{ "id": "particle-size-and-fines", "url": "https://coffearium.org/entries/particle-size-and-fines/", "license": "CC-BY-4.0", "title": "Particle size and fines", "summary": "Ground coffee is a mix of sizes, usually with two peaks – small cell fragments called fines and much larger particles. This spread shapes how fast coffee extracts and how easily water flows through the bed.", "domain": "grinding", "kind": "Concept", "stage": "brew", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-15T00:00:00.000Z", "updated": "2026-10-15T00:00:00.000Z", "added": "2026-10-15T00:00:00.000Z", "brief": [ "Ground coffee has a particle size distribution (PSD), usually with two peaks: one of fines and one near the average particle size.2,3", "Fines are commonly defined as particles below 100 µm. In one study they were cell fragments, with a peak at about 25–50 µm, the size of a coffee cell.1,2", "A coarser grind setting moves the main peak to larger sizes, while the fines peak stays where it is.1", "In espresso, a larger share of fines lowered bed permeability, slowing the flow and lengthening the shot; its effect on extraction efficiency appeared marginal.2", "PSD is measured by sieving, laser diffraction or image analysis; laser diffraction assumes spherical particles.3,12" ], "quickFacts": [ { "label": "Fines", "value": "Particles below 100 µm (common definition)" }, { "label": "Fines peak", "value": "About 25–50 µm, the size of a coffee cell (one study)" }, { "label": "Volume below 100 µm", "value": "9.7% (filter grind) to 29.21% (espresso grind) in one study; 25.52% (unsieved drip-filter grind) and 15.08% (coarser burr grind) in another" }, { "label": "Shape", "value": "Usually bimodal – a fines peak and a main peak" }, { "label": "Measured by", "value": "Sieving, laser diffraction (ISO 13320), image analysis" }, { "label": "Main effect of fines in espresso", "value": "Lower bed permeability – slower flow, longer shot (one study)" } ], "aka": [ "Particle size distribution", "Grind size distribution", "PSD" ], "linkTerms": [ "particle size distribution", "fines", "boulders", "bimodal distribution", "fines migration", "grind size" ], "related": [ "grinding", "grinder-types-and-burrs", "extraction-yield-and-tds", "brew-methods", "espresso-machine", "espresso", "puck-preparation", "extraction-variables", "what-extraction-is", "freshness-and-storage" ], "contested": [ "Fines and extraction efficiency: a coffee science textbook states that small particles expose more surface and extract more efficiently5, and one espresso study found that smaller particles gave more caffeine and trigonelline per gram of beverage.7 A 2024 espresso study that added sieved fines found their effect on extraction efficiency marginal and concluded that they act mainly through bed permeability2; a 2023 study found that surface-based mean sizes did not differ between its grind levels and so could not explain their effect.8", "Sifting out fines: according to a textbook chapter on grinding, sieving the grounds before brewing could avoid the bitter and acidic notes of over-extracted fines and under-extracted boulders.3 In a 2023 French press study, sieved, homogeneous size fractions gave less dissolved solids and fewer bioactive compounds than the bimodal grinds straight from the grinder.11 The two sources discuss different brew methods." ], "corrections": [], "sources": [ { "id": "s1", "title": "Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations", "authors": "Mo, C.; Johnston, R.; Navarini, L.; Suggi Liverani, F.; Ellero, M.", "year": 2023, "publisher": "Scientific Reports 13, 16374", "url": "https://cronfa.swansea.ac.uk/Record/cronfa64681/Download/64681__28732__cf1888c5dbe64008b6a6e7afcc1c36ad.pdf", "doi": "10.1038/s41598-023-42380-y", "tier": "A", "quote": "The first peak is always around 25 ∼ 50µm, because the fines are cell fragments and the cell size is in the range 25 ∼ 50µm.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s2", "title": "The role of fines in espresso extraction dynamics", "authors": "Smrke, S.; Eiermann, A.; Yeretzian, C.", "year": 2024, "publisher": "Scientific Reports 14, 5612", "url": "https://www.nature.com/articles/s41598-024-55831-x", "doi": "10.1038/s41598-024-55831-x", "tier": "A", "quote": "An increase of share of fines decreases coffee bed permeability, leads to reduced flow rates and longer extraction times.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s3", "title": "The Grind – Particles and Particularities", "authors": "von Blittersdorff, M.; Klatt, C.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 13, pp. 311–328", "url": "https://eld.bl.uk/catalog/018161895", "doi": "10.1016/B978-0-12-803520-7.00013-X", "tier": "B", "quote": "The PSD of R&G coffee is mostly bimodal.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s4", "title": "Analysing extraction uniformity from porous coffee beds using mathematical modelling and computational fluid dynamics approaches", "authors": "Moroney, K. M.; O’Connell, K.; Meikle-Janney, P.; O’Brien, S. B. G.; Walker, G. M.; Lee, W. T.", "year": 2019, "publisher": "PLOS ONE 14(7), e0219906", "url": "https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0219906", "doi": "10.1371/journal.pone.0219906", "tier": "A", "quote": "Fines have a larger surface to volume ratio than coarse grains.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s5", "title": "The Brew – Extracting for Excellence", "authors": "Mestdagh, F.; Glabasnia, A.; Giuliano, P.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 15, pp. 355–380", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00015-3", "doi": "10.1016/B978-0-12-803520-7.00015-3", "tier": "B", "quote": "Small particles increase the surface exposed to water, permitting a more efficient extraction process (Andueza et al., 2003).", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s6", "title": "The effect of bean origin and temperature on grinding roasted coffee", "authors": "Uman, 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.", "year": 2016, "publisher": "Scientific Reports 6, 24483", "url": "https://researchportal.bath.ac.uk/en/publications/the-effect-of-bean-origin-and-temperature-on-grinding-roasted-cof/", "doi": "10.1038/srep24483", "tier": "A", "quote": "Furthermore, we elucidate the influence of bean temperature on particle size distribution, concluding that grinding cold results in a narrower particle size distribution, and reduced mean particle size.", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s7", "title": "Time-resolved extraction of caffeine and trigonelline from finely-ground espresso coffee with varying particle sizes and tamping pressures", "authors": "Kuhn, M.; Lang, S.; Bezold, F.; Minceva, M.; Briesen, H.", "year": 2017, "publisher": "Journal of Food Engineering 206, 37–47", "url": "https://portal.fis.tum.de/en/publications/time-resolved-extraction-of-caffeine-and-trigonelline-from-finely/", "doi": "10.1016/j.jfoodeng.2017.03.002", "tier": "A", "quote": "On the one hand, it was found that the particle size significantly affects the extraction kinetics with smaller particles leading to a higher extracted amount of caffeine and trigonelline per collected coffee mass.", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s8", "title": "Influence of Flow Rate, Particle Size, and Temperature on Espresso Extraction Kinetics", "authors": "Schmieder, B. K. L.; Pannusch, V. B.; Vannieuwenhuyse, L.; Briesen, H.; Minceva, M.", "year": 2023, "publisher": "Foods 12(15), 2871", "url": "https://mdpi-res.com/d_attachment/foods/foods-12-02871/article_deploy/foods-12-02871.pdf", "doi": "10.3390/foods12152871", "tier": "A", "quote": "The particle size distributions for the used grinding levels were measured dry and wet by laser diffractometer", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s9", "title": "Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment", "authors": "Cameron, M. I.; Morisco, D.; Hofstetter, D.; Uman, E.; Wilkinson, J.; Kennedy, Z. C.; Fontenot, S. A.; Lee, W. T.; Hendon, C. H.; Foster, J. M.", "year": 2020, "publisher": "Matter 2(3), 631–648", "url": "https://pure.hud.ac.uk/en/publications/systematically-improving-espresso-insights-from-mathematical-mode/", "doi": "10.1016/j.matt.2019.12.019", "tier": "A", "quote": "experimental measurements show a peak in the extraction yield versus grind setting relationship.", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s10", "title": "Uneven extraction in coffee brewing", "authors": "Lee, W. T.; Smith, A.; Arshad, A.", "year": 2023, "publisher": "Physics of Fluids (AIP Publishing)", "url": "https://arxiv.org/pdf/2206.12373v2", "doi": "10.1063/5.0138998", "tier": "A", "quote": "Extraction causes an increase in porosity and thus permeability.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s11", "title": "Using ground coffee particle size and distribution to remodel beverage properties", "authors": "Angeloni, G.; Masella, P.; Spadi, A.; Guerrini, L.; Corti, F.; Bellumori, M.; Calamai, L.; Innocenti, M.; Parenti, A.", "year": 2023, "publisher": "European Food Research and Technology 249(5), 1247–1256", "url": "https://link.springer.com/article/10.1007/s00217-023-04210-3", "doi": "10.1007/s00217-023-04210-3", "tier": "A", "quote": "Moreover, extraction from a very fine homogeneous grind was poorest with respect to these compounds, despite the greater surface in contact with the solvent.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s12", "title": "ISO 13320:2020 Particle size analysis – Laser diffraction methods", "authors": "International Organization for Standardization (ISO)", "year": 2020, "publisher": "ISO", "url": "https://www.iso.org/standard/69111.html", "tier": "A", "quote": "This is because the technique assumes a spherical particle shape in its optical model.", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" } ], "markdown": "## A mix of sizes, not one size\n\nA grinder does not cut coffee into identical pieces. It breaks the beans into many particles of different sizes and shapes.3 How much of the coffee falls into each size class is called the particle size distribution, or PSD.3\n\nA grind setting therefore sets a whole distribution, not one size. Two grinds with the same median size can still differ. According to a 2024 study, different grinding technologies can give different shares of fines at the same median.2 The median is the size below which half of the coffee lies.3\n\nSize matters for two reasons that pull in opposite directions. Small particles expose more surface and let soluble compounds out faster.3,5 But very small particles can block the paths between larger ones, slow the water or clog the filter.3\n\n## Fines and boulders: two peaks\n\nGround coffee usually has a bimodal distribution: plotted as a curve, it shows two peaks instead of one.2,3,11 One peak lies near the average particle size. The other lies among the very small particles, the fines.3\n\n**Fines.** Particles below 100 µm (a tenth of a millimetre) are often called fines.1,2,3 The cut-off is a convention and changes from case to case.3 In a 2023 study, the fines peak always sat at about 25–50 µm.1 Its authors explain this by the bean itself: the fines are fragments of cells, and coffee cells are about that size.1\n\n**Where fines come from.** According to a textbook chapter on grinding, the peak of fines comes from the cellular structure of the roasted bean, not only from the grinder.3 Whenever a bean or a particle cracks, very fine pieces flake off, and this repeats with every further break.3 According to the chapter, fines are unavoidable in most applications.3\n\n**Boulders.** Very coarse particles are sometimes called boulders.3 In the textbook’s account, fines tend to over-extract and boulders to under-extract, so a wide distribution brings both into the same cup.3\n\n
\n
\n

A · Cut through a coffee bed

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    \n
  1. Water in. Water enters the bed from the top and flows through the spaces between particles.5
    2. \n
  2. Boulder. A coarse particle. Much of its coffee sits in intact cells inside it and is less accessible.4
    3. \n
  3. Fines. Small cell fragments, commonly defined as particles below 100 µm.1,2
    4. \n
  4. Pore space. The gaps between particles carry the water. More fines mean a less permeable bed and slower flow.1,2
    5. \n
  5. Channel. A more open path carries more water; extraction opens it further, so small differences can grow.10
    6. \n
  6. Filter. Holds the bed. Fines that move down with the water would collect above it.1
    7. \n
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B · One coarse particle, magnified

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    \n
  1. Broken cells at the surface. Easily accessible coffee, extracted quickly.4
    2. \n
  2. Intact cells inside. Less accessible coffee, extracted slowly.4
    3. \n
  3. Cell-wall fragments. Pieces that flake off at every crack and become fines.1,3
    4. \n
\n
Fines and boulders in a coffee bed, and the cell structure that produces fines. Schematic, not to scale.
\n
\n\n## How grind setting changes the distribution\n\nTurning the grinder coarser moves the main peak to larger sizes.1 The fines peak stays at about the size of a cell.1 What changes is how much of the coffee ends up in each peak.1,2\n\nOne study compared four grinds of the same coffee. About 29% of the volume was below 100 µm in the espresso grind, and about 10% in the drip-filter grind.1 In a second study, an unsieved drip-filter grind held about 26% and a coarser burr grind about 15%.4 So a finer setting adds fines, not only smaller main particles.\n\n
\n
\n
Two mean sizes for six published grinds.1,4 The surface mean d[3,2] is pulled towards the fines; the volume mean d[4,3] lies close to the median.3 The fines peak sat at 25–50 µm for all four grinds of one study.1
\n
\n\nOther factors change the distribution at the same setting:\n\n- **Bean temperature.** In a 2016 study, grinding cold beans gave a narrower distribution and a smaller most common particle size (mode).6\n- **Roast degree.** According to the textbook chapter, darker and faster roasts make beans more porous and brittle. Ground soon after roasting, they give a wide distribution and many fines.3\n- **Grinder.** Different grinders need different settings and give different distributions for the same flow time in espresso, according to the same chapter.3\n\nThe 2016 study found the distribution independent of bean origin and processing method.6\n\n## How particle size is measured\n\nThree methods are common: sieving, laser diffraction and image analysis.3\n\n- **Sieving.** A stack of sieves with shrinking openings is shaken, and the coffee left on each sieve is weighed.3 A 2023 study used six sieves on a vibratory sifter, with a brush to free fines stuck to the coarser particles.11\n- **Laser diffraction.** Dispersed particles pass a laser beam, and software calculates which spheres would scatter the light the same way.3 The international standard ISO 13320 covers sizes from about 0.1 µm to 3 mm.12 The coffee can be dispersed in air (dry) or in a liquid (wet).3,8\n- **Image analysis.** A camera photographs the particles as they pass, and software measures their outlines.3 It also shows their shape, not only their size.3 A 2024 espresso study used a dual-camera instrument of this kind.2\n\nResults from different methods are not directly comparable. Laser diffraction assumes round particles, and for irregular particles its result differs from sieving.12 Larger coffee particles in particular are irregular in shape.3\n\n**By volume or by number.** The two measuring studies cited here report the share of volume in each size class.1,2 Counted by number, fines dominate instead. By our own arithmetic, one particle of 500 µm has the volume of 1,000 particles of 50 µm.\n\n## What fines do in the brew\n\n**Extraction.** Fines have more surface per volume than coarse particles, so they extract faster.4 According to a coffee science textbook, small particles allow more efficient extraction.5 The size of the effect is debated (see Contested).\n\n**Flow.** Fines fill the spaces between larger particles. In a 2024 espresso study, more fines made the bed less permeable, so water flowed more slowly and shots took longer.2 A 2023 study found the same direction: more fines and more surface, lower permeability.1\n\n**Channelling.** Channelling is uneven flow, where water finds easier paths through the bed. In a 2020 espresso study, extraction yield dropped again at the finest grind settings.9 According to the authors, this strongly suggests uneven flow at fine settings.9\n\n**Body and sediment.** According to a coffee science textbook, mouthfeel depends partly on suspended fines, particularly in French press and espresso.5 The same chapter links French press sediment to fines and the metal mesh filter.5 According to the grinding chapter, a certain share of fines is deemed necessary in espresso to build pressure, body and crema in its short brew time.3\n\n## Changing the share of fines\n\nThe distribution is changed mainly through the grind setting. A finer setting raises the share of fines as well as moving the main peak.1,4 In espresso, the share of fines then shows up as a change in flow time.2\n\n**Sifting.** Sieving out part of the grind before brewing narrows the distribution. According to the grinding chapter, this could avoid off-notes from fines and boulders, but it loses coffee and is impractical in a busy café.3 In a 2023 French press study, sieved grinds of one size class gave less dissolved coffee than ordinary bimodal grinds.11 No study opened for this entry tested sifting for pour-over or espresso flavour.\n\n\n\n## Size statistics\n\nA PSD is summarised by a few characteristic sizes. The textbook chapter on grinding uses x10, x50 and x90: the sizes below which 10%, 50% and 90% of the coffee lies; x50 is the median.3\n\nEquivalent diameters compare each particle with a sphere of the same surface or the same volume.3 Averaged over the sample, they give the surface mean d[3,2] (Sauter mean diameter) and the volume mean d[4,3] (De Brouckere mean).3,4,8 The surface mean is strongly pulled down by the fines, while the volume mean lies close to the median.3 In a bimodal grind the two can lie far apart: 27.34 µm and 457.84 µm for one drip-filter grind.4 A ratio of surface to volume gives the specific surface.3\n\n**Number versus volume (our own arithmetic).** The volume of a sphere scales with the cube of its diameter. One 500 µm particle has the volume of 1,000 particles of 50 µm, but those 1,000 particles have 10 times its surface. A number-based distribution is therefore dominated by fines; a volume-based one by the coarse peak. A sample with about 25% of its volume below 100 µm consists, by count, mostly of fines.\n\n**Choice of basis.** In the 2024 espresso study the median was volume-weighted and based on the projected particle area in images.2 The 2023 microtomography study reported its laser-diffraction results as volume shares and specific surface.1 Sieving gives mass fractions directly.3 Comparisons between studies need the same basis and method.\n\n## Measurement in practice\n\n| Method | What it measures | Strengths | Limits | Source |\n|---|---|---|---|---|\n| Sieving | Mass retained between two mesh sizes | Direct mass fractions; simple equipment | Fines stick to coarser particles and need a sieving aid or brush | Blittersdorff & Klatt3; Angeloni et al.11 |\n| Laser diffraction | Volume-equivalent sphere diameters from the scattering pattern | About 0.1 µm to 3 mm; dry or wet dispersion; can run online after industrial grinding | Assumes spherical particles; results for irregular particles differ from other methods | ISO 1332012; Blittersdorff & Klatt3 |\n| Image analysis | Projected outlines of particles | Gives shape (circularity) as well as size | Not stated in the sources opened | Blittersdorff & Klatt3; Smrke et al.2 |\n\nIn a 2023 espresso study, dry and wet laser diffraction of the same grinds gave highly similar distributions.8 Sieve openings and fines definitions do not always match: the 2024 espresso study defined fines as particles below 100 µm but prepared its fines fraction with a 120 µm sieve.2\n\n## Two-domain extraction models\n\nModels of extraction from a bed treat ground coffee as two domains. Easily accessible coffee sits in fines and on the broken surfaces of larger grains; less accessible coffee sits in the intact cells inside larger grains.4 Both are described by first-order rate equations.4 Because fines have a larger surface-to-volume ratio, they extract faster than coarse grains, so different particle sizes extract at different rates.4\n\nMeasurements support a particle-size effect, but its cause is less clear. In a 2017 espresso study, smaller particles gave more caffeine and trigonelline per gram of beverage, while tamping pressure had no detectable effect.7 That study introduced a reduced model with only physically meaningful parameters.7 A 2023 study from the same Munich group measured its grinds by laser diffraction and found that their Sauter mean diameters did not differ significantly between grind levels.8 Surface area alone therefore could not explain the effect of grind level in that study.8\n\nThe 2024 espresso study varied the share of fines directly. Coffee was ground at burr spacings of 160–250 µm; at the three coarsest settings, 1, 2 or 4 g of sieved fines replaced the same mass of coffee in a 20 g dose; shots ran at 9 bar to 40 g of beverage.2 Added fines did not produce outlying combinations of extraction yield and time.2 The authors conclude that fines change bed permeability, not the extraction mechanism, and that the surface-area effect on extraction efficiency seems marginal.2 Fast shots were already efficient: shots of under 10 s and of 15 s reached 17–18% extraction yield.2\n\n## Permeability and channelling\n\nAccording to the brewing chapter, the coffee bed’s permeability decides how water flows; too low a permeability gives high pressure, low flow and long brew times.5 According to the grinding chapter, a wide or bimodal distribution packs to a lower porosity and a higher pressure loss than uniform particles.3 Fines lower permeability in both of the measuring studies opened here.1,2 In the 2024 study, a regression on the full distributions linked particles up to 150 µm to longer extraction time, those above 250 µm to shorter time.2\n\n**Feedback in the bed.** A 2023 model treats channelling as a positive feedback. Extraction dissolves coffee, which raises porosity and permeability, so paths that carry more water open further.10 Small differences in porosity are therefore amplified over the shot.10 In this model, the drop in yield at fine settings comes from one part of the bed being fully extracted, and the authors caution that average yield may be a poor guide to flavour at fine grinds.10 The 2020 espresso study had read the same drop as strong evidence of inhomogeneous flow at fine settings, with poor reproducibility and wasted coffee as results.9\n\nIn the conical geometry of a pour-over, flow is uneven for geometric reasons as well; for a fine drip grind in a cylindrical bed, a model predicted a variation of about 5 percentage points in extraction yield.4 The brew methods entry covers bed shape.\n\n## Fines migration and bed consolidation\n\nFines migration is the idea that water carries fines downwards through the bed. If it happens, fines would collect above the filter and lower the porosity near the bottom, because the filter keeps them from leaving.1 The 2023 microtomography study compared capsule beds before and after extraction.1 After extraction, porosity always decreased from the outlet at the bottom to the inlet at the top: the bed was more open at the bottom, not less.1 Its authors note that fines migration would push the profile the other way.1\n\nAccording to the brewing chapter, the bed also changes as it wets: particles swell and rearrange under flow and pressure, porosity falls and the pressure drop rises; the finer the distribution, the more the bed consolidates.5 No study opened for this entry measured fines migration directly in a pour-over or espresso bed.\n\n## Sifting: what the evidence shows\n\nThe 2023 French press study separated one grind into six sieve fractions: below 124 µm, 125–249, 250–399, 400–599, 600–799 µm and above 800 µm.11 Brews used 25 g of coffee in 250 g of water at 95 °C for 5 minutes.11 The fine grind straight from the grinder gave the highest extraction, 21.4 ± 0.9%; the sieved 125–249 µm fraction gave the lowest.11 The authors call this hard to explain and suggest that compounds may be distributed differently between particle sizes.11 Homogeneous size classes appeared less able to extract the available solutes than bimodal grinds.11\n\nThe textbook chapter on grinding describes the opposite expectation for espresso: a narrower distribution avoids the mix of over-extracted fines and under-extracted boulders.3 The French press study did not test espresso or pour-over, and the chapter reports no sensory measurement.3,11\n\n## Reference values\n\n
\n
\n
Share of the volume below 100 µm in six published grinds, from espresso to coarse grinds.1,4 The two studies used different coffees and grinders; compare within a study.
\n
\n\n| Parameter | Value reported | Context | Source |\n|---|---|---|---|\n| Fines (definition) | Below 100 µm | Common definition; varies by case | Mo et al.1; Smrke et al.2; Blittersdorff & Klatt3 |\n| Fines peak | ≈25–50 µm | Same for four grinds; matches cell size | Mo et al.1 |\n| Espresso grind (type E) | d[3,2] 81.0 µm; d[4,3] 308 µm; 29.21% below 100 µm | Laser diffraction, volume basis | Mo et al.1 |\n| Capsule espresso grind (type H) | d[3,2] 90.1 µm; d[4,3] 341.6 µm; 27.52% below 100 µm | Same | Mo et al.1 |\n| Moka pot grind (type M) | d[3,2] 116 µm; d[4,3] 372 µm; 20.26% below 100 µm | Same | Mo et al.1 |\n| Drip-filter grind (type F) | d[3,2] 214 µm; d[4,3] 673 µm; 9.7% below 100 µm | Same | Mo et al.1 |\n| Drip-filter grind, unsieved (second study) | d[3,2] 27.34 µm; d[4,3] 457.84 µm; 25.52% below 100 µm | Volume share; method not named | Moroney et al.4 |\n| Coarse burr grind (second study) | d[3,2] 37.78 µm; d[4,3] 823.02 µm; 15.08% below 100 µm | Volume share; brew method not named | Moroney et al.4 |\n| Laser diffraction range | ≈0.1 µm to 3 mm | ISO 13320:2020 scope | ISO12 |\n| Espresso fines trial | 1, 2 or 4 g fines in a 20 g dose; 9 bar; 40 g beverage | Burr spacing 160–250 µm | Smrke et al.2 |\n| French press sieve trial | 25 g in 250 g water, 95 °C, 5 min; best yield 21.4 ± 0.9% (unsieved fine grind) | Six sieve fractions | Angeloni et al.11 |\n\n## Open questions\n\n- **Fines migration.** Whether fines move far enough during a brew to change flow has not been measured directly in any study opened for this entry; the one imaging study found a porosity profile opposite to what migration would produce.1\n- **Sifting and flavour.** No peer-reviewed sensory test of sifted grinds for pour-over or espresso was found; the one French press study found sieved fractions extracted less.11\n- **Surface area or permeability.** Two espresso studies found that grind affects the cup mainly through flow, not surface area2,8; how far this holds for low-pressure brews is not established.\n- **Grinder comparisons.** Published distributions for named grinder types at matched settings are scarce in peer-reviewed work; the textbook chapter on grinding shows examples but no tabulated values.3\n\n## Key studies\n\n
\n

Mo et al., 2023 – Microstructure of coffee beds. Measures four grinds of one arabica blend, from espresso to drip filter, by laser diffraction and X-ray microtomography, and simulates flow through the imaged beds. The fines peak stayed at about 25–50 µm, the size of a cell, while the coarse peak moved with the grind; more fines meant lower permeability.

\n

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

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\n\n
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Smrke et al., 2024 – Fines in espresso. Grinds one coffee at six settings and adds sieved fines in steps, then follows flow, yield, aroma and sensory scores. The share of fines mainly controlled bed permeability and shot time, with little effect on extraction efficiency.

\n

Scientific Reports 14, 5612 · doi:10.1038/s41598-024-55831-x

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Moroney et al., 2019 – Uniformity in the coffee bed. Models extraction with two domains of accessible and less accessible coffee, in a cylindrical and a conical bed, and compares two measured grinds. Fines, with their larger surface-to-volume ratio, extract faster than coarse grains; for the fine grind, the model predicted about 5 points of yield variation within a cylindrical bed.

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PLOS ONE 14(7), e0219906 · doi:10.1371/journal.pone.0219906

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Angeloni et al., 2023 – Sieved versus bimodal grinds. Brews French press coffee from sieve fractions and from ordinary grinds. The homogeneous fractions gave less dissolved solids and fewer bioactive compounds than bimodal grinds; a very fine homogeneous grind gave the fewest of these compounds.

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European Food Research and Technology 249(5), 1247–1256 · doi:10.1007/s00217-023-04210-3

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Lee et al., 2023 – Uneven extraction. Models two flow paths through an espresso bed with different porosity. Extraction opens the faster path further, so differences grow during the shot; this can explain why yield falls at very fine grind settings.

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Physics of Fluids · doi:10.1063/5.0138998

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