Particle Size Distribution (PSD)

When coffee is ground, the resulting particles are not uniform. They range from very fine dust (fines, <100 microns) to large chunks (boulders, >1000 microns). The distribution of sizes follows a curve that depends on the grinder type, burr geometry, and roast level. Key PSD metrics include: D50 (median particle size, where 50% of particles are larger and 50% are smaller), D10 (10% of particles are smaller than this), D90 (90% of particles are smaller than this), and the span (D90-D10)/D50, which measures distribution width. A narrow span indicates uniform grinding. PSD affects extraction because smaller particles extract faster (higher surface area to volume ratio) while larger particles extract slower. In a wide PSD, fines are over-extracted (releasing bitter, astringent compounds) while boulders are under-extracted (not contributing their full flavor potential).

The chemistry of Particle Size Distribution (PSD) is defined by the differential extraction kinetics of coffee's 1,000+ chemical compounds. Coffee extraction is a sequential process where compounds dissolve at varying rates based on their molecular weight and solubility. Polar, highly soluble compounds such as organic acids (citric, malic, and quinic) and fruit salts extract first, followed by medium-weight compounds like caffeine and 5-caffeoylquinic acid (5-CQA), and finally heavier, less soluble melanoidins and bitter polyphenols. A wide PSD disrupts this sequence: fine particles (<100 µm) facilitate near-instantaneous surface erosion, quickly releasing bitter-tasting tannins and astringent compounds into the brew. Conversely, larger particles (boulders) are subject to diffusion-limited extraction. Research into intra-bean diffusion suggests that water typically penetrates only about 100 microns into the cellulose matrix; therefore, particles larger than 200 microns in diameter often retain an under-extracted chemical core. This results in a 'bimodal chemical profile' where the beverage simultaneously contains the sharp, sour acidity of under-extracted boulders and the harsh, dry bitterness of over-extracted fines. Additionally, PSD affects the stability of volatile organic compounds (VOCs); ground coffee with a higher proportion of fines possesses a larger total surface area exposed to the atmosphere, which accelerates the oxidation of lipids and the degassing of carbon dioxide, leading to a more rapid loss of aromatic complexity.

The surface area to volume ratio of a particle determines its extraction rate. A sphere of radius r has surface area 4*pi*r^2 and volume (4/3)*pi*r^3, so SA/V = 3/r. Smaller particles (smaller r) have higher SA/V, meaning more surface is available for water to extract from relative to the particle's mass. A particle of 300 microns has 3.3x the SA/V of a 1000 micron particle. Fines (sub-100 microns) have extremely high SA/V and extract almost instantly, often over-extracting. The bimodal distribution typical of burr grinders produces two peaks: one at the target particle size and one in the fines range. This bimodality is inherent to the cutting action of burrs.

PSD is characterized by D10, D50, D90, and span = (D90-D10)/D50. Pour over ideal: D50 ~500-700 microns, span <1.5. Espresso ideal: D50 ~200-300 microns, span <1.5. Burr grinders produce bimodal distributions: a primary peak at the target size and a secondary peak in the fines (<100 microns). Fines content of 10-20% is typical for espresso grinders. High fines increase extraction but also increase bitterness and astringency. Laser diffraction (Malvern, Sympatec) is the gold standard for PSD measurement. Sieve analysis provides a cheaper alternative. The uniformity index (UI = 100 x D10/D90) measures uniformity: higher UI = more uniform.

Invest in a quality burr grinder with uniform particle distribution. Flat burr grinders typically produce narrower PSD than conical burrs. For pour over, use a medium grind (D50 ~600 microns). For espresso, use a fine grind (D50 ~250 microns). If your coffee tastes bitter despite a coarse grind, your grinder may be producing too many fines. If your coffee tastes weak and sour, your grinder may be producing too many boulders (insufficient grinding). WDT (Weiss Distribution Technique) helps distribute fines evenly in espresso pucks, reducing channeling.

• •All burr grinders produce uniform particle sizes. In reality, all burr grinders produce bimodal distributions with a fines peak. Higher-quality grinders produce narrower distributions with fewer fines.
• •Conical burrs are inferior to flat burrs. In reality, each has different PSD characteristics. Conical burrs tend to produce wider distributions with more fines, but some prefer the resulting cup complexity.
• •Sifting (sieve analysis) removes all fines. In reality, sifting removes only the largest fines. Sub-100 micron fines often pass through standard sieves.

• •Corrochano et al. (2015) demonstrated that PSD, not just median particle size, is the primary determinant of extraction uniformity.
• •Research has shown that flat burr grinders typically produce narrower PSD (span ~1.0-1.3) compared to conical burr grinders (span ~1.5-2.0).
• •Studies on espresso extraction have shown that fines content above 20% significantly increases bitterness and astringency.
• •Laser diffraction analysis has revealed that grinder wear and burr alignment significantly affect PSD uniformity.

• •Chapter 5: Extraction Science
• •Chapter 8: Grinding

• •Corrochano, B.R. et al. (2015). 'Grinding Coffee: A Multi-Scale Approach.' Powder Technology.
• •Fasano, A. et al. (2020). 'The Mathematics of Espresso Extraction.' SIAM Journal.
• •Anderson, G. et al. (2019). 'Analysis of Coffee Particle Size Distribution.' Journal of Food Engineering.
• •Uman, E. et al. (2020). 'The Effect of Bean Origin and Temperature on Grinding.' Scientific Reports.

• •Coffee Science Foundation
• •Powder Technology Journal
• •Hoffmann Coffee Grinder Testing
• •Heise Particle Size Analysis