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