The framework of walls stays largely intact; the walls carry micropores.1,5
  2. Central void. A large gas-filled space in each cell – the macropore.1,5
  3. Cytoplasm layer. What is left of the cytoplasm lines the walls, with small voids in it.1
  4. Oil at the surface. Gas pressure pushes freed oil through the walls to the bean surface.1
  5. Crack. Cracks form during roasting; one imaging study found them mainly in the outer regions.7
Green and roasted coffee tissue as the sources describe it. Roasted cells are drawn larger because the bean swells. Schematic, not to scale.1,4,5

How much the bean swells#

As the cells inflate, the whole bean grows. A 2020 paper gives an increase of about 50% to almost 100% in volume.9 In a 2018 drum study, beans reached up to 1.8 times their green volume.8

The bean also loses mass, mostly as water and gas. So its density falls. A 2018 study, citing earlier work, gives about 1.3 g/cm³ for green and about 0.7 g/cm³ for dark-roasted beans.2

11.21.41.61.82× green volumeRange given in a 2020multi-study paper+50% to almost +100%9 Dark roast(2018 degassing study, citingearlier work)up to +80%2 Arabica beans in a drum at200–280 °C(2018)up to 1.8×8
Data table
Point× green volumeSource
Range given in a 2020 multi-study paper+50% to almost +100%1.5–29
Dark roast (2018 degassing study, citing earlier work)up to +80%1.82
Arabica beans in a drum at 200–280 °C (2018)up to 1.8×1.88
Bean volume after roasting relative to green coffee, as reported by three studies. Upper ends are as the sources state them, not measured on the same coffee.2,8,9

According to the textbook chapter, the darker the roast, the more volume and porosity are created, but swelling levels off at some point.1 The bean size and density entry covers green and roasted density in more detail.

Why the pores matter after the roast#

Gas. The pores hold much of the carbon dioxide formed in the roast.1,5 It sits in the large voids and in the porous cell walls, and it leaves slowly during storage.5 The degassing entry describes how.

Oil. Roasting breaks up the oil bodies.1 Gas pressure then pushes the oil through tiny channels in the walls to the surface, faster in dark roasts.1 The coffee oil entry covers this.

Grinding. According to a grinding chapter, darker and faster roasts are more porous and brittle. They grind easily but give many fines.13 In one study, the smallest particles were about the size of a cell: fragments of cells.6

Extraction. According to the textbook chapter, fast-roasted beans generally give more soluble matter on extraction. This may be because more solubles form, because water reaches them more easily in the porous structure, or both.1

Reference values#

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

Parameter Value reported Conditions Source
Cell size 25–50 µm Ground arabica blend; size of the fines peak, read as cell fragments Mo et al.6
Oil bodies 0.2–2.5 µm (typical) Endosperm, five Coffea species, microscopy Crisafulli et al.12
Volume increase About +50% to almost +100% Range given in the introduction of a multi-study paper Münchow et al.9
Volume increase Up to 1.8 × green volume Arabica, beans suspended in the centre of a drum at 200–280 °C, measured every 20 s Bustos-Vanegas et al.8
Volume increase Up to +80% Dark roast (background statement) Smrke et al.2
Density ≈1.3 (green) to ≈0.7 g/cm³ (dark roast) Background values cited from earlier work; printed as kg/m³ in the paper, g/cm³ is our reading (1.3 kg/m³ would be about the density of air) Smrke et al.2
Porosity ≈0.5 Dark roast (background statement) Smrke et al.2
Expansion shape Isotropic (equal in all directions) Drum temperatures above 220 °C Bustos-Vanegas et al.8
Whole-bean degassing time scale (Weibull λ) 153–377 h Nine profiles: three roast degrees × three speeds, 35 °C Smrke et al.2

No study opened for this entry gave cell wall thickness in micrometres or a full series of porosity values against roast degree. The micropore sizes measured by mercury porosimetry are described in an SCA handbook and a doctoral thesis as on the nanometre scale.4,5

Pressure against the cell walls#

Driving force. Water vapour and carbon dioxide form inside the bean faster than they can leave, so pressure builds up; the gas and vapour together drive expansion.1,14 A doctoral thesis describes green coffee as a collection of small reactor units whose thick walls withstand a large pressure build-up.11 Estimates of the internal pressure are discussed in the roast gases and CO₂ entry.

Resistance. The thick walls hold against the pressure.1 According to the textbook chapter, their polysaccharides change gradually between glassy and rubbery states with moisture and temperature; in principle the bean passes from glassy to rubbery and back to glassy, and it swells while rubbery.1 The extent of swelling is therefore the result of gas formation against wall resistance, and the roast profile decides it.1 An SCA handbook places the glass transition around first crack.5

Continuous, not explosive. Unlike popcorn, coffee beans swell steadily rather than in a sudden burst.1 In arabica beans roasted in a drum, expansion was isotropic at drum temperatures above 220 °C, and density fell linearly with moisture content, more steeply at higher temperature.8

Walls intact, tissue cracked. A roasting chapter states that the framework of cell walls remains intact while the cells are excavated.1 A 2008 thesis notes that no cell wall disruption was seen in scanning electron microscopy and attributes this to the rubbery state of the walls at high temperature.11 Cracks do form in the tissue: in microtomography, mainly in the outer regions of the bean.7 First crack and second crack describe the audible events.

Macropores and micropores#

Macropores. The large voids at the centres of the excavated cells are called macropores.4,5 In a cryo-electron micrograph of roasted tissue, a large gas-filled space occupies the centre of each cell, and smaller voids sit in the layer of cytoplasm along the walls.1 The layer thins as roasting continues, because more cell mass turns into gas and water vapour and the cells grow.1

Micropores. The cell walls of roasted coffee are porous too. These micropores are too small to be imaged and are measured indirectly by mercury porosimetry.5 A 2000 thesis review cites microscopy that found no pre-existing channels in the walls of green beans, and states that little was known about how micropores form during roasting.4 Fine micropores are assumed to carry the oil to the surface.4

What becomes of the plasmodesmata. The same thesis states in its literature review that their fate during roasting was unknown. In its own electron micrographs, plasmodesmata channels were still present in some spots of roasted cell walls. Whether they stay open for mass transfer was unclear, and the author did not consider them a key route.4

Cell size. In ground coffee, a 2023 microtomography study found the fines peak at the size of a cell for every grind, and read the fines as cell fragments.6

Fast and slow roasts#

Roast speed changes the structure as well as the flavour.

  • Volume and pores. In a study of two hot-air processes roasted to the same degree, high-temperature roasting gave greater bean volume, greater pore volume and larger micropores in the cell walls than low-temperature roasting.3
  • Density. According to the textbook chapter, fast-roasted beans have much greater volume and porosity and lower density than slow-roasted beans of the same colour.1 A 2008 thesis found the same direction in its laboratory roasts.11
  • Homogeneity. A grinding chapter describes slowly roasted beans as having a more homogeneous matrix and pore sizes and a higher density.13
  • Profile shape. At identical final colour and roasting time, a slow-start, fast-finish profile usually gave more bean volume and porosity than a traditional profile, according to the textbook chapter.1
  • Oil. Fast-roasted beans show a considerably stronger tendency for oil sweating.1

The gravimetric degassing study roasted one arabica to three roast degrees at three speeds each.2 For medium and dark roasts, faster roasts degassed faster; for light roasts there was no such trend.2 The authors consider differences in porosity the probable cause.2

0100200300400h (Weibull scale parameter λ; lower = faster release)Dark · fast153 h2 Dark · medium190 h2 Dark · slow242 h2 Medium · fast190 h2 Medium · medium232 h2 Medium · slow270 h2 Light · fast310 h2 Light · medium377 h2 Light · slow326 h2
Data table
h (Weibull scale parameter λ; lower = faster release)Source
Dark · fast153 h2
Dark · medium190 h2
Dark · slow242 h2
Medium · fast190 h2
Medium · medium232 h2
Medium · slow270 h2
Light · fast310 h2
Light · medium377 h2
Light · slow326 h2
Time scale of degassing from whole beans (Weibull scale parameter λ; lower means faster release) for three roast degrees and three roast speeds of one arabica, at 35 °C. Fast 4–5 min, medium 9–10 min, slow 11–12 min to the same colour.2

Consequences for degassing, grinding and extraction#

Degassing. According to the SCA handbook, gas held in macropores leaves by pressure-driven flow, while gas in the micropores of the walls leaves by slower diffusion.5 The gravimetric study, citing earlier work, states that total pore volume increases almost linearly with roasting time.2 Its authors suggest that more of the gas in dark roasts sits in large pores that grinding destroys, which would explain the larger loss on grinding.2 A water-quench study found no correlation between pore size distribution and degassing behaviour.10 The degassing entry gives the release data.

Grinding. The grinding chapter names roast degree as the most decisive bean property for grinding.13 Porosity and brittleness rise with roast degree, especially in short roasts; such beans grind easily but give a wide particle size distribution with many fines.13 Slowly roasted, denser beans need more energy to grind but grind more evenly.13 Particle size and fines covers the consequences in the bed.

Extraction. In the textbook chapter’s account, fast-roasted beans generally yield more soluble matter, possibly through more solubles, easier access for water in the porous structure, or both.1 No study opened for this entry separated these two causes.

Staling. The SCA handbook lists porosity among the factors that influence how roasted coffee interacts with its environment during storage.5 See freshness and storage.

Measurement methods#

Method What it shows Limits Source
Mercury intrusion porosimetry Pore volume and pore size distribution, including micropores in the cell walls Shows the internal pore system, not surface porosity Schenker et al.3; Baggenstoss11
Volumetry (oil displacement) Bean volume and density Oil must not soak into the pores during weighing Schenker et al.3; Baggenstoss11
Electron microscopy (SEM, cryo-SEM) Cell walls, cytoplasm and voids; oil droplets on the surface Sample preparation can remove cytoplasm Schenker and Rothgeb1
Synchrotron X-ray microtomography Pores and cracks in 3D, pore size distribution by bean region, without cutting Not stated in the sources opened Pittia et al.7
Laboratory X-ray microtomography Macropores; packed beds of ground coffee Resolution too low for the pores inside particles in one study SCA handbook5; Mo et al.6

Different methods, different pores. Microtomography images the macropores; the micropores of the walls are only measured indirectly by porosimetry.5 A 2008 thesis notes that porosimetry gives insight into the internal pore system but no information about surface porosity.11 Porosity figures from different methods are therefore not directly comparable.

Risks and defects#

  • Over-expanded, oily beans. Very fast roasting gives high volume and porosity and a stronger tendency for oil sweating.1 A 2008 thesis notes that fast-roasted coffee is generally suspected to be more affected by lipid oxidation, because more oil migrates to the surface.11
  • Fines from brittle beans. Highly porous, brittle beans give a wide size distribution with many fines.13
  • Faster gas loss. In medium and dark roasts, faster roasts degassed faster from whole beans.2
  • Moisture after quenching. In one study, water-quenched coffees with higher moisture content degassed faster.10 See cooling and quenching.

Open questions#

  • Micropore origin. How micropores form in the cell walls, and how gas and oil move from the bean core to the outside, was not answered in the theses opened for this entry.4,11
  • Plasmodesmata. They were seen in some spots of roasted cell walls, but whether they carry gas or oil is unclear.4
  • Porosity and degassing. One study points to porosity as the probable cause of faster degassing in fast roasts; another found no correlation with pore size distribution (see Contested).2,10
  • Series of values. No study opened for this entry reported porosity across a series of roast degrees and speeds for one coffee in a form that could be charted here.

Key studies#

Schenker et al., 2000 – Pore structure and roasting conditions. Roasted beans in two hot-air processes to the same roast degree and compared them by volumetry, mercury porosimetry and electron microscopy. High-temperature roasting gave greater bean volume, greater pore volume and larger micropores in the cell walls.

Journal of Food Science 65(3), 452–457 · doi:10.1111/j.1365-2621.2000.tb16026.x

Pittia et al., 2011 – Synchrotron microtomography of beans. Imaged green and roasted beans in 3D without cutting them. Roasting created a porous structure through the whole bean, with pore shapes and sizes varying by region and cracks mainly in the outer regions.

Journal of Food Science 76(2), E222–E231 · repository record

Bustos-Vanegas et al., 2018 – Physical properties during roasting. Followed volume, surface area and density of arabica beans, suspended in the centre of a drum, every 20 s at five temperatures. Final volume reached up to 1.8 times the green volume, and expansion was isotropic above 220 °C.

Industrial Crops and Products 112, 839–845 · doi:10.1016/j.indcrop.2017.12.015

Smrke et al., 2018 – Degassing by roast degree and speed. Weighed whole beans of one arabica, roasted to three degrees at three speeds, as they degassed at 35 °C. Darker and, for medium and dark roasts, faster roasts released gas faster; the authors point to porosity as the probable cause.

Journal of Agricultural and Food Chemistry 66(21), 5293–5300 · doi:10.1021/acs.jafc.7b03310

Mo et al., 2023 – Cells in ground coffee. Measured four commercial grinds of a 100% arabica blend (one of them dark-roasted) by laser diffraction and X-ray microtomography. The fines peak sat at 25–50 µm for every grind, the size of a cell, which the authors read as cell fragments.

Scientific Reports 13, 16374 · doi:10.1038/s41598-023-42380-y

Contested
  • Whether cell walls break: a roasting textbook chapter describes the framework of cell walls as remaining intact1, and a 2008 thesis reports no evidence of cell wall disruption in electron microscopy11; a heat-transfer study describes porosity rising through both the destruction of cells and the degradation of the intercellular matrix.14
  • Pore structure and degassing: a gravimetric study attributes faster degassing of faster roasts probably to differences in porosity2; a water-quench study found that pore size distribution did not correlate with degassing behaviour.10 The studies measured different things – total degassing kinetics against mercury porosimetry of the cell-wall pores.
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Change log

  • – The fate of the plasmodesmata during roasting is no longer described as unknown; the text now reports that the 2000 thesis found them in some spots of roasted cell walls, with their role in mass transfer unclear. Reason: Verification against Schenker 2000 (Diss. ETH 13620); "unknown" is the thesis's literature review, its own results (section 4.2.3) found the channels in roasted beans.
  • – Porosity of about 0.5 and density of about 1.3 to 0.7 g/cm³ are now presented as values a 2018 study cites from earlier work, not as its own measurements. Reason: Smrke et al. 2018 give these values in the introduction with a reference to earlier work; they did not measure porosity or density.

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. ASmrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C. (2018). Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee. Journal of Agricultural and Food Chemistry 66(21), 5293–5300. doi:10.1021/acs.jafc.7b03310 · other copies
  3. ASchenker, S.; Handschin, S.; Frey, B.; Perren, R.; Escher, F. (2000). Pore structure of coffee beans affected by roasting conditions. Journal of Food Science 65(3), 452–457. doi:10.1111/j.1365-2621.2000.tb16026.x · other copies
  4. BSchenker, S. (2000). Investigations on the Hot Air Roasting of Coffee Beans (Diss. ETH No. 13620). ETH Zürich (doctoral thesis).
  5. BSmrke, S.; Sage, E.; Wellinger, M.; Yeretzian, C. (2018). The Coffee Freshness Handbook (ISBN 978-0-9995807-2-1). Specialty Coffee Association, Santa Ana.
  6. AMo, C.; Johnston, R.; Navarini, L.; Suggi Liverani, F.; Ellero, M. (2023). Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations. Scientific Reports 13, 16374. doi:10.1038/s41598-023-42380-y · other copies
  7. APittia, P.; Sacchetti, G.; Mancini, L.; Voltolini, M.; Sodini, N.; Tromba, G.; Zanini, F. (2011). Evaluation of microstructural properties of coffee beans by synchrotron x-ray microtomography: a methodological approach. Journal of Food Science 76(2), E222–E231.
  8. ABustos-Vanegas, J. D.; Corrêa, P. C.; Martins, M. A.; Baptestini, F. M.; Campos, R. C.; de Oliveira, G. H. H.; Nunes, E. H. M. (2018). Developing predictive models for determining physical properties of coffee beans during the roasting process. Industrial Crops and Products 112, 839–845. doi:10.1016/j.indcrop.2017.12.015 · other copies
  9. 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
  10. ABaggenstoss, J.; Poisson, L.; Luethi, R.; Perren, R.; Escher, F. (2007). Influence of Water Quench Cooling on Degassing and Aroma Stability of Roasted Coffee. Journal of Agricultural and Food Chemistry 55(16), 6685–6691. doi:10.1021/jf070338d · other copies
  11. BBaggenstoss, J. (2008). Coffee roasting and quenching technology – formation and stability of aroma compounds (Diss. ETH No. 17696). ETH Zürich (doctoral thesis). doi:10.3929/ethz-a-005666902 · other copies
  12. ACrisafulli, P.; Navarini, L.; Silizio, F.; Pallavicini, A.; Illy, A. (2013). Ultrastructural Characterization of Oil Bodies in Different Coffea Species. Tropical Plant Biology 7, 1–12. doi:10.1007/s12042-013-9132-2 · other copies
  13. Bvon Blittersdorff, M.; Klatt, C. (2017). The Grind – Particles and Particularities. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 13, pp. 311–328. doi:10.1016/B978-0-12-803520-7.00013-X · other copies
  14. 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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