Grinding Finer Won't Make Your Coffee Stronger — Oregon Chemists Have the Receipts

Warm morning light falling across a wooden kitchen counter with a coffee grinder, beans, and brewing gear mid-routine

Reviewed by the Cascadia Bean Supply roasting and sourcing team.

The "Grind It Finer" Reflex

Ask ten people how to make their coffee stronger and at least seven will say the same thing: grind it finer. More surface area, more contact with water, more flavor pulled out of the bean — it sounds like basic kitchen physics, and for a narrow range of grind sizes, it's true. But a run of coffee-chemistry research out of, of all places, a university physics and chemistry department in Eugene, Oregon, has spent the last several years quietly dismantling the idea that finer is simply better. Grind too fine past a certain point, it turns out, and you don't get a stronger cup. You get a weaker, less consistent, more bitter one — and a very specific kind of mess on your counter. The story of why is one of the more satisfying pieces of real coffee science we've come across, and it changes how we think about the last five feet between a bag of beans and a cup.

The 2020 Study That Upended Espresso Logic

In January 2020, the journal Matter published a paper from Jamie Foster, a mathematician at the University of Portsmouth, and Christopher Hendon, a computational chemist at the University of Oregon, modeling exactly what happens as water moves through a bed of ground coffee during espresso extraction (PopSci, 2020). Their model, which borrowed mathematical tools from battery research, predicted something that ran against conventional cafe wisdom: grinding finer and finer doesn't keep increasing how much flavor you extract. Past a certain point, extraction actually drops.

The mechanism, per the model, comes down to clogging and uneven flow. Very fine grounds pack together tightly enough that water can't move through the bed evenly. It finds easier paths through looser pockets, over-extracting those areas into bitterness while leaving denser, clogged pockets under-extracted and sour. The average extraction number on a refractometer might look fine; the cup itself tastes like both problems at once. The researchers' proposed fix was almost the opposite of the industry default: use less coffee, grind it coarser, and let the shot run fast — on the order of 7 to 15 seconds, rather than the 25-to-35-second pull most cafes train baristas to chase.

They didn't just publish and move on. A coffee shop in Oregon that adopted the approach reportedly cut the amount of ground coffee it used by 25% over the following year, and the team estimated that if U.S. cafes broadly adopted coarser, lower-dose shots, it could add up to roughly $1.1 billion in savings annually, largely from using less coffee per drink without sacrificing quality (PopSci, 2020). A later writeup of the group's expanded, multi-country follow-up work described the same core recipe — something like a 15-gram dose instead of 20, ground noticeably coarser, run fast — as producing shots that still tasted good while using meaningfully less coffee (Bean Scene Magazine).

It's worth being precise about what this does and doesn't say. This is espresso-specific modeling, built around espresso's pressurized, fast, tightly packed extraction. It is not a blanket claim that every brew method should be ground coarser, and the researchers themselves have described this as a starting point for further work, not a finished theory of flavor. But the core finding — that there's a point past which finer grinding actively hurts extraction instead of helping it — is one of the more robust, well-modeled results in recent coffee science.

Why "Uneven" Beats "Fine" as the Real Culprit

A 2023 follow-on paper in Physics of Fluids, from William Lee and colleagues at the University of Huddersfield, dug further into the mechanism (Lee, Smith & Arshad, arXiv preprint of the Physics of Fluids study). Rather than treating the coffee bed as one uniform mass, they modeled it as two parallel pathways with slightly different starting porosity — a simple stand-in for the uneven packing that happens in any real grind. Their model found that uneven flow shows up at every grind size, fine or coarse. What changes at fine grinds is that one of those pathways can run out of soluble coffee to give up entirely, stalling its contribution while the other pathway keeps extracting (and over-extracting). That's the moment total extraction yield starts to fall, and it's also the moment a shot is most likely to taste like a strange mix of sour and bitter at once, because you're drinking from two very differently extracted pathways blended into one cup.

The practical upshot is a slightly uncomfortable one for anyone who likes a tidy rule: a single "extraction percentage" number can hide a genuinely uneven shot. Two espressos with an identical measured extraction yield can taste completely different depending on how evenly the water actually moved through the grounds. Grind size matters less in isolation than how uniformly that grind packs and lets water through it — which is part of why dialing in a new bag of beans is as much trial-and-error as formula.

The Static Problem Nobody Talks About

The same University of Oregon group, this time with volcanologists Josef Dufek and Joshua Méndez Harper, took an unusual next step: they borrowed instruments normally used to measure electrical charge generated by exploding volcanic ash and put them under a coffee grinder (Oregon Public Broadcasting, 2024). Grinding coffee, they confirmed, generates meaningful static electricity, which is why fine grounds cling to the inside of a grinder, leap onto countertops, and refuse to settle into a portafilter or dripper basket cleanly. That's not just a tidiness problem — clinging, scattered fines mean an inconsistent, unmeasured amount of coffee actually makes it into your brew, and lost fines are disproportionately likely to be the ones most responsible for over-extraction.

Their fix, published in Matter in December 2023, measured something home baristas have passed around as folk wisdom for roughly a decade: adding a single drop of water to your beans before grinding meaningfully cuts the static charge and the dust it produces (OPB, 2024). Dark roasts, which lose more of their internal moisture during roasting, build up more and more negative charge, so they see the biggest benefit; light roasts, which can even charge positively, still benefit, just less dramatically. It's a rare case of a kitchen trick getting its "why" explained years after coffee nerds were already doing it.

Not All Burrs Produce the Same Grind

Grind size settings are also less universal than the little numbers on a grinder's dial suggest. Research out of the Coffee Excellence Center at ZHAW (Zurich University of Applied Sciences), led by Dr. Samo Smrke, measured the share of "fines" — the smallest, sub-100-micron particles in a grind — produced by different burr geometries, and found real structural differences: conical burrs tend to produce roughly 25 to 35% fines, flat burrs roughly 15 to 30%, and roller grinders as little as 5 to 15% (ZHAW Coffee Excellence Center). The same research found that extraction time tracked the share of fines more closely than it tracked the median particle size — raising the fines fraction from 10% to 15% added roughly 10 extra seconds to a shot, independent of what the "grind setting number" said.

In plain terms: two grinders set to what looks like "the same" grind size can produce meaningfully different cups, because the full distribution of particle sizes they produce — not just the average — is doing a lot of the flavor work. That's a big part of why a grind setting that works perfectly on one machine rarely transfers cleanly to another.

Practical Takeaways

  • Don't chase "fine" as a proxy for "strong." If a shot or cup tastes thin or sour despite a fine grind, the fix may be going coarser, not finer — especially for espresso, where the research base is strongest.
  • Weigh your dose instead of eyeballing the hopper. A consistent gram weight of beans matters more than any dial number, since the same setting can yield different grinds shot to shot if dosing drifts.
  • Try a drop of water before grinding (the "RDT" trick). A single drop or light spritz on your beans before grinding cuts static and fines scatter, especially with darker roasts — and it's now backed by a published mechanism, not just forum lore.
  • Know your burrs. Conical, flat, and roller burrs produce meaningfully different fines content at "the same" setting. If you switch grinders, expect to re-dial your grind from scratch rather than copying old numbers.
  • Judge by taste and time, not just extraction math. A shot or brew can hit a textbook extraction percentage and still taste uneven. Watch the flow and taste the result, rather than trusting one number in isolation.
  • Treat pour-over and drip findings as directional, not proven. Most of this research is espresso-specific; the uneven-extraction mechanism likely applies more broadly, but it hasn't been measured the same way outside pressurized brewing.

What This Means for Roast-to-Order, Single-Origin Coffee

This research is a useful reminder of something we lean on heavily at Cascadia Bean Supply: grind size is only one variable in a system, and it behaves differently depending on everything else around it — the bean's density, how recently it was roasted, how much moisture it's carrying, even the specific geometry of the burrs doing the grinding. We roast each single-origin lot to order in small batches, which means the beans in your bag haven't spent months losing the internal moisture and structural consistency that roasting imparts, and degassing slowly and predictably rather than erratically off a shelf. That doesn't make grind size irrelevant — you still have to dial in your own grinder, and the physics above applies to any bean, from anyone — but it does mean you're not fighting staleness and inconsistency on top of everything else. Fresh, evenly roasted beans give the variable you can control (grind) a fair shot at actually mattering, instead of masking problems that started well before the grinder.

Our honest advice, consistent with the research above: buy whole bean, grind just before brewing, and don't assume the smallest setting on your grinder is doing you any favors. Dial in by taste, not by instinct.

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