tags: [] - coffee/brewing - coffee/equipment aliases: - Particle size distribution - Grind distribution - Coffee particle distribution
Grind Size Distribution¶
Tags: #coffee/brewing #coffee/equipment Aliases: Particle size distribution, Grind distribution, Coffee particle distribution Related: Coffee Grinders MOC | ../Maps of Content/Grind Size MOC | Extraction | Flow Rate | Particle Uniformity Status: ✅ Complete
Overview¶
Grind size distribution describes the spread of particle sizes produced when coffee is ground — not just the average particle size, but the full range from the finest dust to the largest fragments. All grinders produce a distribution of particle sizes rather than uniform particles of a single size, and the shape of this distribution (how narrow or wide the spread, how large the proportion of ultra-fine particles) directly affects extraction uniformity, flavour clarity, and the optimal grind setting. Burr geometry, material, and speed are the primary determinants of grind distribution quality.
Why Distribution Matters¶
If all coffee particles were the same size, they would all extract at the same rate and the brewer could control extraction simply by adjusting grind size. In practice, every grinder produces a range:
- Main peak (modal size): The target particle size that the brewer intends to produce
- Fines: Ultra-small particles far below the target size; extract rapidly and often over-extract during normal brew time
- Boulders: Particles larger than the target; extract slowly and often under-extract
A wide distribution with many fines and boulders makes it difficult to simultaneously avoid over-extraction (from fines) and under-extraction (from boulders). The resulting cup can taste simultaneously bitter and sour — the "muddy" or "complex in the wrong way" character associated with poor grinders.
Bimodal Distribution¶
Most grinders produce a bimodal distribution — two peaks: 1. Primary peak: The intended coarse particle size (set by burr gap) 2. Secondary fine peak: Ultra-fine particles generated by fracturing at crystal boundaries in the bean
The secondary peak in a bimodal distribution is inevitable at some level because coffee cell walls shatter differently at fine burr gaps. High-quality grinders minimise the secondary peak; low-quality grinders produce a pronounced secondary fine peak.
Factors Affecting Distribution¶
| Factor | Effect on distribution |
|---|---|
| Burr geometry (flat vs. conical) | Flat burrs typically produce narrower distribution; conical burrs wider |
| Burr size (diameter) | Larger burrs → more uniform grinding; better distribution |
| Burr material | Steel vs. ceramic affects fracture patterns and fines generation |
| RPM (burr speed) | Higher speed → more heat → more fines; lower speed generally produces cleaner distribution |
| Bean density | Denser beans (high altitude, light roast) produce more fines at equivalent grind settings |
| Roast level | Dark roasts are brittle; generate more fines at equivalent settings |
Measurement¶
Grind distribution can be measured by: - Laser diffraction particle analysis (PSD): Laboratory standard; measures the full particle size distribution curve - Sieve analysis: Physical sieves of known micron ratings; separates fractions by weight - Visual inspection: Course approximation; experienced grinder reviewers use this - Brewing analysis: Indirect assessment by tasting and measuring extraction across dial-in settings
Implications for Espresso vs. Filter¶
Espresso: Fines fill the spaces between coarser particles, increasing puck resistance and contact time. Some fines are necessary for proper espresso flow resistance. Excess fines cause channelling and bitterness.
Filter coffee: Fines extract rapidly during the early pour stages and contribute bitterness and muddiness. Paper filtration prevents fines from entering the cup, but they still extract during contact time. Metal filters pass fines into the cup.
Key Facts¶
- Grind size distribution describes the full range of particle sizes from a grind setting — not just the average
- All grinders produce a bimodal distribution; a prominent fine secondary peak worsens cup quality
- Fines over-extract and cause bitterness; boulders under-extract and cause sourness; both worsen with poor grinders
- Larger, slower burrs generally produce narrower (more uniform) distributions
- Laser diffraction is the laboratory standard for measuring distribution; sieve analysis is the accessible alternative
Related Notes¶
- Particle Uniformity
- ../Maps of Content/Grind Size MOC
- Extraction
- Flow Rate
- Coffee Grinders MOC
References¶
- Specialty Coffee Association — Grinder Standards
- Rao, S. & Perger, M. (2018). The Physics of Filter Coffee. Scott Rao.
Changelog¶
| Date | Change |
|---|---|
| 2026-04-28 | Note created |
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