Retention
What it is and why it matters
- Grounds left in the grinder either stay stuck and accumulate, or get swapped into the next dose ("exchange"). Under about 1 g is fine as long as it isn't exchange 15:21.
- Retained chaff and fines that end up in the next dose taste dry and papery, like old green tea 35:14.
- Zero retention is a myth. Some mass leaves as gas, so an exact dose-in/dose-out usually means old grounds were exchanged 32:00. It also differs between units and with grind size, with fine espresso grounds clinging more than coarse ones 32:00 32:43.
- Retention builds to a steady state, so cleaning a grinder resets it and frequent cleaning can make it look worse until crevices refill 15:42.
- Even good grinders hold a few tenths of a gram, and about 0.4 g on a budget grinder is normal 5:35.
What drives it
- Burr orientation matters. Vertical or angled burrs eject better and retain less than horizontal ones, which often rely on sweeper arms 35:44 6:35 8:14 2:36. Near-horizontal burrs with an inner ridge trap grounds 5:37. Horizontal burrs without an auger depend on high RPM and a knocker, and at low RPM grounds linger 8:43. Even with large burrs and an auger, Lance Hedrick prefers vertical 12:42.
- Nothing beneath the burrs means almost nothing to retain 19:46.
- Very low RPM increases retention 8:12. Under roughly 200 rpm the burrs spin too slowly to fling grounds out 7:37.
- Declumpers and tapered chutes can clog and hold coffee 17:28 25:30, though a declumper also reduces static spray 25:46.
- Hopper dosing always retains because bellows work poorly with a full chamber 32:43. He also prefers single dosing because beans oxidise in a hopper 8:45.
What to do
- Bellows help nearly every grinder 6:42 9:08, and even the Niche Zero gives up another 0.2 to 0.3 g 3:05. Bellows plus RDT gets almost everything out 28:53. For dose-in/dose-out precision he adds a camera-lens blower 11:40.
- Run a few beans through when moving between filter and espresso settings 39:33.
- Fines collectors on the Timemore Sculptors let you knock out and discard the clingy fraction. He weighs an extra 0.5 to 0.75 g for filter and throws that part away 19:18 5:36.
- Timed dosing is imprecise because flow changes as the hopper empties. Coarse grinds time more consistently than espresso grinds 28:17.
Static
- Grinding builds charge by rubbing (tribo-electric) and by fracturing. The Hendon group paper Lance Hedrick reviewed found fracturing dominates 6:09.
- In that paper, fines under about 100 µm and boulders over about 300 to 500 µm tended to be negative and mid-size particles positive, so opposite charges cling together into clumps 9:13. Roasted beans with over about 2% internal moisture tended towards a positive net charge, and green moisture didn't predict it 8:42.
- Ambient humidity above about 60% reduces static, with little effect between 20% and 45% 29:00.
- Some grinders are notoriously static. The DF64 makes huge amounts 10:45.
RDT (a few drops of water on the beans)
- What it is. A light spritz or a few drops of water on the whole beans just before grinding, to reduce static and retention 2:37. It also stops chaff and fines flying 2:34.
- What the research showed. Added water broke up clumps (the peak particle shifted smaller), made espresso shots run up to roughly 50% longer and increased TDS, with the biggest effect around 20 µL per gram 31:05. Water also reduces retention because it suppresses charge where beans fracture 49:04 29:00.
- His own test, grinder by grinder. On a Kafatek Monolith Max and a Niche Zero the effect differed. The Niche improved at 10 µL/g, declined at 20 and collapsed pucks at 40 34:12 35:13. Start small, around 5 to 10 µL/g (roughly 0.1 to 0.2 g on 20 g), and judge by taste or refractometer 39:23.
- Ionizers are not the same. Ionizers or waiting for static to fade can't undo clumps that have already formed, so they don't replicate the water effect 47:33.
Ionizers, chaff and when to skip RDT
- Built-in ionizers (Fellow Ode Gen 2, Option-O Lagom 01) greatly reduce static and can make RDT unnecessary 2:34 7:09.
- Lance Hedrick isn't a fan of ionizers that clog and need cleaning, and accepts chaff mess because he wants chaff out of the brew 10:14.
- For filter he sometimes skips RDT deliberately so chaff clings to the grinder and can be wiped away, which reduces papery flavour 10:45 11:46 15:49. Blowing chaff out of light roasts protects floral notes 4:32.
- Ionizers and light-roast chaff don't mix well. A light RDT helps at coarse grinds, while dark and fine grinds make little mess 14:14.
James Hoffmann's view
- RDT from the start. His 2017 static hack was a wet spoon handle stirred through the weighed beans before grinding, with no harm to grinder, beans or drink 0:35. Later he uses one spray or a wet teaspoon routinely, unless a manufacturer advises against it 2:33. Several grinders he reviewed effectively need it (Levercraft Ultra, Niche Duo, DF54) 7:08 5:36 9:12.
- The 2023 Hendon paper. At about 20 µL of water per gram (three or four sprays for 18 g) the paper saw much slower espresso flow and higher extraction on an EK43 6:09 5:39. Hendon's explanation is that oppositely charged fines stick to larger particles, and neutralising the charge lets fines distribute evenly 7:09. Hoffmann reproduced a big slowdown on a DF64 Gen 2 (28 s to about 38 s) but none on a Eureka Mignon Oro, and speculates that grinders which bottleneck at fine settings generate more charge 11:43 12:14 13:46. Ionizers act too late to undo clumps formed in the burrs 10:11. This matches Lance Hedrick's finding that the effect is grinder-specific 34:12.
- Too much water can make beans stick in shallow funnels or clog the burrs 12:53 6:43.
- Retention and exchange. Getting the same weight out as in doesn't prove zero retention, because old coffee may be swapped for new 3:05 5:44. To test for exchange he grinds coarse, returns to espresso and sees if the next shot runs fast 11:08 18:28. Weigh beans in and grounds out when learning a grinder 1:33.
- Purging. Retaining grinders need 5 to 10 g purged after a setting change 10:07, and old commercial designs could hold 20 to 30 g 16:26. Low-retention grinders (Niche Zero) let you change setting and grind the next dose straight away 10:16.
- Bellows reduce retention but blow mess around. Hoffmann finds them a lazy addition and prefers a light tap 8:11 6:41.
- Leaving grounds in the bin for 10 to 15 s and shaking it lets static settle 3:02.
Daddy Got Coffee's view
- Two kinds of retention. Grounds get exchanged into the next dose, or stick in nooks and rarely come out 19:25. On-demand grinder chutes can hold several grams that sit overnight and go stale, making the first shot inconsistent, so purge about a shot's worth first 10:09 10:41.
- Buildup drifts the grind. On a Niche Zero, even about 0.4 g of buildup shifts grind size until it's cleaned. Dry-clean roughly monthly, more often with oily dark roasts 11:22 1:31.
- RDT is grinder-specific. It's effectively required on a Zerno and a big help on the Aries, but unnecessary on the Lagom Casa and Femobook A4Z, where it can make beans stick 17:55 9:11 5:39 5:08. One spray was the sweet spot on a Bentwood and Weber Key 12:22 18:58. Some cheap grinders reportedly rust inside from regular spritzing 5:07.
- Ionizers reduce static without removing it, and humidity still plays a large part 16:53 6:36.
Kyle Rowsell's view
- Low retention is the trait that matters most in a grinder you switch between filter and espresso 3:01.
- Plasma generators plus declumpers cut static sharply on newer 64 mm grinders compared with the original DF64 23:02. His day-to-day routine is in the grinder workflow page.
The Coffee Chronicler's view
- What counts is the grounds swapped into the next dose rather than the total left inside. The Coffee Chronicler measured 0.2 g total on a DF54 against 1.1 g on an Encore ESP 5:40, the same exchange distinction Lance Hedrick and James Hoffmann draw.
- On the DF64 Gen 2, a plasma generator and a reworked declumper reduced static and mess 7:37. Whatever RPM does to the distribution, it visibly changes static and mess 7:36. Grinding finer on an Encore ESP brought more clumping, heat and retention 7:44, and grinders without a chute drop grounds more evenly into the cup 4:49.
Brian Quan's view
- High retention mostly matters when you change coffees, because old grounds get swapped into the next dose 3:31.
- Denis Basaric of Kafatek told Brian Quan that a good burr coating is smooth, safe for food contact and helps static drain away. A DLC coating performed badly for static in Basaric's tests 10:14.
For Notchbook
RDT changes the effective grind (de-clumping makes it act finer and can lengthen espresso shots), so if a user starts or stops using it, or changes the amount, the pinned setting may need dialling in again.
Summarised in our own words from the creators' videos, with every point linked to the moment it comes from. Notchbook isn't affiliated with or endorsed by any of these creators, so please watch the originals for full context. Sources
Keep reading
- Burr alignment
- Astringency
- Conical vs flat, and burr geometry
- Grind size and extraction
- Particle size distribution and fines
- Seasoning new burrs
- Slow feeding and RPM
- Zero point and grinder scales
- Grinder workflow: single dosing, WDT, RDT, purging and storing beans
- Troubleshooting: what to change when a cup tastes wrong
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