Bimodal vs. unimodal particle distribution: what really lies behind syrupy mouthfeel
Bimodal vs. unimodal particle size distribution: what really lies behind a syrupy mouthfeel
“Unimodal” is one of the best-selling words in grinder marketing. We looked into what the measurements actually show – and learned along the way that, in that form, the term doesn’t really exist. What does exist is a clean physical explanation for what we experience as body.
Every espresso grind is bimodal, often even trimodal. A unimodal espresso distribution has never been measured in the scientific literature – the correct term would be “fines-reduced”, not “unimodal”.
Fines fill the gaps between the large particles, lower permeability and lengthen extraction. That is well documented. That they create body is not.
As far as we know today, a syrupy mouthfeel comes from several phases at once: dissolved solids, emulsified oil droplets, suspended cell wall fragments and the foam. It isn’t simply a question of viscosity.
What bimodal actually means
Bimodal means the particle size distribution has two peaks: one at the very finest particles, the so-called fines, and one at the coarse fraction. By convention, particles below 100 micrometres count as fines – that is an agreement, not a constant of nature.
A asks: how much coffee mass sits at this size? Two hills — fines and coarse fraction. B asks: how many particles are this size? Only one hill is left; the coarse fraction all but disappears.
Curve shapes schematic, positions based on measured values: fines peak 40–50 µm (Corrochano et al. 2015), peaks at 13 µm and 257 µm (Uman et al. 2016), 99% of particles below 100 µm by number (Cameron et al. 2020).
Both charts show exactly the same grind. The top one counts mass, the bottom one counts particles – and suddenly two hills have become one. This is exactly where most misunderstandings in forums begin: two people talk about the same grinder and mean two different charts. The orange curve at the top is what fines-reduced burr geometries achieve: a smaller fines peak. It doesn’t disappear. Analysing 300 distributions, Gagné found that the position of the fines peak differs between different grinders by up to 8 micrometres – real, but small.
Peer-reviewedA warning about interpreting the numbers, one that very nearly tripped us up ourselves: the much-quoted line “99 per cent of all particles are smaller than 100 micrometres” from Cameron et al. (2020) is number-weighted, not mass-weighted. By mass, fines are a small minority. Yet they account for around 80 per cent of the accessible surface area – and that 80 per cent is an estimate, not a measured surface area. Also: different measuring methods – laser diffraction, dynamic image analysis, sieving – produce different figures. Values from two studies can’t be compared directly.
Why unimodal espresso doesn’t exist
This is where our research made us rethink the most. In all the peer-reviewed literature we went through, there is not a single measured unimodal particle size distribution at espresso grind size. All of them are bimodal, and Gagné’s analysis of 300 distributions shows that the data are actually better described by three overlapping log-normal distributions than by two.
The mechanism behind this can be found in Cameron et al. (2020): as the grind is set finer, the proportion of fines rises – but their size stays constant. Particles keep breaking until they are small enough to pass through the gap between the burrs. The fines peak is a product of fracture mechanics, not of the gap width. Grinding finer produces more fines, not fewer.
The only experimentally documented narrowing of the distribution comes from a different direction: Uman et al. ground beans at minus 196 degrees in liquid nitrogen. The modal value fell by 31 per cent, and the fines cut-off from 70 to 61 micrometres. Narrower, yes – unimodal, no.
If someone sells you a unimodal grinder, what they actually mean is: the fines peak is smaller. That is a real, measurable property and genuinely worth something. It just isn’t what the word claims. That’s why we prefer to say “fines-reduced” – it describes what really happens. Others in the industry are more relaxed about it, and that’s fine too.
Fines, permeability and the extraction curve
This is where the evidence gets really solid. Corrochano et al. (2015) systematically measured the permeability of coffee beds: it ranges from 2.59 × 10−14 to 3.36 × 10−13 square metres – a factor of around 13 between the extremes, from particle size distribution and bed density alone. At the same bulk density, a coarsely ground bed was 4.4 times more permeable than a fine one.
The most revealing figure: the measured bed porosity was between 0.12 and 0.33 – well below 0.36, the value for a random packing of equally sized spheres. This is the direct physical evidence that fines fill the interstices between the coarse particles.
On the left, the water finds continuous gaps. On the right, the fines settle into the interstices and narrow every path.
The bars show the extremes of the measured permeability: 3.36 × 10⁻¹³ versus 2.59 × 10⁻¹⁴ m², a factor of around 13 (Corrochano et al. 2015). Particle sizes are heavily exaggerated in the illustration — real fines are about ten times smaller than drawn here. The 300–800 µm range in the legend is a rough guide for an espresso grind — estimated, not measured in this figure.
Smrke, Eiermann and Yeretzian (ZHAW Wädenswil) state the same finding directly in 2024: a higher fines content lowers permeability, slows the flow and lengthens extraction time.
But that doesn’t mean “more fines equals more extraction”. Cameron et al. (2020) found a non-monotonic curve: extraction yield rises with a finer grind only up to a point and then falls again. The reason is agglomeration and uneven bed density, which partly clog the flow. At the finest setting, the gap between calculated and actually measured yield was 13.1 per cent – the share of the bed that, by calculation, the water no longer reaches. At somewhat coarser settings this gap shrinks to 6.1 and 2.6 per cent. So the effect isn’t a law of nature but a consequence of grinding too fine.
The authors’ conclusion is especially instructive: the same yield of 22 per cent can be reached with two quite different grind sizes – but the chemical composition of the two shots is not the same. Two espressos with an identical refractometer reading can taste completely different. You can play this through in the grinder simulator.
What syrupy body is physically made of
Now for the real question. In 2011, Illy and Navarini described in Food Biophysics what an espresso consists of as a physical system – and the figures are surprisingly concrete:
| Phase | Key figure |
|---|---|
| Dissolved solids | around 52.5 g/l |
| Emulsified oil droplets | 90% smaller than 10 µm, volume fraction 0.2–0.3% |
| Suspended cell wall fragments | 2–5 µm, around 150 mg per cup, which works out at about 5 g/l for a 30 ml cup (calculated from 150 mg and 30 ml, not measured) |
| Total lipids | Arabica 1.80 mg/ml, Robusta 0.55 mg/ml |
| Crema | Foam density 0.40–0.60 g/ml, bubbles 10–150 µm |
What surprised us in our research: we found no published measurement of the shear viscosity of espresso. Illy and Navarini, like Ferrari et al., deal with interfacial properties, not the viscosity of the drink. Any specific millipascal-second figure you read about espresso is, as far as we know, unsupported.
That points to a particular reading: in terms of concentration, the dissolved solids alone sit below the threshold at which people can reliably detect differences in viscosity at all. What carries the syrupy impression is probably the dispersed phases – oil droplets, microparticles, foam – plus the cross-modal link to sweetness and aroma that we described on the sensory page. That is a line of reasoning, not proof, and we are deliberately labelling it as such.
What this means for choosing a grinder
Community measurementThe popular dichotomy – conical equals bimodal equals full-bodied, flat equals unimodal equals clear – doesn’t stand up to measurement. Gagné did find that conical burrs are on average less unimodal and less uniform, but with a large overlap: you can easily find a flat burr set that is less unimodal than a conical one. And one finding directly contradicts the usual narrative: unimodality and uniformity correlate positively. You don’t trade one for the other.
One more finding that undercuts the marketing: Smrke et al. (2024) varied the fines content systematically and had the results assessed in sensory tests. Result: no sensory disadvantage from a higher fines content. That puts the whole “fines are bad” story on thin ice – and the counterclaim that fines guarantee body along with it. We simply don’t know for sure yet.
In practice, the way we see it, that means: choose your grinder by what you can measure and experience – uniformity, retention, handling, grind adjustment resolution, burr size. We’ve described how burr size comes into it under Burr sizes explained, and compared the top models under Weber EG-1 vs. Lagom P80 vs. Lagom 01. You’ll find the whole range under coffee grinders.
Frequently asked questions
What is the difference between a bimodal and a unimodal particle size distribution?
Bimodal means two peaks: a fines peak and a coarse-fraction peak. Unimodal would mean just one peak. In the peer-reviewed literature, a unimodal distribution has never been measured at espresso grind size – all of them are bimodal or even trimodal. The correct term for grinders with a small fines peak is “fines-reduced”.
Do fines create more body in espresso?
What is documented is that fines lower permeability, slow the flow and lengthen extraction time. There is no causal evidence that they create more body. Smrke et al. (2024) found neither a sensory disadvantage nor a demonstrated body advantage with a higher fines content.
Where does a syrupy mouthfeel in espresso come from?
As far as we know today, from several phases at once: around 52.5 g/l of dissolved solids, emulsified oil droplets below 10 µm, around 150 mg of suspended cell wall fragments per cup and the crema foam, with a density of 0.40 to 0.60 g/ml. We couldn’t find a published viscosity measurement of espresso.
Do conical burrs give more body than flat burrs?
As a general rule, that doesn’t stand up to measurement. In an analysis of 300 particle size distributions across 24 espresso grinders, conical burrs were on average less unimodal, but the overlap between the burr types was large. The burr type alone says little about body.
Does bean origin affect the particle size distribution?
No. Uman et al. (2016) tested coffees from Guatemala, El Salvador, Tanzania and Ethiopia using analysis of variance: the differences were not significant. What clearly changes the distribution is the bean temperature during grinding – beans ground cold produce a narrower distribution.
Roastery tip
If you want to taste the difference between fines-rich and fines-reduced, pick a coffee with plenty of structure. Cozy Chocolate shows the dense, round side, Wild Peach the clear, fruity one. The same coffee on two different grinders is more eye-opening than any curve – and that is the only measurement that really counts in the end. Our recommendation, one of many.
