{ "id": "grinding", "url": "https://coffearium.org/entries/grinding/", "license": "CC-BY-4.0", "title": "Grinding", "summary": "Why roasted coffee is ground before brewing – it breaks open cells and shortens the paths that solubles must travel – and what grind size, freshness, heat, moisture and static change in the cup.", "domain": "grinding", "kind": "Process", "stage": "brew", "topics": [ "science", "history" ], "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": [ "According to a coffee science textbook, grinding turns roasted beans into particles with a large surface, open pores and short paths that make the solubles accessible.1", "In a 2016 extraction model, broken cells at particle surfaces release their contents fast, while intact cells inside larger particles release theirs by slow diffusion.2", "According to a coffee science textbook, adjusting the grind changes percolation speed and contact time; too fine a grind may hinder percolation.3", "In one study, up to 75% of the gas trapped in freshly roasted coffee escaped during grinding and within the following 90 s.7", "In a 2016 study, grinding cold beans gave a narrower particle size distribution and a smaller most common particle size (mode).9" ], "quickFacts": [ { "label": "What grinding does", "value": "Opens cells, enlarges the surface, shortens diffusion paths" }, { "label": "Grind by method (textbook)", "value": "Turkish fine; pour-over, espresso and moka pot medium; French press and cold brew coarse" }, { "label": "Measured grind peaks (one study)", "value": "Espresso ≈400 µm; lungo setting ≈600 µm; French press and filter machine ≈1,000–1,025 µm" }, { "label": "Gas lost on grinding", "value": "Up to 75% of trapped gas within 90 s (freshly roasted, espresso fineness; one study)" }, { "label": "Bean temperature", "value": "Colder beans gave a narrower, finer grind (one study)" }, { "label": "Static charge", "value": "Depends mainly on the moisture inside the beans (one study)" } ], "aka": [ "Coffee grinding", "Grind size", "Grind setting" ], "linkTerms": [ "grinding", "grind size", "grind setting", "ground coffee", "roast and ground coffee", "grinding on demand" ], "related": [ "grinder-types-and-burrs", "particle-size-and-fines", "brew-methods", "extraction-yield-and-tds", "espresso-machine", "degassing", "freshness-and-storage", "what-extraction-is", "extraction-variables", "espresso", "puck-preparation" ], "contested": [ "Grind size and extraction yield in full immersion: a coffee science textbook states that small particles expose more surface and permit a more efficient extraction3, while an equilibrium study of full immersion brews found that grind size had only a minor effect, with yields of about 19–23% across median particle sizes of 579–1,311 µm.4 The textbook describes the rate of extraction in general; the study measured brews left to reach equilibrium.", "How much gas grinding releases at once: a grinding chapter describes a large part of the trapped gas as liberated instantly when the larger pores are opened1, while a gravimetric study measured up to 75% for freshly roasted coffee at espresso fineness, within 90 s.7 Roast, grind and method differ; the degassing entry compares further measurements." ], "corrections": [], "sources": [ { "id": "s1", "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 smaller the particle, the bigger is the specific surface, and the better is the extractability.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s2", "title": "Asymptotic analysis of the dominant mechanisms in the coffee extraction process", "authors": "Moroney, K. M.; Lee, W. T.; O’Brien, S. B. G.; Suijver, F.; Marra, J.", "year": 2016, "publisher": "SIAM Journal on Applied Mathematics 76(6), 2196–2217", "url": "https://researchrepository.ul.ie/server/api/core/bitstreams/265842cc-fa84-4a4d-ab2e-01860768631f/content", "doi": "10.1137/15M1036658", "tier": "A", "quote": "The kernels of larger grains consist of intact cells with a much higher mass transfer resistance.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s3", "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": "s4", "title": "An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee", "authors": "Liang, J.; Chan, K. C.; Ristenpart, W. D.", "year": 2021, "publisher": "Scientific Reports 11, 6904", "url": "https://www.nature.com/articles/s41598-021-85787-1", "doi": "10.1038/s41598-021-85787-1", "tier": "A", "quote": "The coffee grind particle size did have a very minor effect on TDS.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s5", "title": "Comparison of nine common coffee extraction methods: instrumental and sensory analysis", "authors": "Gloess, A. N.; Schönbächler, B.; Klopprogge, B.; D’Ambrosio, L.; Chatelain, K.; Bongartz, A.; Strittmatter, A.; Rast, M.; Yeretzian, C.", "year": 2013, "publisher": "European Food Research and Technology 236, 607–627", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/4251/2/2013_Gloess-etal_Comparison-of-nine-common-coffee-extraction-methods.pdf", "doi": "10.1007/s00217-013-1917-x", "tier": "A", "quote": "The particle size distributions of the respective milling degrees 3 and 7 of Espresso Grinder KED 640 (Ditting, Switzerland) and of milling degree 8 of the retail grinder KR 805 (Ditting, Switzerland) were analyzed with laser diffraction at DELICA AG (Switzerland).", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s6", "title": "Analysis of Volatile Compounds Released during the Grinding of Roasted Coffee Beans Using Solid-Phase Microextraction", "authors": "Akiyama, M.; Murakami, K.; Ohtani, N.; Iwatsuki, K.; Sotoyama, K.; Wada, A.; Tokuno, K.; Iwabuchi, H.; Tanaka, K.", "year": 2003, "publisher": "Journal of Agricultural and Food Chemistry 51(7), 1961–1969", "url": "https://pages.uoregon.edu/chendon/coffee_literature/old_literature/2003%20J.%20Agric.%20Food%20Chem.,%20Volatiles%20released%20during%20grinding.pdf", "doi": "10.1021/jf020724p", "tier": "A", "quote": "the volatile compounds generated during the grinding of roasted coffee beans were rich in nutty- and smoke-roast aromas", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s7", "title": "Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee", "authors": "Smrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C.", "year": 2018, "publisher": "Journal of Agricultural and Food Chemistry 66(21), 5293–5300", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/12175/2/2017_Smrke-etal_Roasted-coffe-degassing-time-resolved-gravimetry.pdf", "doi": "10.1021/acs.jafc.7b03310", "tier": "A", "quote": "Compared to the amount of degassing from whole beans, up to 75% of the trapped gas was released during and immediately after grinding (within 90 s, until the first measurement), for freshly roasted coffee.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s8", "title": "Protecting the Flavors – Freshness as a Key to Quality (chapter 14)", "authors": "Yeretzian, C.; Blank, I.; Wyser, Y.", "year": 2017, "publisher": "The Craft and Science of Coffee (ed. B. Folmer), Academic Press, 329–353", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00014-1", "doi": "10.1016/B978-0-12-803520-7.00014-1", "tier": "B", "quote": "In general, these processes are accelerated when beans are ground and when stored at elevated storage temperature.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s9", "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.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s10", "title": "Water content of roasted coffee: impact on grinding behaviour, extraction, and aroma retention", "authors": "Baggenstoss, J.; Perren, R.; Escher, F.", "year": 2008, "publisher": "European Food Research and Technology 227(5), 1357–1365", "url": "https://agris.fao.org/search/ru/records/65df955b7c7033e84bee3b18", "doi": "10.1007/s00217-008-0852-8", "tier": "A", "quote": "Coffees with high moisture content exhibited coarser particles upon grinding", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s11", "title": "Moisture-controlled triboelectrification during coffee grinding", "authors": "Méndez 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.", "year": 2024, "publisher": "Matter 7(1)", "url": "https://par.nsf.gov/servlets/purl/10508770", "doi": "10.1016/j.matt.2023.11.005", "tier": "A", "quote": "We demonstrate that whole-bean coffee charging primarily depends on the moisture content of the beans themselves.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s12", "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": "maximum extraction yield is reached for espressi with > 40 s extraction time", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s13", "title": "Turkish coffee culture and tradition", "authors": "UNESCO Intangible Cultural Heritage", "year": 2013, "publisher": "UNESCO", "url": "https://ich.unesco.org/en/RL/turkish-coffee-culture-and-tradition-00645", "tier": "A", "quote": "The freshly roasted beans are ground to a fine powder;", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s14", "title": "Station 3, Room 1, 1st Floor: Coffee Grinders and Technology (audio guide)", "authors": "Kaffeemühlenmuseum Wiernsheim", "publisher": "museum.de", "url": "https://www.museum.de/audioguide/422/3/EN", "tier": "B", "quote": "Even the earliest coffee mills had a grinding mechanism that was manually adjustable.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s15", "title": "Coffee Mill (object record, Enterprise Manufacturing Company)", "authors": "National Museum of American History, Smithsonian Institution", "publisher": "Smithsonian Institution", "url": "https://si.edu/object/coffee-mill:nmah_312114", "tier": "A/B", "tierNote": "A for its own catalogue data (maker, dates made, patent dates, materials); B otherwise", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s16", "title": "We Consumers – Tastes, Rituals, and Waves", "authors": "Morris, J.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 19 (p. 465 cited)", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00019-0", "doi": "10.1016/B978-0-12-803520-7.00019-0", "tier": "B", "quote": "In the 1870s, the majority of coffee was still sold loose in the green for home roasting and grinding;", "accessed": "2026-10-15T00:00:00.000Z" } ], "markdown": "## Why coffee is ground\n\nA whole roasted bean gives up little to water. Grinding breaks it into small particles with a large surface, open pores and short distances for the soluble substances to travel.1 These substances are then accessible for extraction.1\n\n**Surface area.** The smaller a particle, the larger its surface per unit of volume (its specific surface).1 For an idealised sphere, surface divided by volume equals 6 divided by the diameter, so halving the diameter doubles the specific surface (geometry; our arithmetic). Small particles expose more surface to water and extract more efficiently, according to a coffee science textbook.3\n\n**Opened cells.** Roasted coffee is made of cells. In a 2016 model of coffee extraction, the surfaces of ground particles consist mostly of broken cells, whose contents are released quickly.2 The core of a larger particle still consists of intact cells, which release their contents much more slowly.2 The finest particles, called fines, are fragments of broken cells.2\n\n**Two speeds of extraction.** In the same model, extraction runs in two stages. A rapid start from the broken surfaces is followed by much slower diffusion out of the intact grains.2 In one espresso study, shots of under 10 s and of 15 s already reached 17–18% extraction, more than 80% of the highest yield observed.12 The authors take the maximum yield for their recipe to be reached with shots longer than 40 s.12\n\n
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A · Whole roasted bean

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  1. Surface. Water reaching a whole bean meets only its outside; grinding creates the surface, open pores and short paths that make the solubles accessible.1
    2. \n
  2. Closed cells. Intact cells resist the transfer of their contents much more than broken ones.2 The bean is drawn cut open only to show them.
    3. \n
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B · Ground coffee

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  1. Broken surface cells. Grinding opens the cells at the particle surface; their contents dissolve rapidly.2
    2. \n
  2. Fines. Small fragments of broken cells, produced during grinding.2
    3. \n
  3. Intact core. Solubles leave by slow diffusion through the grain.2 The smaller the particle, the shorter this path.1
    4. \n
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What grinding does to a roasted bean: it opens cells at every new surface and leaves intact cells inside larger particles. Schematic, not to scale.
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\n\n## What grind size controls\n\nGrind size changes how fast coffee extracts. In a percolation brew, it also sets how long the water stays in contact with it.\n\n**Flow through the bed.** In a pour-over, adjusting the grinder changes how fast water percolates and how long it is in contact with the coffee.3 A finer grind leaves smaller spaces between the particles, which raises the resistance of the bed and slows the flow.3 Too fine a grind may hinder an efficient distribution of water, possibly leading to over-extraction.3 Fines can also clog the pores between larger particles or in filters, and pass into the cup.1\n\n**Contact time.** According to the grinding chapter of a coffee science textbook, the longer the contact time, the coarser the coffee should be ground.1 It states that very fine particles may be over-extracted, which gives higher bitterness, and that a short contact time, as in espresso, limits this.1\n\n**Full immersion.** In immersion brews left to reach equilibrium, grind size changed the result only slightly. In one equilibrium study (brew ratio 15, 99 °C), the strength of full immersion brews was 1.36 ± 0.09% total dissolved solids (TDS) over median particle sizes of 579–1,311 µm.4 Finer grinds gave slightly higher extraction yields, but the spread stayed within about 19–23%.4 See Contested.\n\n**Not size alone.** To match the same flow time, different grinders need different settings, average sizes and size distributions, according to the grinding chapter.1 The shape and spread of the distribution are covered in the particle size and fines entry.\n\n## Grind size by method\n\nEach brew method needs its own grind size.3 Textbooks describe sizes in words; studies report them as measured particle sizes.\n\n- **In words.** A textbook lists boiled (Turkish) coffee as fine; pour-over, espresso and moka pot as medium; French press and cold brew as coarse.3 For Turkish coffee, freshly roasted beans are ground to a fine powder.13\n- **Espresso.** The grinding chapter names espresso as possibly the only method where a certain share of fines is deemed necessary, for pressure build-up, body and crema.1\n- **Measured.** In a nine-method comparison, the espresso grind peaked at about 400 µm. A setting close to the one used for lungo peaked at 600 µm.5 The coarsest setting, used for French press, Karlsbad pot and filter machine, peaked at 1,000–1,025 µm.5 An aroma study chose 400–800 µm as suitable for paper drip brewing.6\n\n
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Grind sizes in published studies. Rows 1–3: peak of the size distribution by laser diffraction.5 Row 4: a stated size range for paper drip.6 Row 5: median sizes tested in full immersion.4 The measures differ; compare within a row.
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\n\nGrinder settings are not sizes. Grinders differ, so a setting number on one says little about the particle size on another.1 How grinders and burrs work is covered in the grinder types and burrs entry.\n\n## Freshness after grinding\n\nGround coffee loses gas and aroma far faster than whole beans.\n\n**Gas.** In one study, up to 75% of the trapped gas escaped from freshly roasted coffee during grinding and the next 90 s.7 The ground coffee then went on releasing gas, up to 3.0 mg/g for arabica and 3.9 mg/g for robusta.7 Why roasted coffee holds gas is explained in the degassing entry.\n\n**Aroma.** Evaporation of aroma compounds, aroma reactions and oxidation all run faster once beans are ground, according to a freshness chapter.8 The chapter reports an earlier headspace study in which several key aroma compounds fell markedly shortly after grinding.8 In a 2003 study, volatiles captured during grinding itself were richer in nutty and smoky-roast aromas than the headspace of ground coffee.6\n\n**Grinding on demand.** Grinders in cafés and homes usually grind at the moment the coffee is needed, which preserves its freshness best.1 At a point of sale, and even less at home, there is no protective nitrogen atmosphere during or after grinding.1 For a barista grinding small amounts, the grinding chapter sees no need to let the grounds rest.1 How to store coffee is covered in the freshness and storage entry.\n\n\n\n## Extraction kinetics: surfaces and cores\n\n**Two domains.** An asymptotic analysis of coffee extraction attributes the fast early stage to fine particles and to broken cells, both with low mass transfer resistance.2 After grinding, the surfaces of the grains consist mostly of broken cells.2 The kernels of larger grains are intact cells with a much higher resistance, and their coffee leaves by slow diffusion through the tortuous grain.2 The authors report a significantly higher initial outflow concentration for fine grinds than for coarser ones.2\n\n**What this means for grind size.** Because the fast stage comes from fines and broken cells, a finer grind extracts faster at the start.2 The textbook view that small particles extract more efficiently3 describes this rate effect. At equilibrium, in full immersion, the end point depended little on grind size.4\n\n**Bed properties.** According to the grinding chapter, a finer grind packs more densely, lowers the porosity of the bed and raises the pressure drop during percolation.1 A wide or bimodal distribution lowers porosity further than uniform particles do.1 Smaller particles also release gas faster and become more sensitive to oxidation.1 The behaviour of espresso beds under pressure is covered in the espresso machine entry.\n\n**Fines in espresso.** In a 2024 espresso study with six grinder settings (burr spacings of 160–250 µm), most of the extraction happened early: shots under 10 s and of 15 s gave 17–18% yield, and the maximum came only above 40 s.12 For the one coffee tested, the authors judged the flavour profile best at about 30 s and 19–20% yield; this was not a systematic sensory test.12 The role of fines in detail belongs to the particle size and fines entry.\n\n## Heat and bean temperature\n\n**Heat during grinding.** According to the grinding chapter, warmer ground coffee gives faster flow times.1 It was once explained by thermal expansion of the grinding mechanism; the chapter states that more recent studies found modern grinders dimensionally stable, and that the bean itself deforms more plastically when warmer or more humid, giving a coarser grind.1 A 2016 study proposes that burrs warming over a working day explain why baristas adjust the grind, through warming of the particles at the burr surface; it did not measure burr temperature.9 An aroma study could not exclude a temperature rise during grinding as a cause of its results.6 No study opened for this entry measured the temperature rise in a coffee grinder, so no figure is given.\n\n**Bean temperature.** The 2016 study ground the same coffee at room temperature, from a freezer, from dry ice and from liquid nitrogen, on one flat-burr grinder.9 Colder beans gave a narrower particle size distribution and a smaller most common particle size (mode).9 The authors suggest a shattering transition in some components of the bean between room and freezer temperature, and call this a possibility still under study.9 They expect the finer grind to extract faster and the more uniform one to waste less of the bean, but did not test extraction or flavour.9 According to the grinding chapter, the finest industrial grinding preferably adds liquid nitrogen, which makes the bean more brittle and protects aroma.1\n\nWhether grinding frozen beans improves the cup has not been tested in any study opened for this entry (see Open questions). Storage in the freezer is covered in the freshness and storage entry.\n\n## Moisture and roast degree\n\n**Moisture.** In a 2008 study, roasted coffees with a high moisture content gave coarser particles on grinding and needed an equilibration time before grinding.10 Coffees with low water content extracted more effectively, and percolation through them was slower.10 Higher moisture also lowered aroma stability during closed storage.10 According to the grinding chapter, water used to quench a roast first wets the bean surface and needs a resting time to spread evenly before grinding.1\n\n**Roast degree.** The grinding chapter names roast degree as the most decisive property of the beans for grinding.1 Darker roasts, especially fast ones, are more porous and brittle and grind easily, but give a wider distribution with more fines.1 Slowly roasted beans are denser and more homogeneous; they need more energy to grind but give a more even result.1\n\n## Static and retention\n\n**Static.** Grinding charges coffee particles electrically, and charged boulders and fines cling together.11 In a 2024 study, the charging of whole-bean coffee depended mainly on the moisture inside the beans.11 Lighter roasts with more than 2% internal moisture (basis not stated) charged positively; drier coffees, which tended to be darker roasts, charged mainly negatively.11 A small amount of water added to the beans before grinding reduced the charge and immediately reduced clumps of boulders and fines.11 In the same study, an espresso (18.0 g in, 45.0 g out) ground with added water reached 8.9% TDS against 8.2% without, and the shot ran nearly 50% longer.11\n\n**Retention.** The grinding chapter lists, among the requirements for grinders, avoiding segregation and the build-up of fines, for example by reducing electrostatic charging.1 No study opened for this entry measured how much ground coffee stays in a grinder between doses.\n\n**Dosing.** Point-of-sale grinders dose in three ways: pre-weighed beans, a timer, or a load cell that stops the grinder at a set mass.1 The chapter describes grinder development moving towards weight dosing, to lock the ratio of coffee to beverage.1\n\n## Reference values\n\n| Parameter | Value | Conditions | Source |\n|---|---|---|---|\n| Grind peak, espresso | ≈400 µm (full width at half maximum 220 µm) | Laser diffraction; semi-automatic machine set-up | Gloess et al.5 |\n| Grind peak, lungo | ≈600 µm | Same study; measured at setting 7, lungo brewed at 7.25 | Gloess et al.5 |\n| Grind peak, coarsest setting | 1,000–1,025 µm | Same study; French press, Karlsbad pot, filter machine | Gloess et al.5 |\n| Particle size for paper drip | 400–800 µm | Chosen for an aroma study | Akiyama et al.6 |\n| Full immersion TDS across grind sizes | 1.36 ± 0.09% | Median particle sizes 579–1,311 µm, brew ratio 15, 99 °C, at equilibrium | Liang et al.4 |\n| Full immersion yield across grind sizes | About 19–23%, slightly higher when finer | Same series | Liang et al.4 |\n| Gas lost during and just after grinding | Up to 75% of trapped gas | Freshly roasted, espresso fineness, within 90 s | Smrke et al. 20187 |\n| Gas released by grounds afterwards | Up to 3.0 mg/g (arabica); up to 3.9 mg/g (robusta) | Espresso fineness | Smrke et al. 20187 |\n| Espresso yield, early shots | 17–18% at under 10 s and 15 s; > 80% of the maximum observed | 20 g to 40 g, 9 bar, burr spacings 160–250 µm | Smrke et al. 202412 |\n| Espresso with water added before grinding | 8.9% TDS vs 8.2% dry; shot nearly 50% longer | 18.0 g in, 45.0 g out, 7 bar, 94 °C | Méndez Harper et al.11 |\n\n## History of grinding\n\nOnly dated facts with a source that meets our rules are given; the history of grinder designs is incomplete here.\n\n- **Mortar and pestle.** Before the coffee mill, roasted beans were pulverised with a mortar and pestle, according to a coffee grinder museum.14 The museum describes this method as still used in the Ethiopian coffee ceremony (as of 2026).14\n- **Fine powder and stones.** Turkish coffee is made from beans ground to a fine powder.13 According to the grinding chapter (2017), stone grinders are traditionally used for Turkish coffee.1\n- **Mills.** Even the earliest coffee mills had a manually adjustable grinding mechanism, according to the museum.14 A cast-iron coffee mill with a steel grinder in the Smithsonian collection, made in Philadelphia between 1876 and 1898, carries patent dates of 21 October and 9 December 1873.15\n- **From home to factory.** A history chapter describes coffee in the United States as once sold mostly green, for roasting and grinding at home, and later pre-roasted and usually ready-ground.16\n- **Industrial and on-demand grinding.** According to the grinding chapter (2017), the roller grinder can still be considered state of the art for industrial grinding, while café and home grinders grind on demand.1 No source opened for this entry dates the first electric coffee grinder.\n\n## Open questions\n\n- **Frozen beans in the cup.** Grinding cold narrowed the particle size distribution in one study, which did not test extraction or flavour.9 No peer-reviewed comparison of brews from frozen and room-temperature beans was found.\n- **Heat in practice.** How much a grinder warms in service, and how much that changes the grind, has not been measured in any study opened for this entry.1,9\n- **Retention.** No published measurement of grinder retention was found.\n- **Grind and flavour.** The grinding chapter calls for more scientific studies of the effect of grinding on extraction and cup quality, which it describes as empirically perceived.1\n\n## Key studies\n\n
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Moroney et al., 2016 – Dominant mechanisms in coffee extraction. Analyses a multiscale extraction model mathematically. Ground particles have surfaces of mostly broken cells and kernels of intact cells; extraction starts fast from the broken cells and fines and then slows to diffusion out of the intact kernels.

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SIAM Journal on Applied Mathematics 76(6), 2196–2217 · doi:10.1137/15M1036658

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Uman et al., 2016 – Bean origin and temperature. Grinds coffees of different origins and processes, and one coffee at four temperatures down to liquid nitrogen, and measures the particle size distributions. Origin and processing did not change the distribution, while colder beans gave a narrower, finer grind.

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Scientific Reports 6, 24483 · doi:10.1038/srep24483

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Smrke et al., 2018 – Gravimetric degassing. Weighs whole and ground roasted coffee continuously in containers with a capillary outlet. Grinding freshly roasted coffee to espresso fineness released up to three quarters of the trapped gas within 90 s.

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

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Méndez Harper et al., 2024 – Static in coffee grinding. Measures the charge on ground coffee across many coffees. Charging depended mainly on the moisture inside the beans, and a little water added before grinding reduced both charge and clumping and raised espresso strength in the authors’ test.

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Matter 7(1) · doi:10.1016/j.matt.2023.11.005

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Liang et al., 2021 – Full immersion at equilibrium. Models and measures full immersion brews to equilibrium. Grind size changed strength and yield only slightly across a wide range of median particle sizes.

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Scientific Reports 11, 6904 · doi:10.1038/s41598-021-85787-1

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