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Grinding · Grinding

What affects grinding

The same grinder setting does not always give the same grind. Roast degree, water in the bean, bean temperature and the grinder itself shift the particle sizes; several commonly named factors have not been measured.

Across the chain: Science

Source-checkedby Coffearium verification pass· Last source-checked 27 Oct 2026· Prepared with AI assistance· 13 sources (11 tier A)· 1 contested· Updated 27 Oct 2026· Literature last searched 27 Oct 2026
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Contents
  • In a 2005 Colombian study, darker roasts gave significantly smaller particles at the same medium grind setting; the authors relate this to the greater brittleness of darker coffee.2
  • Roasted coffee with more water ground into coarser particles and needed time to equilibrate before grinding; drier coffee did not.4
  • Above a critical water activity of about 0.86, water softens roasted beans and removes their typical brittleness.6,7
  • In a 2016 study, cooling beans from room temperature to about −200 °C narrowed the particle size distribution and reduced the mode of its number distribution by 31%.3
  • Sieves and laser diffraction can disagree: for fine grinds in one study, sieving gave about 380–440 µm whatever the roast, laser diffraction about 110–310 µm.2

Same setting, different grind#

A grinder setting fixes the gap between the burrs, not the size of the particles that come out. The grinding entry explains why coffee is ground, and grinder types and burrs explains how the gap is set. This entry gathers what has been measured about the other factors.

They fall into two groups. Some belong to the beans: roast degree, the water they hold, their temperature and their age. Others belong to the grinder: the gap, the speed of the burrs, how the beans are fed in and how worn the burrs are. According to the grinding chapter of a coffee science textbook, both groups strongly influence the grinding result.1

Most of the evidence comes from a handful of studies, each with one grinder and a few coffees. Where a factor has not been measured, this entry says so.

Roast degree#

According to the textbook chapter, roast degree is the property of the beans that matters most for grinding.1 Roasting makes beans lose strength and toughness and become progressively more brittle, as a 2001 texture study found.5 A brittle bean breaks into smaller pieces under the same load, which is how the study below explains its results.2

A 2005 study from Cenicafé, the Colombian coffee research institute, measured this directly. It roasted three grades of Colombian coffee to four roast degrees and ground them all on one disc mill at the same settings.2 At the medium setting, each darker roast gave significantly smaller particles than the one before.2 The authors explain this by brittleness: the darker the roast, the more brittle the coffee.2

500550600650µm (mean diameter)Light roast · sieves607–6222 Medium roast · sieves598–6132 Dark roast · sieves586–5972 Very dark roast · sieves572–5842 Light roast · laserdiffraction611–6332 Medium roast · laserdiffraction594–6182 Dark roast · laserdiffraction567–6022 Very dark roast · laserdiffraction524–5712
Data table
Pointµm (mean diameter)Source
Light roast · sieves607–622606.85–622.422
Medium roast · sieves598–613598.14–612.832
Dark roast · sieves586–597586.43–596.572
Very dark roast · sieves572–584572.4–583.712
Light roast · laser diffraction611–633610.76–632.612
Medium roast · laser diffraction594–618593.54–617.542
Dark roast · laser diffraction567–602566.77–602.42
Very dark roast · laser diffraction524–571524.05–570.752
Mean particle size of the same medium grind setting at four roast degrees, measured by sieves and by laser diffraction. Each span covers three Colombian coffee grades.2

Fast and slow roasts. A 2000 study roasted beans to the same roast degree at high and at low temperature. The high-temperature beans had more pore volume and larger micropores in their cell walls.8 According to the textbook chapter, such porous, brittle beans grind easily but give a wide distribution with many fines. Slowly roasted, denser beans need more energy but grind more evenly.1 No study consulted for this entry measured the size distribution of fast and slow roasts side by side.

In practice. Changing to a darker coffee at the same setting gives a finer grind in the study above.2 How much a barista must adjust depends on the grinder and the coffee; no general rule has been measured.

Water in the bean#

Roasted coffee holds little water, but beans take up part of the water used for quenching, according to the textbook chapter.1 In a 2008 industrial study, roasts quenched with different amounts of water reached 2.3–8.8 g/100 g of water (basis not stated).4 The coffees with more water ground into coarser particles.4 They also needed time to equilibrate before grinding gave good results; the drier coffees did not.4 The abstract gives no resting time, and the full paper could not be accessed. Cooling and quenching covers how water gets into the bean.

Why water matters. Water changes how a roasted bean breaks. In a 2007 study, roasted beans grew stronger and tougher as they took up water, up to a critical water activity of about 0.86.6 Above it, the bean matrix softened and lost its typical brittleness.6 Water activity (aw) describes how available the water in a food is, on a scale from 0 to 1; moisture and water activity explains it.

00.10.20.30.40.50.60.70.80.91water activity (aw) at which the beans were equilibratedStrength and toughnessrise with water(2007 study)0–0.866 Water-induced hardeningends(2011 study)0.757 Beans soften; brittlenessis lost0.86–0.956,7
Data table
water activity (aw) at which the beans were equilibratedSource
Strength and toughness rise with water (2007 study)0–0.866
Beans soften; brittleness is lost0.86–0.956, 7
Water-induced hardening ends (2011 study)0.757
How the mechanical behaviour of roasted beans changed with water activity in two laboratory studies of beans held at set water activities.6,7

Static. Water inside the beans also sets how strongly the grounds charge during grinding, according to a 2024 study.9 Charge makes fines and larger particles cling together and to the grinder.9 The static and retention entry covers this.

Bean temperature and bean age#

Temperature. A 2016 study ground the same coffee from four temperatures: liquid nitrogen, dry ice, a freezer and room temperature.3 Colder beans gave a narrower particle size distribution and smaller particles.3 Cooling from room temperature to about −200 °C reduced the most common particle size in the number distribution (the mode) by 31%.3 The biggest change came between room temperature and the freezer.3

Warm coffee behaves the other way. According to the textbook chapter, warmer ground coffee gives faster flow times. The chapter explains this by the bean appearing to deform more plastically when warm or humid, which gives a coarser grind.1 No study consulted for this entry measured the temperature of beans or burrs during café service.

Age. Freshly roasted beans hold gas from roasting, according to the textbook chapter1; the degassing entry covers how much and how fast it escapes. No study consulted for this entry measured how the particle size distribution changes with the number of days since roasting.

The grinder: gap, speed, feeding and wear#

Gap. The grind setting is the main control. A finer setting moves the main peak of the distribution to smaller sizes, while the peak of fines stays at about the size of a coffee cell, 25–50 µm.13 Settings on one grinder say little about another; grinder types and burrs covers this.

Burr speed. The textbook chapter lists speed, with position and parallelism of the burrs, among the settings a grinder should allow.1 No peer-reviewed study consulted for this entry measured how burr speed in revolutions per minute (rpm) changes the particle size distribution. The temperature study above ran its grinder at one speed only.3

Feeding. For industrial roller grinders, the chapter lists a consistent, not too high feeding rate among the measures for a narrow distribution.1 No study consulted for this entry compared beans fed from a full hopper with single doses in a café or home grinder.

Wear. In roller grinders, fines rise steadily over the service life of the rolls at a constant average size, according to the textbook chapter.1 A university study of espresso burrs reports that wear and damage made the distribution less precisely controllable.10 Its abstract gives no figures. Grinder care covers cleaning and burr replacement.

How the grind is measured#

Every result above depends on how the particles were measured. The three common methods are sieving, laser diffraction and image analysis; particle size and fines describes them.

Sieves and fine grinds. The 2005 study measured the same grinds with sieves and with laser diffraction.2 For medium grinds, the two methods gave mean diameters that did not differ significantly.2 For fine grinds they diverged. Sieving gave 378.50–438.23 µm for every roast degree, while laser diffraction gave 114.13–310.33 µm, smaller the darker the roast.2 The authors conclude that the sieve method does not measure the real particle size of fine grinds.2 They attribute this to sieves clogged by the oil in roasted coffee.2 For coarse grinds, the sieve stack covered only about 60% of the distribution.2

0100200300400500µm (mean diameter)Light roastlaser2 sieves2 Medium roastlaser2 sieves2 Dark roastlaser2 sieves2 Very dark roastlaser2 sieves2
Data table
Pointµm (mean diameter)Source
Light roastlaser271.31–310.332
Light roastsieves378.5–423.222
Medium roastlaser242.72–292.292
Medium roastsieves394.37–433.342
Dark roastlaser181.7–207.042
Dark roastsieves397.49–438.232
Very dark roastlaser114.13–177.832
Very dark roastsieves396–413.412
The same fine grinds measured by sieves and by laser diffraction, at four roast degrees. Sieving missed the roast effect, which the authors attribute to sieves clogged by coffee oil.2

Laser diffraction and shape. Laser diffraction calculates the diameters of spheres that would scatter light the same way.1,11 ISO 13320 notes that the technique assumes spherical particles, so results for irregular particles differ from those of other methods.11 In the 2005 study, laser values for coarse grinds lay well below the sieve values: 566.90–650.51 µm against 770.94–906.76 µm.2 The paper gives no explanation beyond the incomplete coverage of the sieves.

Image analysis. A 2024 espresso study measured particles with a dual-camera imaging instrument instead.12 Its burr spacings and size statistics are not directly comparable with sieve or laser data from other studies.

What this means for the evidence. A roast or water effect of a few tens of micrometres, as in the medium grinds above, is smaller than the gap between methods for fine grinds.2 Comparisons are therefore safe only within one study, one method and one grinder.

Why roast and water change fracture#

A brittle, glassy matrix. Roasted beans are weaker and less tough than green beans.6 In the 2001 study, their brittleness was linked to water loss, to structural change from pyrolysis and to lower density.5 At a similar water content, dried and roasted beans differed only slightly in fracture resistance.5 Glass–rubber transition explains the glassy state.

Two effects of water. A 2011 study combined compression tests, calorimetry and NMR on roasted beans.7 From the monolayer up to a water activity of 0.75, water hydrated the solid matrix and hardened the beans.7 Once hydration was complete, water acted as a plasticiser and the compressive modulus fell.7 Above 0.86, hardness fell and the elastic modulus fell further.7 The 2007 study placed the critical water activity of roasted beans at about 0.86.6 The two studies agree on softening above 0.86 but differ on where hardening ends.

From texture to grind. These texture studies did not grind the beans. The link to particle size comes from the 2008 grinding study, in which wetter coffee ground coarser.4 According to the textbook chapter, lighter coffees with more residual moisture give a narrower distribution but need more energy.1

Cold fracture. The 2016 study suggests that some components of the bean pass a shattering transition between room temperature and freezer temperature, and calls this a possibility.3 It did not measure the transition. The same study found the distribution independent of origin and processing for four lightly roasted coffees.3 In the 2005 study, the larger-bean export grade gave significantly larger particles than lower grades (see Contested).2

Reference values#

Factor Reported effect Conditions Source
Roast degree, medium grind (sieves) Light 606.85–622.42 µm; medium 598.14–612.83 µm; dark 586.43–596.57 µm; very dark 572.40–583.71 µm Three Colombian grades, one disc mill, fixed setting; roast degrees significantly different (Tukey, 5%) Guevara-Barreto & Castaño-Castrillón2
Roast degree, medium grind (laser) Light 610.76–632.61 µm; medium 593.54–617.54 µm; dark 566.77–602.40 µm; very dark 524.05–570.75 µm Same grinds; laser diffraction in isopropanol Guevara-Barreto & Castaño-Castrillón2
Fine grind, sieves vs laser Sieves: light 378.50–423.22, medium 394.37–433.34, dark 397.49–438.23, very dark 396.00–413.41 µm. Laser: light 271.31–310.33, medium 242.72–292.29, dark 181.70–207.04, very dark 114.13–177.83 µm Same study; sieves clogged by oil Guevara-Barreto & Castaño-Castrillón2
Coarse grind, sieves vs laser Sieves 770.94–906.76 µm; laser 566.90–650.51 µm Same study; sieves covered about 60% of the distribution Guevara-Barreto & Castaño-Castrillón2
Water content after quenching 2.3–8.8 g/100 g (basis not stated); wetter coffee ground coarser and needed equilibration time Industrial roasts Baggenstoss et al.4
Critical water activity, roasted beans ≈0.86; brittleness lost above it Light, medium and dark roasts; compression Pittia et al. 20076
Water-induced hardening, roasted beans Up to aw 0.75; further softening above 0.86 Compression, calorimetry, NMR Rocculi et al.7
Bean temperature Mode of the number distribution −31% from room temperature to ≈−200 °C; distribution narrower Flat burrs, 1,480 rpm, fixed setting, 20 g Uman et al.3
Origin and processing No effect on the distribution Four lightly roasted arabica coffees Uman et al.3
Fines peak across grind settings ≈25–50 µm, the size of a cell Four grinds, espresso to drip filter Mo et al.13
Burr speed, feeding, bean age Not measured No study consulted for this entry –

Open questions#

  • Burr speed. No peer-reviewed measurement of particle size against burr rpm was found. The textbook chapter treats speed as a setting worth adjusting, without data.1
  • Single dosing versus hopper. Whether the weight of beans above the burrs changes the grind has not been measured in any study consulted for this entry.
  • Days since roasting. How degassing and slow changes in the bean alter the grind over the first weeks has not been measured in the studies consulted.
  • Burr wear in cafés. One study reports less precise control with worn burrs10, but no open data give the mass ground before the distribution shifts.
  • Fast versus slow roasts. Pore structure differs8, but no study consulted ground fast and slow roasts of the same colour and compared their distributions.
  • Effect in the cup. None of the grinding studies above tested extraction or flavour; the effect of fines on flow is covered in particle size and fines.12

Key studies#

Guevara-Barreto & Castaño-Castrillón, 2005 – Grind of Colombian roasted coffee. Roasts three Colombian coffee grades to four roast degrees, grinds them at three settings on one disc mill and measures the result by sieves and laser diffraction. Darker roasts and smaller-bean grades gave smaller particles, and sieving failed for fine grinds because oil clogged the sieves.

Cenicafé 56(1), 5–18 · full text (Spanish, English abstract)

Uman et al., 2016 – Bean origin and temperature. Grinds four lightly roasted coffees, and one coffee from four temperatures, on the same flat-burr grinder. Origin and processing made no difference, while colder beans gave a narrower distribution and a smaller mode.

Scientific Reports 6, 24483 · doi:10.1038/srep24483

Baggenstoss et al., 2008 – Water content and grinding. Quenches industrial roasts with different amounts of water and follows grinding, espresso extraction and storage. Wetter coffee ground coarser and needed a rest before grinding; drier coffee extracted more effectively.

European Food Research and Technology 227(5), 1357–1365 · doi:10.1007/s00217-008-0852-8

Pittia et al., 2007 – Water activity and texture. Equilibrates raw and roasted beans across the full range of water activity and compresses them. Water strengthened roasted beans up to a critical water activity of about 0.86 and softened them above it, removing their brittleness.

Journal of Texture Studies 38(1), 116–134 · doi:10.1111/j.1745-4603.2007.00089.x

Rocculi et al., 2011 – Hardening and softening by water. Combines compression tests, calorimetry and NMR on green and roasted beans. Water first hardens the bean matrix, then acts as a plasticiser once the matrix is hydrated.

Journal of Agricultural and Food Chemistry 59(15), 8265–8271 · doi:10.1021/jf201333a

Contested
  • The coffee itself, beyond roast: a 2016 study found the particle size distribution independent of the origin and processing of four lightly roasted arabica coffees3, while a 2005 study found that Colombian export-grade coffee (excelso), with larger beans, gave significantly larger particles than lower grades.2 The studies compared different properties (origin and process versus bean size grade) on different grinders.
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Change log

  • – The 31% reduction with cold grinding is now given as the mode of the number distribution. Reason: Uman et al. 2016 report the mode of the number distribution (Fig. 4b), not of the volume distribution most grind data use.
  • – Removed the unused source Gloess et al. 2013 and added citations to three uncited statements (water uptake from quenching, gas in fresh beans, charged particles clinging). Reason: Every factual statement needs a cited source; the gas statement was narrowed to what the cited chapter says.

Sources

  1. 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 copiesQuoted passage: a description; Textbook chapter on grinding by two industry authors; general account of industrial and point-of-sale grinding, partly citing earlier work..
  2. AGuevara-Barreto, R. A.; Castaño-Castrillón, J. J. (2005). Caracterización granulométrica del café colombiano tostado y molido. Cenicafé 56(1), 5–18.Quoted passage: the authors’ own result; Colombian arabica, three grades (excelso, consumo, 50:50 consumo–pasilla), four roast degrees, one disc mill at fine, medium and coarse settings; sieving (NTC 2441) and laser diffraction; laboratory..
  3. AUman, E.; Colonna-Dashwood, M.; Colonna-Dashwood, L.; Perger, M.; Klatt, C.; Leighton, S.; Miller, B.; Butler, K. T.; Melot, B. C.; Speirs, R. W.; Hendon, C. H. (2016). The effect of bean origin and temperature on grinding roasted coffee. Scientific Reports 6, 24483. doi:10.1038/srep24483 · other copiesQuoted passage: the authors’ own result; Four arabica coffees (three washed, one natural) roasted light to medium; one flat-burr grinder with Turkish burrs at 1,480 rpm, fixed setting; 20 g doses; beans held 2 h at four temperatures; laser diffraction; 31% is the mode of the number distribution..
  4. ABaggenstoss, J.; Perren, R.; Escher, F. (2008). Water content of roasted coffee: impact on grinding behaviour, extraction, and aroma retention. European Food Research and Technology 227(5), 1357–1365. doi:10.1007/s00217-008-0852-8 · other copiesQuoted passage: the authors’ own result; Industrial-scale normal and long roasts quenched with different amounts of water, giving 2.3–8.8 g/100 g water; abstract only..
  5. APittia, P.; Dalla Rosa, M.; Lerici, C. R. (2001). Textural changes of coffee beans as affected by roasting conditions. LWT – Food Science and Technology 34(3), 168–175. doi:10.1006/fstl.2000.0749 · other copiesQuoted passage: the authors’ own result; Laboratory roasts at 170 and 200 °C and dehydrated controls at 90–105 °C; mechanical tests on whole beans; abstract only..
  6. APittia, P.; Nicoli, M. C.; Sacchetti, G. (2007). Effect of moisture and water activity on textural properties of raw and roasted coffee beans. Journal of Texture Studies 38(1), 116–134. doi:10.1111/j.1745-4603.2007.00089.x · other copiesQuoted passage: the authors’ own result; Raw beans and light, medium and dark roasts equilibrated at aw 0–0.95; uniaxial compression; laboratory; abstract only..
  7. ARocculi, P.; Sacchetti, G.; Venturi, L.; Cremonini, M.; Dalla Rosa, M.; Pittia, P. (2011). Role of Water State and Mobility on the Antiplasticization of Green and Roasted Coffee Beans. Journal of Agricultural and Food Chemistry 59(15), 8265–8271. doi:10.1021/jf201333a · other copiesQuoted passage: the authors’ own result; Green and roasted beans equilibrated at set water activities; compression tests, sorption isotherms, calorimetry and NMR; laboratory; abstract only..
  8. 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 copiesQuoted passage: the authors’ own result; Hot-air roasts at high and low temperature to the same roast degree; volumetry, mercury porosimetry, electron microscopy; abstract only..
  9. AMéndez Harper, J.; McDonald, C. S.; Rheingold, E. J.; Wehn, L. C.; Bumbaugh, R. E.; Cope, E. J.; Lindberg, L. E.; Pham, J.; Kim, Y.-H.; Dufek, J.; Hendon, C. H. (2024). Moisture-controlled triboelectrification during coffee grinding. Matter 7(1). doi:10.1016/j.matt.2023.11.005 · other copiesQuoted passage: the authors’ own result; About 30 commercial coffees plus in-house roasts of different roast degree and moisture; one grinder with 98 mm steel flat burrs; laboratory charge measurements..
  10. BGriffiths, C.; Adams, G. C.; Rees, A. (2018). Wear Monitoring of Coffee Grind-On-Demand Burrs Using Precision Sieving and Laser Diffraction. International Journal of Food and Bioscience 1(1), 34–41.Quoted passage: the authors’ own result; Espresso grind-on-demand burrs; precision sieving and laser diffraction; abstract only, no numbers used..
  11. AInternational Organization for Standardization (ISO) (2020). ISO 13320:2020 Particle size analysis – Laser diffraction methods. ISO.Quoted passage: a definition; International standard for laser diffraction particle sizing; scope page only..
  12. ASmrke, S.; Eiermann, A.; Yeretzian, C. (2024). The role of fines in espresso extraction dynamics. Scientific Reports 14, 5612. doi:10.1038/s41598-024-55831-x · other copiesQuoted passage: the authors’ own result; One coffee ground at six burr spacings (160–250 µm) on one grinder; espresso 20 g to 40 g at 9 bar; image analysis of particle size..
  13. 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 copiesQuoted passage: the authors’ own result; Four grinds of one medium-roast arabica blend (espresso to drip filter); laser diffraction (Mastersizer 3000) and X-ray microtomography..

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”.

The literature for this entry was last searched on 27 Oct 2026. A statement that no source consulted for this entry shows something refers to the sources above; newer or missing work may exist.

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