You likely view fines as an error. It is that powdery residue lingering in your grinder’s chute or layering the burr chamber, which makes your portafilter basket appear hazy before brewing starts. Your gut tells you the grinder is flawed. You might try a coarser setting or hunt for different burrs, believing a superior piece of gear reduces that leftover debris.
You get some of that dust in every single shot you pull. It’s just what happens when burrs shear beans. A 2024 Zurich University of Applied Sciences study in Scientific Reports finally used real controls to examine those particles. The mechanism they discovered alters how that dust actually affects your brew.
Fines are not some defect your grinder misses. They are a metric, but you likely haven’t figured out how to interpret the data.
📋 Table of Contents
- What Fines Actually Are
- The Study: Fines Change Permeability, Not the Extraction Ceiling
- Scott Rao’s Happy Medium, and Why Nobody’s Quantified It
- Burr Geometry: Where Your Fines Percentage Actually Comes From
- Reading Your Own Shot for a Fines Problem
- The Turbo Shot Wrinkle
- Troubleshooting: The Most Common Knockouts
- The Closing Bell
What Fines Actually Are
Regardless of the price, your burr set always does two things at once. It cuts some beans to your exact target size, but it also crushes a smaller portion into a fine dust. Once you pack that puck, those tiny particles act less like coarse sand and more like flour.
That shape is technically called a bimodal particle size distribution. You have two overlapping groups in one grind: a large cluster at your target size and a smaller group falling far below that. For decades, coffee scientists used laser diffraction to measure this. Now, there is a newer, mechanistic explanation for how that second, smaller group affects your shot when water hits it at nine bars of pressure.
The Study: Fines Change Permeability, Not the Extraction Ceiling
Samo Smrke, André Eiermann, and Chahan Yeretzian conducted this study to treat fines as a standalone variable. You should know they used a Bentwood Vertical 63 grinder and one Costa Rican arabica for the baseline. To do this, they took identical doses and intentionally added extra fines, specifically 1, 2, and 4 grams, to a base sample. They pulled three shots of every mix to track the final yield, extraction time, and flow rate.
Two main points anchor this work. First, your flow slows down when you add fines because coffee bed permeability drops. The regression model showed a definite positive link between the percentage of particles under 150 microns and shot duration. More fines mean a slower drip and a longer pull. Second, adding extra fines to a dose didn’t send extraction yield into some weird territory. Standard espresso pulls about 20% of soluble solids, and these fines-spiked shots stayed right in that ballpark; they just took longer to get there.
Your grind’s fines dictate water speed way more than they control the amount of flavor extracted.
This shift changes how you view “too many fines.” It isn’t just about some vague, muddy, over-extracted taste. The actual problem is a puck that drains so slowly that the prolonged water contact pulls out astringency. Or, the puck clogs in spots, forcing water to detour around blocks and channel through the open gaps. You end up with a thin, sour, uneven shot, all coming from that same root cause.
Scott Rao’s Happy Medium, and Why Nobody’s Quantified It
Long before that Zurich data dropped, pro baristas already had a handle on this. Scott Rao’s “happy medium” theory of fines has been a staple of dial-in talk since 2023, backed by thirty years of experience and more than a million shots. He argues that you need just enough fines to plug the holes between larger particles. This blocks the low-density paths that let water rush through unevenly, which stops the brew from pulling out too much bitterness and astringency. If you strip those fines out, those gaps stay wide. Water just takes the easiest route and channels right through, over-extracting one thin strip of the puck while leaving the rest of the bed alone.
Go too far the other way and those gaps meant for fines begin piling up, causing local clogs. Since the water cannot pass through a blocked area, it finds a way around. You get channeling again. It comes from a different source but looks the same.
Rao admits his theory has a gap. Even with years of hype around particle-size measurement, “no one has quantified optimal fines levels with any precision.” That specific number still doesn’t exist. The Zurich data doesn’t give you that figure. Instead, it explains why his gut feeling worked: permeability. You can track that variable in your own portafilter without needing a particle counter.
Burr Geometry: Where Your Fines Percentage Actually Comes From
Your fines baseline is decided by burr geometry before you even adjust the grind-size dial. Conical burrs use a ring and a cone to crush beans. This action creates a broader, bimodal spread where a larger fines tail exists next to your target particles. Flat burrs work differently. They shear beans between two parallel discs, which cuts that tail into a tighter distribution closer to unimodal at that same nominal setting.
We put the DF54’s 54mm flat burr set to the test against the conical grinders that once owned this price point. You get more consistent shots from a tighter particle size distribution because it creates uniform resistance through the puck. That matters way more than hunting for a tiny bit of extra grind-size precision on burrs that can’t actually deliver it. The Niche Zero vs. DF64 Gen 2 comparison looks at this same fight from a different angle, pitting two flat-burr machines against each other to see who controls that variable better.
Conical burrs aren’t dead. You get more fines, and that extra tail can actually give your espresso more sweetness and body if you dial it in. That is why some baristas stick with conicals even when flat burrs look better on a spec sheet. Your burr geometry decides your fines baseline before you even start tweaking your technique.
Reading Your Own Shot for a Fines Problem
You lack a laser diffraction particle analyzer, though that tool would verify these facts. You do not require one. The evidence from the Zurich data and Rao indicates the issues are right there in your own basket.
If your shot begins with a rush but then visibly drags and dims, ending in a compressed, gummy puck, you’ve got too many fines choking the flow. Coarsen your grind by one notch. If you use a conical burr grinder, keep an eye on the roast level. Brittle, dark roasts shatter into more fines than dense, light ones at the same setting. A dark, oily espresso blend like Bob and Weave Dark Blend produces more fines at the same collar position than a lighter roast like Black Eye Breakfast Blend. That is why a setting that works for one blend might choke on the other.
If your shot tastes sour and pale or spurts and flows unevenly, even with a standard total time, you probably lack fines rather than having too many. Under-extraction tastes nearly the same as channeling through gaps between coarse particles. That is why you might try a finer grind to fix channeling and only make things worse. Just grinding finer doesn’t ensure the correct fines end up in the right spots; you can easily overshoot and trigger the clogging failure mode mentioned above.
Check the used puck instead of relying solely on the timer. If the disc is compressed, dry, and tight, it tells you one thing; if it is cratered and wet in spots, it means the opposite.
The Turbo Shot Wrinkle
If you have noticed “turbo shots” popping up in dial-in discussions, there is a detail from this study you need to grasp. Extractions that wrap up within 10-15 seconds still reach a 17-18% extraction yield. That is over 80% of the highest yield the researchers saw during the entire trial. Some baristas used this data to start grinding coarser on purpose, making short shots their standard routine.
Don’t just mimic this technique; get why it works. You need to stop blurring the line between two distinct metrics. Flow rate tracks permeability and depends on your fines percentage. Extraction yield is a different beast. A quick shot doesn’t mean your yield automatically drops.
Troubleshooting: The Most Common Knockouts
| Symptom | Likely Cause | Fix |
|---|---|---|
| Your shot kicks off too quickly, then it chokes and goes dark toward the finish. You’ve got a compressed gummy puck. | Too many fines clogging locally | Go coarser with your grind. Otherwise, see if that darker roast is creating too many fines at your current setting. |
| Your brew is spurting and the flow is inconsistent. Even though your total time is on point, the coffee tastes sour and looks pale. | Your fines are lacking, so the water is just carving paths through the puck. | Tighten your grind. It looks like under-extraction, but you need to move the opposite way to fix it. |
| Your settings are the same, but one bean flows while the other chokes. | Your grinder isn’t broken. The gap in fines comes from the roast level. | Adjust your grind for each bean. You need to do this specifically when switching from dark, brittle roasts to light, dense ones. |
| You’re still getting channeling even though your puck is tamped flat. | Your conical burr’s broader fines tail is hitting an inconsistent dose. | Switch to a flat-burr grinder. Otherwise, make your single-dose consistency better by using a dosing routine that is shorter and more repeatable. |
| Your brew is running too quick, finishing in less than 15s. It tastes watery. | Your grind is too chunky for that basket. Fines aren’t the issue here. | Adjust your grind size smaller and run another test. Do that before you decide you are actually pulling a “turbo shot.” |
The Closing Bell
Rao nailed the truth about fines for years using nothing but his palate. Now, Zurich researchers have labeled that hunch permeability and assigned it a specific value you should track in your own basket.
Stop hunting for a lower fines count with a new burr set this week. Instead, run two consecutive shots using the same bean. Set one slightly coarser and the other slightly finer than your standard dial. Examine the used pucks rather than relying solely on the flavor of the drink. A cratered, patchy mess and a tight, dry puck provide different answers. Figure out which one you have. Once you do, dialing in the grind is no longer guesswork.
Use the puck to dial in. Stop relying on some random rule of thumb you saw once and failed to verify on your own gear.
