Particle-size-distribution (PSD) data from a grinder shouldn't be summarized by a single median value alone. The tail of the distribution — where very fine or very coarse particles sit — can carry important information about extraction performance that a single median number can't reveal.

PSD needs to be read as a distribution

A complete PSD result is a curve or table describing the mass or particle-count percentage at each size interval, not a single number. Two samples with the same median size (X50) can still have different distribution shapes, leading to different extraction behavior when brewed.

What tail distribution means

Tail distribution refers to the portions of the distribution curve at either end, corresponding to particles significantly smaller or larger than the bulk of the sample's mass. Both the fine tail (fines) and the coarse tail (boulders) belong to this group, even though they usually make up only a small proportion of the sample's mass compared with the central portion of the distribution.

Why the fines fraction deserves attention

Research by Smrke, Eiermann, and Yeretzian on the role of fine particles in espresso extraction dynamics found the proportion of very fine particles can significantly affect the extraction process, even though they make up only a small fraction of the sample by mass. The proposed mechanism relates to fine particles having a large surface area per unit mass and potentially altering how water moves through the coffee bed.

How to read the tail when X50 values are close

When comparing two samples with nearly identical median size, looking at the tail of the distribution can help reveal differences that the median index misses. A sample with a thicker fine tail, even with an equivalent X50, can produce a different flow rate or extraction efficiency than a sample with a thinner fine tail.

Raw material can also change the tail

Research on the effect of bean origin and temperature on grinding roasted coffee shows characteristics of the bean before grinding, such as brittleness related to roast level or bean temperature at the time of grinding, can affect the shape of the resulting particle-size distribution, including the tail. This means a change in the tail doesn't necessarily always originate from the grinder itself.

PSD doesn't automatically predict sensory outcome

A particle-size distribution, even when the tail has been analyzed in detail, is still just physical size data. It doesn't automatically tell you how a drinker will rate the resulting brewed cup. To know that, you still need additional data on extraction efficiency and, if needed, direct sensory evaluation.

Using the tail to track consistency

In daily operations, tracking the tail of the distribution across measurements can help detect early signs that a grinder is starting to produce more fine particles than usual, for example due to worn blades or an incorrectly adjusted grind gap. This is a highly practical use of PSD data, focused on detecting change over time rather than searching for one absolute ideal distribution shape.

Practical conclusion

Reading PSD data only through the median size can miss important information sitting in the tail of the distribution, especially the fine-particle ratio. Tracking the tail over time, cross-referenced with extraction efficiency and raw-material characteristics, is a more practical approach than relying on a single median number alone.

References

• Smrke, 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.
• Particle Size Distribution Model for Optimizing Coffee Grind Consistency (2025). Journal of Food Process Engineering, 48, e70129.
• Effect of particle size distribution on mass transfer during solid-fluid extraction and its application to coffee brewing (2025). Journal of Food Engineering, 394, 112511.
• The effect of bean origin and temperature on grinding roasted coffee (2016). Scientific Reports.