What is coffee extraction?

Coffee extraction is the transfer of soluble compounds from ground coffee into water. Roughly 28-30 % of the bean is soluble, yet only 18-22 % should end up in the cup for a balanced brew. Below that window coffee tastes sour and grassy; above it, bitter and astringent.

Extraction is a physico-chemical cascade where hot water dissolves and diffuses compounds out of each ground particle: organic acids first (citric, malic, quinic), then caramelised sugars and Maillard products, and finally heavier bitter phenolics. Each family has its own kinetics, which is why the same coffee can taste utterly different depending on contact time and grind size.

The American chemist Ernest Earl Lockhart codified this in the 1950s at MIT through the Coffee Brewing Control Chart, commissioned by the Coffee Brewing Institute. He proved that ground coffee usefully yields only 18-22 % of its dry mass: below that you stay in sour-grassy territory, above it you slide into dry bitterness. That corridor, called extraction yield (EY), became the backbone of the SCA Golden Cup. You measure it by multiplying TDS (percentage of dissolved solids in the cup, read on a VST or Atago refractometer) by beverage mass, then dividing by the dry dose.

Four levers drive extraction: grind size (finer particles expose more surface), water temperature (90-96 °C for filter methods; boiling water has lost dissolved oxygen), contact time (25-30 s espresso, 3-4 min V60, 4 min French press, 12-24 h cold brew), and turbulence (agitation, pressure, pour pattern). Quebec physicist Jonathan Gagné, author of 'The Physics of Filter Coffee' (2021), showed that the bimodal particle-size distribution of a Mahlkönig EK43 explains its reputation: fines diffuse fast, boulders keep flow controlled.

In Belgium, the old filter-coffee tradition served with a speculoos embeds these principles intuitively: soft Brussels tap water, medium grind, a generous 60 g/L ratio to offset darker roasts. The third-wave scene in Ghent and Antwerp has leaned on refractometry since roughly 2015, drawing on Scott Rao's 'Coffee Brewing Handbook' and Matt Perger's Barista Hustle syllabus — both now canonical on how to read a shot or a pourover scientifically.

Compound families and their order of extraction