What is particle size distribution (PSD) in coffee?
PSD (Particle Size Distribution) describes the range of particle sizes produced by a coffee grinder. A grinder creates particles of varying sizes — from very fine (< 100 µm, called 'fines') to coarse (> 1000 µm, 'boulders'). The width and shape of this distribution directly influences extraction quality and uniformity: a narrow PSD (particles of similar size) produces more uniform and predictable extraction than a wide PSD.
PSD is one of the most studied parameters by coffee scientists and engineers since the 2010s, notably through the work of Christopher Hendon (chemist, University of Oregon) and collaborations with grinder manufacturers such as Mahlkönig, Feld Company and Anfim.
How a PSD forms: when a grinder's burrs or discs mill coffee beans, they do not produce particles of a single size but a continuous spectrum. Flat or conical burr grinders typically produce: — 'Boulders' (> 800-1200 µm): very large particles, under-extracted — 'Target' particles (the desired zone by method: 250-650 µm for espresso, 500-1200 µm for filter) — 'Fines' (< 100-200 µm): very small particles that extract very rapidly and can create astringency
An ideal PSD is narrow and centred on the target size, with few fines and few boulders. Inexpensive home grinders produce wide, irregular PSD — often the primary difference between a €50 and a €500+ grinder.
The influence on extraction is multifactorial. Fines extract in seconds (they contribute body but also astringency if excessive). Boulders extract very little, diluting the cup with underdeveloped aromas. A wide PSD therefore inevitably creates uneven extraction: some particles are over-extracted when others are barely touched.
Grinders that produce narrow PSD (valued by specialty cafés): flat-burr grinders like the Mahlkönig EK43 or Mazzer Robur, hardened steel disc grinders from Feld Company (espresso), 98 mm burr grinders from Lagom and Anfim. These grinders reduce fines while maintaining a tight target zone.
PSD is measured with laser particle analysers (Malvern diffractometer, Mastersizer) — laboratory equipment giving a full statistical distribution. In professional practice, PSD is evaluated indirectly through cup results and espresso behaviour (flow, resistance, puck autopsy). Vibrating sieve analysis at different mesh sizes is an accessible artisan alternative.
PSD fractions and their extraction impact
- Fines (< 100 µm): ultra-fast extraction, body, astringency if excessive
- Fine-medium particles (100-300 µm): fast-extracting, acids and sugars
- Espresso target zone (250-500 µm): standard extraction in 25-30 s at 9 bar
- Filter target zone (400-1200 µm): extraction in 3-5 min, good yield 18-22%
- Boulders (> 1200 µm): poorly extractable, dilute and flatten the profile
- Narrow PSD = uniform extraction = cleaner and more predictable cup
- Wide PSD = heterogeneous extraction = simultaneously bitter and sour
Why the shape of the distribution matters as much as its centre
A grinder's particle size distribution (PSD) is not simply characterised by its median particle size. The full distribution — represented as a histogram of particle sizes — has a shape that tells you more. A narrow, tall, symmetric distribution (low standard deviation) means nearly all particles will extract at similar rates and reach target EY simultaneously. A wide, flat distribution with a heavy tail toward small particles (many fines) means some particles over-extract while others under-extract, producing the confused flavour described as 'muddy' or 'average' even when median particle size is correct.
Laser diffraction particle size analysis — the laboratory technique used to characterise grinder output by researchers at the University of Bath and commercial labs like those at the Global Coffee Platform — shows that single-dose grinders like the Niche Zero produce narrower distributions than commercial multi-dose grinders at comparable price points. This is partly mechanical design (the Niche's conical burr geometry reduces fines generation) and partly a function of single-dosing itself: a static bed of beans fed slowly into a conical burr experiences fewer re-grinds than continuously fed multi-dose commercial grinders where particles can contact the burrs multiple times.
Going deeper
Bimodal distribution — a histogram with two peaks, one at fine particles and one at coarse — appears in many consumer grinders and is the specific enemy of even extraction. The two peaks represent two populations extracting at completely different rates. Some specialty barista communities address this with sifting: using a mesh screen (typically 400 µm pore size for espresso) to remove fines before dosing, intentionally narrowing the distribution. The technique improves extraction uniformity measurably but adds time and generates waste. Equipment manufacturers have responded by designing burr geometries specifically to reduce fines generation, making the sifting workaround less necessary in high-end grinders released post-2020.