The Science of Grinding: Particle Size and Coffee Extraction

Grinding is one of the most critical variables in coffee brewing, as particle size directly determines the surface area available for extraction. When coffee is ground, the exposed area increases exponentially; a 10 g dose of whole beans has minimal contact with water, but once ground to 400 μm particles, the surface area can exceed 1 m². This increased surface enables water to dissolve soluble compounds—acids, sugars, oils, and bitter alkaloids—in a controlled manner.

The extraction process follows the principle of mass transfer: hot water acts as a solvent and diffuses into coffee particles, extracting compounds in stages. First, acids (citric, malic) and sugars dissolve; then, more complex compounds like lipids and chlorogenic acids; finally, bitter phenols and tannins are extracted. If the grind is too fine or contact time is too long, over-extraction occurs, yielding bitter and astringent flavours. Conversely, if the grind is too coarse or contact time is too short, under-extraction results in sour and weak profiles.

For espresso, a very fine grind (200–400 μm) is needed. The high pressure (9 bar) forces water through the coffee cake in 25–30 seconds, achieving optimal extraction around 19–22 % total dissolved solids. A standard recipe uses 18 g of coffee for 36 g of espresso. To calibrate your grinder, adjust finer if extraction is under 25 seconds (under-extracted), or coarser if over 30 seconds (over-extracted).

Pour-over methods like the V60 require a medium-fine grind (600–800 μm). Brew time is typically 2:30–3 minutes with a ratio of 15 g of coffee per 250 g of water at 92–93 °C. The paper filter slows the flow and retains oils, producing a clean, bright cup. If the brew finishes in under 2 minutes, the grind is too coarse; if it exceeds 3:30, it's too fine.

French press uses a coarse grind (800–1000 μm) to prevent particles passing through the metal mesh. Brew time is 4 minutes with a ratio of 60 g/L. The longer immersion extracts oils and body, yielding a rich, full-bodied cup. If the coffee tastes muddy, grind coarser; if weak, grind finer or extend brew time.

Grinder quality is crucial. Blade grinders produce uneven particle distribution, leading to simultaneous over- and under-extraction. Burr grinders—flat or conical—produce uniform particles. Studies show that conical burrs generate bimodal distribution (fines and boulders), enhancing body, while flat burrs yield unimodal distribution, emphasising clarity. For specialty coffee, invest in a quality burr grinder; adjust grind size in small increments and taste the difference to find your ideal setting.