{ "id": "grinder-types-and-burrs", "url": "https://coffearium.org/entries/grinder-types-and-burrs/", "license": "CC-BY-4.0", "title": "Grinder types and burrs", "summary": "How coffee grinders break beans – blades, conical burrs, flat burrs and industrial roller mills – and how the shape, gap, wear and temperature of the grinding parts change particle sizes, fines, heat and retention.", "domain": "grinding", "kind": "Equipment", "stage": "brew", "topics": [ "equipment", "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": [ "Blade grinders chip beans apart and give a defined grind size only if the grinder has a sieve; burr grinders break beans in a gap between two grinding parts, called burrs.1,7", "In conical and flat burr grinders, one burr turns and one is held; the distance between them sets the grind size.7,8,9", "Industrial grinding mostly uses roller mills: pairs of rolls turning at different speeds, often in several stages, followed by a normaliser.1,10,11", "For a given median grind size, different grinding technologies can give a different share of fines, particles below 100 µm.4", "Weighing the grounds with a load cell in the grinder replaces dosing by time, which only estimates the dose.1,12" ], "quickFacts": [ { "label": "Blade grinder", "value": "Steel blades chip beans; no defined size without a sieve (textbook)" }, { "label": "Burr grinders", "value": "Conical (a cone working against a second burr) or flat (two facing discs); one burr turns, one is held" }, { "label": "Industrial grinding", "value": "Roller mills with one or more pairs of rolls; normaliser below the rolls" }, { "label": "Grind size set by", "value": "Gap between the burrs or rolls, adjusted along the axis or across the rolls" }, { "label": "Bean temperature", "value": "Mode of the size distribution 31% smaller from room temperature to about −200 °C (one study)" }, { "label": "Static", "value": "Charge set mainly by the beans’ internal moisture; a little water on the beans cut retention to near zero (one study)" }, { "label": "Dosing", "value": "Pre-weighed beans, timer, or load cell that stops the grinder at a set mass" } ], "aka": [ "Coffee grinder", "Burr grinder", "Coffee mill" ], "linkTerms": [ "burr grinder", "burr grinders", "blade grinder", "conical burr", "conical burrs", "flat burr", "flat burrs", "roller mill", "roller grinder", "grinding burrs", "normaliser", "grind by weight", "grinder retention" ], "related": [ "grinding", "particle-size-and-fines", "degassing", "espresso-machine", "brew-methods", "extraction-yield-and-tds", "freshness-and-storage", "puck-preparation", "espresso" ], "contested": [ "Why grinders drift as they warm up: a grinding chapter states that warmer ground coffee flows faster because the bean deforms more plastically, and that modern grinders stay dimensionally stable1; a 2016 study proposes warming of particles at the interface with a warm burr as the reason baristas re-adjust during the day.3 Both point to the coffee rather than the mechanism, but neither measured burr or particle temperature in use." ], "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": "A defined grind size can only be achieved when the grinder is equipped with a sieve with a specific mesh size.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s2", "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": "Depending on the type (and quality) of the grinder and on the coffee blend and roasting degree, various particle size distributions will be obtained.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s3", "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": "We find that the particle size distribution is independent of the bean origin and processing method.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s4", "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": "For a given median grind size, different grinding technologies can yield a different share of fines (particles < 100 µm).", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s5", "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": "s6", "title": "Wear Monitoring of Coffee Grind-On-Demand Burrs Using Precision Sieving and Laser Diffraction", "authors": "Griffiths, C.; Adams, G. C.; Rees, A.", "year": 2018, "publisher": "International Journal of Food and Bioscience 1(1), 34–41", "url": "https://cronfa.swansea.ac.uk/Record/cronfa45569", "tier": "B", "tierNote": "Journal article in a university repository (author version); peer review is not stated, so it is treated as B", "quote": "the results demonstrate that there is variation in the precision control of particle size distribution.", "abstractOnly": true, "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s7", "title": "Coffee grinder (US Patent 5,201,474)", "authors": "Midden, W. E. (assignee Bunn-O-Matic Corporation)", "year": 1993, "publisher": "United States Patent and Trademark Office; filed 3 August 1990, granted 13 April 1993", "url": "https://patents.google.com/patent/US5201474A/en", "tier": "A/B", "tierNote": "A for how the burrs are built; B for its background on home grinders", "quote": "adjusting the axial spacing or gap between the rotatable burr 28 and stationary burr 26", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s8", "title": "Coffee-mill (US Patent 461,784)", "authors": "Morgan, C. (assignor to the Arcade Manufacturing Company)", "year": 1891, "publisher": "United States Patent Office; filed 26 March 1891, granted 20 October 1891", "url": "https://patents.google.com/patent/US461784A/en", "tier": "A", "quote": "either raises or lowers it, thus adjusting the millto grind to any desired degree of fineness.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s9", "title": "Conical burr grinder (WO 2011/109873 A1)", "authors": "Psarologos, C.; Davenport, D.; Hoare, R. (applicant Breville Pty Ltd)", "year": 2011, "publisher": "World Intellectual Property Organization; priority 12 March 2010, published 15 September 2011", "url": "https://patents.google.com/patent/WO2011109873A1/en", "tier": "A", "quote": "The spacing between the upper burr and the lower burr determines the grind size.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s10", "title": "Bi-modal roller grinder (US Patent 8,091,813 B2)", "authors": "Kirschner, J.; Heinsz, L. J. (assignee The Coca-Cola Company)", "year": 2012, "publisher": "United States Patent and Trademark Office; priority 7 September 2005, granted 10 January 2012", "url": "https://patents.google.com/patent/US8091813", "tier": "A/B", "tierNote": "A for how the grinder is built; B for its background on burr grinders", "quote": "The grinder 250 also may include a densifier 310.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s11", "title": "Roller mill for coffee (US 2012/0037735 A1)", "authors": "Holl, W.; Simon, G. (applicant Bauermeister Zerkleinerungstechnik GmbH)", "year": 2012, "publisher": "United States Patent and Trademark Office; priority 16 August 2010, published 16 February 2012", "url": "https://patents.google.com/patent/US20120037735", "tier": "A/B", "tierNote": "A for how the mill is built (rolls, gap, speed ratio); B for its background and taste claims", "quote": "driving the first and second grinding rollers in opposition with a roller peripheral velocity ratio from 1:1.5 to 1:4", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s12", "title": "Coffee grinder with weight control device of the dose of ground coffee (WO 2020/089706 A1)", "authors": "Faggioni, G.; Faggioni, C. M.; Vatti, S. G.; Vatti, F. P.; Faggioni, M. (applicant DRM SRL)", "year": 2020, "publisher": "World Intellectual Property Organization; priority 30 October 2018, published 7 May 2020", "url": "https://patents.google.com/patent/WO2020089706A1/en", "tier": "A/B", "tierNote": "A for how the weighing grinder is built; B for its background on dosing by time and grinding on demand", "quote": "The load cell 16 and the overlying pressure bar 15 are housed inside the base of the coffee grinder 1.", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s13", "title": "Coffee grinder-doser apparatus with independent fork and weighing device with feedback (US Patent 11,058,258 B2)", "authors": "Alessi, R.; Oddera, M.", "year": 2021, "publisher": "United States Patent and Trademark Office; filed 31 May 2018, granted 13 July 2021", "url": "https://patents.google.com/patent/US11058258B2/en", "tier": "A/B", "tierNote": "A for how the apparatus is built; B for its background on grind-by-time dosing", "quote": "in the conventional solutions of the grind-by-time type the duration of grinding is fixed", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s14", "title": "Variable speed coffee grinder (US 2016/0220067 A1)", "authors": "Teahan, M.", "year": 2016, "publisher": "United States Patent and Trademark Office; priority 30 January 2015, published 4 August 2016", "url": "https://patents.google.com/patent/US20160220067A1/en", "tier": "A/B", "tierNote": "A for how the drive is built; B for its background statements on grinding speed, which are not measurements", "quote": "a variable speed motor whose speed and starting torque characteristics can be adjusted via a programmable controller 16", "accessed": "2026-10-15T00:00:00.000Z" }, { "id": "s15", "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" } ], "markdown": "## Blades and burrs\n\nA grinder breaks roasted beans into particles small enough to brew. The grinding entry explains why; this entry covers the machines.\n\n**Blade grinders.** A blade grinder works like a kitchen blender.1 Steel blades spin and chip the beans apart.1 According to a coffee science textbook, a defined grind size is only possible if the grinder has a sieve with a set mesh.1\n\n**Burr grinders.** A burr grinder feeds beans into a gap between two grinding parts with teeth, called burrs.7 One burr is driven and turns; the other is held still.7,9 The spacing between the burrs sets the grind size.7,9 Turning a dial moves one burr closer to the other or further away.9\n\nGrinding always makes particles of many sizes and shapes. This spread of sizes is called the particle size distribution (PSD).1 Most ground coffee has two peaks: one of fines and one near the average size.1,4 Fines are often defined as particles below 100 µm.1,4 The type and quality of the grinder change this spread.2 The particle size and fines entry explains how the spread is measured and what it does in the cup.\n\n## Conical, flat and roller\n\n**Conical burrs.** A cone-shaped burr works against a second burr; one turns and the other is held.8,9,14 In one patented grinder, an outer burr turns around a stationary inner burr.14 In a mill patented in 1891, a crank turned the grinding cone on its spindle.8 The grinding shell was fixed to the body, and raising or lowering the spindle set the fineness.8 According to the textbook, conical burrs are made of tool steel or ceramics. Small sets are mainly found in home grinders, larger ones in busy espresso bars.1\n\n**Flat burrs.** Two toothed discs face each other.7 In one patented design, the teeth are angled so that the two burrs cut like scissors.7 The grooves are deepest near the centre and shallow towards the edge.7 Beans are fed in from above, and the ground coffee leaves all around the rim of the turning burr.7 According to the textbook, flat burrs are used everywhere, from home grinders to packing lines in small and medium roasteries.1\n\n**Roller mills.** Industrial plants grind between rolls rather than burrs.1,11 Two rolls turn towards each other, with a grinding gap between them.11 Several pairs can follow each other in stages.1,10 According to the textbook, the roller mill was still the state of the art for industrial coffee grinding as of 2017.1\n\n
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A · Conical burrs

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  1. Beans in. From the hopper above the burrs.8
    2. \n
  2. Inner cone. Turns on a spindle, driven by a crank or a motor.8,9
    3. \n
  3. Gap. Where the beans are broken; moving one burr along the axis sets the grind size.8,9
    4. \n
  4. Outer burr. Held still; in the 1891 mill, a grinding shell fixed to the body.8
    5. \n
  5. Grounds out. Below the burrs.
    6. \n
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B · Flat burrs

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  1. Beans in. Fed into the burrs from above.7
    2. \n
  2. Stationary burr. Its axial position sets the gap.7
    3. \n
  3. Rotating burr. Driven by the motor.7,9
    4. \n
  4. Gap and teeth. Grooves deepest near the centre, shallower towards the rim; angled teeth cut like scissors.7
    5. \n
  5. Grounds out. All around the rim of the rotating burr.7
    6. \n
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C · Roller mill

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  1. First pair of rolls. Breaks the beans; roll surfaces may be grooved or smooth.1,11
    2. \n
  2. Second pair. A further grinding stage; industrial mills can have several.1,10
    3. \n
  3. Gap and speed difference. The rolls of a pair turn towards each other at different peripheral speeds, shown by the arrows of different length.1,11
    4. \n
  4. Normaliser. Mixes and beats the grounds below the rolls and sets their density.1
    5. \n
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Three ways to grind coffee, each cut through its grinding parts. Coffee is shown in colour. Schematic, not to scale.
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\n\n## What changes the grind\n\n**The gap.** The distance between burrs or rolls is the main setting.7,9,11 Settings do not transfer between grinders: to reach the same espresso flow time, different grinders need different settings and give different average sizes.1\n\n**Burr design.** Grinder makers work on the design of the teeth, the casing around the burrs and the spout to shape the PSD, according to the textbook.1 For a given median size, different grinding technologies can give a different share of fines.4 No peer-reviewed study opened for this entry compared conical and flat burrs at matched settings.\n\n**Speed.** The textbook lists the speed of the burrs as a setting a grinder should let the barista adjust.1 Variable-speed drives exist in patented grinders.14 No study opened for this entry measured how burr speed changes the PSD.\n\n**Alignment.** Grinders should allow precise, repeatable setting of the parallelism and position of the grinding tools, according to the textbook.1 No study opened for this entry measured how misaligned burrs change the PSD.\n\n**Wear.** In industrial roller mills, worn rolls give a wider PSD and more fines at the same average size, according to the textbook.1 A university study of grind-on-demand burrs reports that damage and wear made the PSD less precisely controllable.6\n\n**Temperature.** Warmer ground coffee gives faster flow times, according to the textbook. A warmer bean seems to deform more and break coarser.1 In a 2016 study, colder beans gave a narrower PSD and a smaller most common particle size (mode).3\n\n**The beans.** In the same study, four single-origin coffees gave the same PSD whatever their origin and processing.3 According to the textbook, roast degree is the bean property that matters most. Darker and faster roasts are more porous and brittle, and give a wide PSD with many fines.1\n\n## Hand and electric\n\nHand grinders use the same mechanisms as electric ones; the 1891 patent above describes a crank-turned conical mill.8 According to the textbook, small conical burrs are mainly used in home grinders.1 No study opened for this entry compared the PSD of hand and electric grinders.\n\n## Dosing: doser, on demand and by weight\n\n**Grinding on demand.** Café and home grinders mostly grind at the moment the coffee is needed, which best preserves freshness, according to the textbook.1 Ground coffee loses gas fast. In one study, up to 75% of the trapped gas in fresh coffee escaped during grinding and within 90 s.15 No source opened for this entry describes how doser grinders are built. They are therefore not covered further here.\n\n**Three ways to dose.** According to the textbook, point-of-sale grinders dose in one of three ways.1\n\n- **Pre-dosed:** a weighed amount of beans goes in, and all of it is ground.1\n- **By time:** a timer runs the motor for a set time, which gives roughly the right amount.1 One patent states that the dose is calculated from the grinding time, while the real grinding speed varies.12 Another names grind size, coffee type, wear, humidity and temperature as causes of variation.13\n- **By weight:** a load cell weighs the ground coffee and stops the grinder at the set mass.1 In one patented grinder, the load cell sits in the base, under the fork that holds the portafilter.12\n\nWeight dosing helps keep the ratio of coffee to beverage fixed when the espresso machine stops at a set beverage weight.1\n\n\n\n## How burrs break beans\n\n**Fracture and fines.** A bimodal PSD comes not only from the grinder but from the bean’s structure.1 Whenever a roasted bean cracks, very fine particles below 100 µm flake off, and this repeats with every further break.1 In a 2016 laser-diffraction study, some particles were produced in their final size by the first fracture of the whole bean, and only a few small particles came from that first fracture.3 The textbook adds that with other techniques the two peaks can merge, especially when grinding with liquid nitrogen.1\n\n**Cutting versus crushing.** In industrial roll grinders, cutting tends to produce rounder particles and a narrow PSD; pressure and shear give irregular shapes and a wider PSD.1 The textbook attributes this to an older coffee technology handbook and states that the best configuration is found empirically.1 In the patented flat burr above, angled teeth on both burrs give a scissors-like cutting action.7\n\n**Conical and flat in one example.** The textbook shows PSDs from several flat burr grinders and a conical grinder, each set to give the same espresso flow time.1 In its account, the conical grinder gave a wider PSD and needed a clearly coarser median to reach the same flow time.1 One of the chapter’s two authors works for a grinder maker, the curves are drawn without numbers, and no peer-reviewed comparison was found; this entry therefore gives no fines share by burr type.\n\n**Why the share of fines matters.** In a 2024 espresso study, more fines reduced the permeability of the coffee bed, slowing flow and lengthening extraction.4 Within the conditions tested, fines mainly changed permeability rather than how efficiently coffee was extracted.4\n\n## Heat in the grinder\n\nAccording to the textbook, warmer ground coffee gives faster flow times. It was once explained by thermal expansion of the grinding mechanism, but grinders were later found to stay dimensionally constant; the bean itself seems to become more plastic when warm or humid and to break coarser.1 Centrifugal and pin grinders keep some particles in the chamber longer than needed, and tend to produce more heat and a wider PSD than roller grinders.1\n\nThe 2016 study tested bean temperature on a commercial flat-burr grinder fitted with its Turkish burr set, turning at 1,480 rpm at a fixed setting.3 Beans came from liquid nitrogen, dry ice, a freezer and room temperature.3 Cooling from room temperature to about −200 °C reduced the mode of the size distribution by 31%, and the PSD became narrower, with the biggest change between room temperature and −19 °C.3 The authors also marked a fine particulate cutoff, a diameter they defined arbitrarily. Their figure caption gives it as 61 ± 3 µm at −196 °C, 63 ± 3 µm at −79 °C, 73 ± 3 µm at −19 °C and 70 ± 3 µm at 20 °C.3 The paper’s text gives the coldest temperature as −200 °C.3 They suggest that particles warm at the interface with a warm burr and that the burrs grow warmer with use; they call the fracture mechanism behind the temperature effect hard to describe.3\n\n
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The authors’ arbitrarily defined fine particulate cutoff for one coffee ground at four bean temperatures on the same flat-burr grinder and setting; the mode of the distribution fell by 31% from room temperature to about −200 °C.3 Measured by laser diffraction; ±3 µm for each value.
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\n\nNo study opened for this entry measured burr or coffee temperature during normal café grinding, so no temperature rise is given.\n\n## Static and retention\n\nCoffee charges as it is ground. In a 2024 study, the charge of the grounds depended mainly on the moisture inside the beans, which predicted charge slightly better than roast colour.5 Drier coffees charged largely negatively.5 The charge grew as coffee was ground finer; finer settings mean more fracturing, longer contact with the burrs and more fines.5 Negatively charged coffees reached up to about 120 nC per gram.5 The grinder had steel flat burrs of 98 mm.5\n\nAdding a little water to the whole beans before grinding reduced charging systematically.5 It left near-zero grounds retained in the grinder and broke up electrostatic clumps of fines and boulders.5 In espresso brewed from 18.0 g of coffee to 45.0 g of beverage, the shot took nearly 50% longer and reached 8.9% total dissolved solids (TDS) rather than 8.2%.5 According to the textbook, avoiding segregation and fouling of fines, for example by reducing electrostatic charging, is one of the requirements a grinder should meet.1\n\nRetention (coffee left inside the grinder between doses) was not measured in grams in that study, and no other study opened for this entry measured it.\n\n## Industrial grinding for packaged coffee\n\n**Roll stages.** In one patented roller grinder, three pairs of rolls follow each other: crusher rolls, finish rolls spaced to give the target size, and fines rolls, one of them adjustable.10 The textbook describes industrial roller grinders with several stages.1 It describes axial grooves that are asymmetrical, with a sharp and a dull edge, and facing rolls can be paired sharp to sharp, sharp to dull or dull to dull.1 Grooved roll surfaces are usual; one patent instead prefers smooth rolls.11\n\n**Speed difference.** The rolls of a pair turn at different speeds. One patent specifies a ratio of peripheral speeds from 1:1.5 to 1:4 and, for fine coffee powder, a gap of 0.03–0.05 mm.11 According to the textbook, newer roller grinders give each roll its own drive, intended to tailor the PSD, reduce fines and spread wear.1\n\n**Bimodal by design.** A roller grinder can also make two populations on purpose. In the textbook’s account, the lowest pair of rolls can be shifted so that part of the coffee passes without further grinding.1 The textbook cites a patent application for this; the patent granted on that application describes a bi-modal roller grinder.1,10\n\n**Normalising.** Below the rolls, a normaliser mixes, kneads or beats the grounds with paddles or hammers.1 It breaks remaining large particles, cuts up released silverskin, reduces fines by binding them to larger particles, and sets the density of the grounds for silos, packs and capsules.1 In the patented roller grinder, a densifier with blades forms the grinds into a more uniform size and shape.10 Where fines and chaff must be removed entirely, as for soluble coffee, air separation is used.1\n\n**Wear in industry.** At a constant average size, fines rise steadily over the service time of the rolls, according to the textbook; the gaps are re-adjusted and grooved rolls are eventually re-sharpened or replaced.1 Monitoring the fines is described as the best way to follow roll wear.1\n\n**Gas.** Grinding releases trapped gas, including carbon monoxide; the textbook recommends ventilating grinders.1 The degassing entry covers how much gas is released.\n\n## Reference values\n\n| Parameter | Value | Context | Source |\n|---|---|---|---|\n| Fines (definition used) | Particles below 100 µm | Volume share in espresso study; textbook definition | Smrke et al.4, von Blittersdorff & Klatt1 |\n| Bean origin and processing | No effect on PSD | Four single-origin coffees, one flat-burr grinder | Uman et al.3 |\n| Bean temperature | Mode −31% from room temperature to ≈−200 °C; PSD narrower | Same grinder, 1,480 rpm, fixed setting | Uman et al.3 |\n| Fine particulate cutoff by bean temperature | 61, 63, 73, 70 µm (± 3 µm) at −196, −79, −19, 20 °C | Cutoff defined arbitrarily by the authors; one coffee, laser diffraction; values from the figure caption | Uman et al.3 |\n| Static charge | Up to ≈120 nC/g (negative charge) | Grinding experiments, steel 98 mm flat burrs | Méndez Harper et al.5 |\n| Water on beans, espresso | Shot ≈50% longer; 8.9% TDS vs 8.2% | 18.0 g in, 45.0 g out | Méndez Harper et al.5 |\n| Roll speed ratio | 1:1.5 to 1:4 | One roller-mill patent | Holl & Simon11 |\n| Roll gap, fine powder | 0.03–0.05 mm | Same patent | Holl & Simon11 |\n| Roller stages (patent example) | 3 pairs: crusher, finish, fines | Bi-modal roller grinder | Kirschner & Heinsz10 |\n| Gas lost on grinding | Up to 75% of trapped gas within 90 s | Freshly roasted, espresso fineness | Smrke et al. 201815 |\n\nThe textbook also gives burr diameters, throughputs, roll sizes, service lives and densities of ground coffee. Each rests on that one chapter, so they are left out under our number rule.\n\n## Open questions\n\n- **Conical versus flat.** No peer-reviewed study opened for this entry measured the share of fines for conical and flat burrs at matched median size. Claims that one type gives more fines or more “body” remain untested here.\n- **Burr speed.** A patent background states that slower grinding is more precise, but gives no measurement.14 No study opened for this entry tested it.\n- **Alignment and wear in cafés.** How far a burr can be out of parallel before the PSD changes, and how many kilograms a burr set grinds before it does, lack open, peer-reviewed data.\n- **Heat in use.** Burr and coffee temperatures during a busy service have not been measured in the studies opened here.\n- **Hand grinders.** No study opened for this entry compared hand and electric grinders.\n\n## Key studies\n\n
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Uman et al., 2016 – Bean origin and temperature. Grinds four single-origin coffees on one flat-burr grinder and measures the PSD by laser diffraction; origin and processing made no difference. Grinding the same coffee from liquid nitrogen, dry ice, freezer and room temperature showed that colder beans give a narrower distribution and a smaller mode.

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Scientific Reports 6, 24483 · doi:10.1038/srep24483

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Méndez Harper et al., 2024 – Static during grinding. Measures the charge of ground coffee across roasts, moisture contents and grind settings. Internal moisture predicted charge slightly better than roast colour; a little water on the beans cut charge, clumping and retention, and changed espresso flow and strength.

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

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Smrke et al., 2024 – Fines in espresso. Grinds one coffee at several burr spacings, adds sieved fines and follows extraction over the shot. Fines lowered bed permeability and lengthened shots but hardly changed extraction efficiency; the paper notes that grinding technologies differ in their share of fines at a given median.

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

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