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Grinding · Grinding

Static and retention

Grinding charges coffee particles with static electricity, so grounds clump and cling inside the grinder. What sets the charge, what a little water on the beans does, and how much coffee stays behind between doses.

Across the chain: Science · Equipment

Source-checkedby Coffearium verification pass· Last source-checked 27 Oct 2026· Prepared with AI assistance· 8 sources (4 tier A)· 1 contested· Updated 27 Oct 2026· Literature last searched 27 Oct 2026
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Contents
  • In a 2024 study, the static charge of freshly ground coffee depended mainly on the water inside the beans; coffees below about 2% water charged negatively.1
  • Charge grew as coffee was ground finer, and most of it came from fracturing the beans rather than from rubbing.1
  • A little water on whole beans before grinding cut charge and clumping in two studies; no sensory test of its effect on flavour was consulted.1,2
  • Grinder retention is ground coffee left inside the grinder. In one test, about 12% of a 10 g dose stayed behind; water at 10 µL/g lowered this to about 2.5%.2
  • Retained grounds held more fines than the dose that left the grinder; no study consulted measured how stale retained grounds change the next cup.2

Why ground coffee becomes charged#

When beans break in a grinder, the particles carry away electric charge.1 This is static electricity, also called triboelectric charging. Charged particles attract one another and stick to surfaces, so grounds form clumps and cling to the grinder.2,3

In a 2024 laboratory study of about 30 coffees, the charge depended mainly on the water left inside the roasted beans.1 Water content predicted the charge better than roast colour did.1 Coffees with less than about 2% water charged negatively.1 Positive charge appeared only in very light roasts.1 Dark roasts hold less water, so in practice they tended to charge negatively.1

Finer settings gave more charge in the same study.1 A finer grind breaks each bean into more pieces. The authors found that fracture, not rubbing, produced most of the charge.1 How roast degree and water change the grind itself is covered in What affects grinding.

Clumps#

A clump is a small lump of fines and larger particles held together by charge.1 Clumps change how water reaches the coffee in a brew, so extraction can vary.2 Breaking clumps in an espresso basket, for example with a needle tool, is described in Puck preparation.

A little water before grinding#

Some baristas add a few drops of water to the whole beans before grinding. They use a spray bottle or a dropper.3 The practice is known in coffee forums as the Ross droplet technique (RDT).3

Two laboratory studies tested it on one flat-burr grinder. Water on the beans reduced the charge and the clumps at once.1,2 Less coffee stuck inside the grinder.2 Espresso made from the wetted beans ran slower and came out stronger.1,2

The amounts are small. In one study, the authors put the water needed at less than 0.5 mL; their espresso shots used 18.0 g of coffee.2 Too much water may cause caking or corrosion in the grinder, the authors warn; they did not observe this themselves.2

What the studies did not test matters as much. Neither study tasted the coffee, so the effect on flavour is not known.1,2 Both used a single grinder model.1,2

0102030405060water added, µL per g of beansRetention fell from ≈12%to ≈2.5%102 Charge reduced by at least50–60%20–302 0.5 mL, if on one 18 gdose(our calculation)282 Upper bound named in theabstract(0.05 mL/g)502
Data table
water added, µL per g of beansSource
Charge reduced by at least 50–60%20–302
Retention fell from ≈12% to ≈2.5%102
0.5 mL, if on one 18 g dose (our calculation)282
Upper bound named in the abstract (0.05 mL/g)502
Water added to whole beans before grinding, in microlitres per gram of beans, and what it did in a 2024 study on one flat-burr grinder. The 0.5 mL mark is our conversion of the authors’ figure (0.5 ÷ 18 ≈ 0.028 mL/g), assuming it refers to one 18 g espresso dose; the paper does not state the dose it refers to.2

Retention: coffee that stays in the grinder#

Grinder retention is the ground coffee left inside a grinder after a dose. It collects in the space around the burrs and in the chute that leads to the cup.2,7 Static makes fine particles stick to the grinder, so charge and retention are linked.2

Retention matters in two ways. First, the dose that leaves the grinder is lighter than the beans that went in.2 Second, old grounds left inside can reach a later dose, as the background of a grinder patent describes.7 Ground coffee loses gas and aroma fast.5

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Where grounds stay behind in a flat-burr grinder

  1. Hopper. Holds whole beans; for single dosing, one weighed dose.4
  2. Throat. The opening between hopper and burrs.
  3. Stationary burr. The disc that does not turn; its position sets the gap.8
  4. Rotating burr. The disc driven by the motor.8
  5. Grounds retained in the burr chamber. Rich in fines; knocked out by hand.2
  6. Exit chute. Grounds can cling between burrs and chute.7
  7. Cup or portafilter. Receives the dose.7
Section through a flat-burr grinder showing where ground coffee is reported to stay behind: in the burr chamber and in the exit chute. Point at a number or a line of the legend to highlight the part. Schematic, not to scale.2,4,7,8

Single dosing#

Single dosing means weighing the beans for one brew and grinding all of them. A coffee science textbook calls this predosed grinding.4 Grinding on demand, just before brewing, best preserves freshness, according to the same chapter.4

Single dosing only works as intended when little coffee stays behind. Otherwise part of today’s dose remains in the grinder, and earlier grounds may come out with it. One patented grinder drops the grounds straight down into the portafilter, with nothing in between, to avoid retention.7 No study consulted for this entry compared retention across grinder designs.

Charge: sign, size and source#

Measurement. In the 2024 study, a Faraday cup under the grinder chute collected the grounds, and an electrometer read their net charge.1 Each test ground 1 g of beans.1 Because some coffee stayed in the space between the burrs, the charge was divided by the mass that reached the cup, giving the charge-to-mass ratio (Q/m) in nC/g.1 Water content of the roasted beans was measured with a moisture meter (basis not stated), and roast colour on the Agtron scale.1 All grinding was done on one flat-burr grinder with stock 98 mm steel burrs.1

Sign. Q/m changed sign at about 2% water (basis not stated): drier coffees charged negatively, wetter ones positively.1 By colour, positive charging occurred only above Agtron 70.1 Water content was the better predictor of the two.1 Negatively charged coffees reached up to about 120 nC/g; positively charged ones generally stayed below 50 nC/g.1

Grind setting. The magnitude of Q/m increased as coffee was ground finer.1 Coarser settings gave less charge, whether the coffee charged positively or negatively.1

Fracture versus friction. Seven coffees were ground once, then passed through the grinder a second time at a much coarser setting that left the particle sizes nearly unchanged.1 The second pass, with friction but little new fracture, gave up to 90% less charge; most reground coffees carried 5–10 nC/g.1 The authors conclude that most charging comes from fracturing.1 They could not separate rubbing of coffee on coffee from rubbing on the burrs.1

Electrons, not ions. Salt solutions (NaCl at 0.5 and 1.0 M) suppressed charge no better than pure water.1 The authors read this as evidence that charge moves as electrons rather than as ions.1 The mineral content of the added water therefore made no difference to charge in that study.1

Room humidity. Humidity of the room air affected charging only above about 60% relative humidity (RH).1 Water inside the beans, not in the air, set the charge in the range tested.1

Four ways to reduce charge#

A follow-up study tested four approaches on the same grinder type: waiting after grinding, water on the beans, and two kinds of ioniser at the chute.2

  • Waiting. Charge decayed with a time constant of about 15 s for a light roast and about 65 s for a darker, drier roast (1.0% residual water).2 The authors note that waiting risks loss of volatiles and quality, and that charge dissipates over minutes.2
  • Water. Adding 20–30 µL of water per gram of beans reduced charge by at least 50–60%.2 Water also prevented clumps and kept fines from sticking to the grinder.2
  • Ionizers. A balanced ioniser cut charge by at least about 50% in dark roasts and up to 90% in light roasts.2 A unipolar high-voltage ioniser had to be tuned to each coffee.2 The high-voltage ioniser, which acts on grounds at the chute, did not shift the particle size distribution or lower retention; the balanced ioniser was tested on charge only.2
  • Only water changed the espresso. Water gave longer shots and at least 15% higher concentration for dark roasts.2 With the high-voltage ioniser, TDS rose only from 7.76% to 8.02%.2

Inside the grinder, relative humidity rose from 40% to as much as 75% for a few tens of seconds when wetted beans were ground, then returned to room level within minutes; no condensation was detected.2

Water and the espresso#

In the 2024 Matter study, espresso was brewed from 18.0 g of coffee to 45.0 g of beverage at 7 bar and 94 °C.1 Without water the shot reached 8.2% TDS; with water on the beans it reached 8.9% and ran nearly 50% longer.1 Water on the beans shifted the particle size distribution, because clumps no longer formed and fines no longer stuck to the grinder.2 A larger share of fines lowers bed permeability and slows flow, as a separate espresso study showed.6 That is one plausible link to the slower shots; the grinding studies did not test it directly.2,6

The Matter authors expect markedly different flavour profiles from wetted beans, but neither study ran a sensory test.1,2 No study consulted for this entry measured the effect of water before grinding on flavour, aroma or crema.

Measuring retention#

How it was measured. In the 2024 iScience study, 10 g of whole beans were ground, and the mass left in the grinding cavity was compared with the mass expelled.2 The paper does not describe the weighing step in detail. Retained fines had to be knocked out of the grinder mechanically.2

What was found. Without treatment, about 12% of the dose stayed in the grinder.2 With 10 µL of water per gram of beans, about 2.5% stayed.2 With the high-voltage ioniser, retention was about the same as without treatment.2 For this grinder, retained material can amount to hundreds of milligrams of dry coffee.2 On a 10 g dose, 12% is about 1.2 g and 2.5% about 0.25 g (our calculation from the reported shares).2 The Matter study, with water on the beans, describes near-zero retention.1

02468101214% of the dose retainedNo treatment≈12%2 High-voltage ioniser atthe chute≈12%, as without treatment2 Water on the beans, 10µL/g≈2.5%2
Data table
% of the dose retainedSource
No treatment≈12%2
High-voltage ioniser at the chute≈12%, as without treatment2
Water on the beans, 10 µL/g≈2.5%2
Share of a 10 g dose left inside one flat-burr grinder after grinding, without treatment, with a high-voltage ioniser at the chute and with water on the beans (10 µL/g). Values are approximate as reported in the text of the study.2

Retained grounds are not a sample of the dose. Grounds left in the grinding cavity were rich in fines.2 A dose that leaves fines behind therefore has a coarser size distribution than the whole grind. Fines govern flow in espresso.6 No study consulted for this entry measured how much retained fines from one dose shift the size distribution of the next.

Stale grounds in the next dose#

Ground coffee loses gas fast. In a gravimetric study, up to 75% of the gas trapped in freshly roasted coffee escaped during grinding and within the first 90 s.5 The Degassing and Freshness and storage entries give the full data. The background of a 2014 grinder patent application gives the concern behind retention: ground coffee quickly loses its volatiles and, left too long, becomes stale.7

The size of this effect in the cup is not established. No study consulted for this entry measured how long grounds stay in a grinder in daily use, how much they exchange with each new dose, or whether a taster can detect it. The patent describes baristas purging the grinder to clear old grounds, which wastes beans.7 No study consulted for this entry measured what purging removes.

Reference values#

Parameter Value Conditions Source
Sign change of charge At ≈2% water in the beans (basis not stated); negative below ≈30 roasted coffees; one flat-burr grinder, 98 mm steel burrs Méndez Harper et al. 2024a1
Positive charge by colour Only above Agtron 70 Same study Méndez Harper et al. 2024a1
Largest charge ≈120 nC/g (negative) Same study Méndez Harper et al. 2024a1
Charge after a second, coarse pass Up to 90% lower Seven coffees; particle sizes nearly unchanged Méndez Harper et al. 2024a1
Room humidity with an effect on charge Above ≈60% RH Same study Méndez Harper et al. 2024a1
Charge decay after grinding Time constant ≈15 s (light roast); ≈65 s (darker roast, 1.0% water) Grind and wait Méndez Harper et al. 2024b2
Water to cut charge 20–30 µL/g: charge at least 50–60% lower Water on whole beans Méndez Harper et al. 2024b2
Retention, untreated ≈12% of a 10 g dose One flat-burr grinder, 98 mm burrs Méndez Harper et al. 2024b2
Retention, water on beans ≈2.5% at 10 µL/g Same Méndez Harper et al. 2024b2
Humidity in the grinder 40% rising to ≤75% RH for a few tens of seconds Wetted beans Méndez Harper et al. 2024b2
Espresso, dry vs wetted beans 8.2% vs 8.9% TDS; shot nearly 50% longer 18.0 g in, 45.0 g out, 7 bar, 94 °C Méndez Harper et al. 2024a1
Espresso concentration, dark roasts ≥15% higher with water Same grinder Méndez Harper et al. 2024b2
Espresso TDS with a high-voltage ioniser 7.76% → 8.02% Mean of five shots Méndez Harper et al. 2024b2

Risks and limits#

  • Too much water. The authors warn that excess water may cause caking or corrosion and could allow bacterial growth in the grinding chamber; they had not observed these.2 No study consulted for this entry measured burr corrosion after long-term use of wetted beans.
  • One grinder. Both studies used one flat-burr grinder with steel burrs.1,2 The Matter authors expect other burr materials to charge differently.1
  • Unusual coffees. The iScience authors note that some coffees may fall outside the range they tested.2
  • Requirement for grinders. A grinding textbook chapter lists, among the requirements a grinder should meet, avoiding the build-up of fines, for example by reducing electrostatic charging.4 Cleaning retained grounds out of a grinder is covered in Grinder care.

Open questions#

  • Flavour of water before grinding. No sensory study of the technique was consulted.
  • Retention across grinder designs. Retention was measured on one grinder only; no peer-reviewed comparison of designs was consulted.
  • Exchange of retained grounds. How much old ground coffee reaches a new dose in daily use, and whether it can be tasted, is not established.
  • Fines left behind. How retained fines change the size distribution of the dose that leaves the grinder was not measured.

Key studies#

Méndez Harper et al., 2024 – Water inside the beans sets the charge. Grinds about 30 roasted coffees on one flat-burr grinder and measures the charge with a Faraday cup. Internal water predicted the sign and size of the charge better than roast colour; fracture produced most of it, and water added to the beans cut charge and clumping and made espresso slower and stronger.

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

Méndez Harper, Bumbaugh and Hendon, 2024 – Four ways to remove static. Compares waiting, water on the beans and two ionisers on the same grinder type. Ionizers reduced charge, but the high-voltage ioniser did not lower retention; only water lowered retention, from about 12% to about 2.5% of a 10 g dose, and changed espresso concentration.

iScience 27, 110639 · doi:10.1016/j.isci.2024.110639

Smrke et al., 2018 – Gas lost at grinding. Follows the weight loss of whole and ground roasted coffee over time. Up to 75% of the trapped gas left freshly roasted coffee during grinding and in the first 90 s, which shows how fast ground coffee changes once it is made.

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

Contested
  • Concentration of the espresso ground with water: the 2024 study in Matter reports 8.9% total dissolved solids (TDS) against 8.2% without water1, while a summary of the same work by the same authors in a trade magazine gives 8.7%.3 This entry gives the peer-reviewed value.
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Change log

  • – Ionizer results narrowed to the high-voltage ionizer for particle size, retention and espresso. Reason: The iScience study tested those outcomes only with the high-voltage ionizer; the balanced ionizer was tested on charge only.
  • – The 0.5 mL of water is no longer stated as the amount needed for an 18 g dose; the conversion now states its assumption. Reason: The iScience study does not tie the 0.5 mL figure to a stated dose.
  • – Stationary and rotating burr in the grinder legend re-sourced to a burr-grinder patent. Reason: The textbook chapter cited before does not describe which burr turns.

Sources

  1. 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; Laboratory grinding of about 30 roasted coffees on one flat-burr grinder (98 mm steel burrs), charge by Faraday cup; espresso brewed with and without water on the beans; United States..
  2. AMéndez Harper, J.; Bumbaugh, R. E.; Hendon, C. H. (2024). Strategies to mitigate electrostatic charging during coffee grinding. iScience 27, 110639. doi:10.1016/j.isci.2024.110639 · other copiesQuoted passage: the authors’ own result; Laboratory tests of water, two kinds of ionizer and waiting, on one flat-burr grinder (98 mm burrs); three coffees, each roasted light and dark; espresso concentration measured, no sensory test..
  3. BMéndez Harper, J.; Hendon, C. H. It’s Electric: Understanding – and Reducing – Static Electricity During Grinding. SCA News 25(21), Specialty Coffee Association.Quoted passage: a description; Summary for practitioners of the authors' grinding studies; one grinder..
  4. 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 industrial and point-of-sale grinding..
  5. ASmrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C. (2018). Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee. Journal of Agricultural and Food Chemistry 66(21), 5293–5300. doi:10.1021/acs.jafc.7b03310 · other copiesQuoted passage: the authors’ own result; Gravimetric degassing of arabica and robusta, whole and ground at espresso fineness, 35 °C; laboratory, Switzerland..
  6. 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 several burr spacings, sieved fines added back; espresso, laboratory, Switzerland..
  7. A/BRyan, J. L.; Greeve, A. J. M.; Papi, L. (assignee Van Deuce Pty Ltd) (2019). Coffee grinder (US 10,426,296 B2). United States Patent and Trademark Office.Quoted passage: a description; Patent for a grinder that discharges grounds straight down into the portafilter; filed 19 Dec 2014, granted 1 Oct 2019..
  8. A/BMidden, W. E. (assignee Bunn-O-Matic Corporation) (1993). Coffee grinder (US Patent 5,201,474). United States Patent and Trademark Office; filed 3 August 1990, granted 13 April 1993.Quoted passage: a description; Patent for a burr coffee grinder with one stationary and one motor-driven disc-like burr; filed 1990..

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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