The Science of Extraction
Extraction is the process of dissolving soluble compounds out of roasted, ground coffee and into water. A coffee bean is mostly insoluble plant matter — cellulose, fiber, structural carbohydrate — but roughly 28–30% of its mass can, in principle, dissolve. Brewing is the controlled act of taking some of that soluble fraction. How much you take, and which compounds, decides whether the cup is sour, sweet, balanced, or bitter. Everything else in pour over is in service of steering this one process.
#The Physics: Three Things Happening at Once
When water contacts a coffee particle, three mechanisms drive flavor out:
- Dissolution — soluble solids (acids, sugars, flavor compounds) leave the particle surface and enter the water.
- Diffusion — compounds deeper inside the particle migrate outward to the surface to be dissolved, the slow rate-limiting step.
- Erosion — agitation and flow physically wash material off the surface.
Extraction is driven by a concentration gradient: water grabs compounds fastest when it is fresh and "empty," and slows as it loads up. This is exactly why percolation brewing like pour over — where fresh water continually passes through the bed — extracts efficiently, and why immersion methods like the French press, sitting in increasingly saturated water, extract more gently.
#What Controls the Rate ⚙️
| Lever | Effect on extraction | Note |
|---|---|---|
| Grind size | Finer = more surface area = faster | Grind Size and Surface Area |
| Temperature | Hotter water dissolves faster | The Role of Temperature in Extraction |
| Time | Longer contact = more extracted | Brew Time and Total Contact Time |
| Agitation | Stirring/turbulence speeds erosion | Agitation and Turbulence |
| Water chemistry | Minerals help bind flavor | Water Chemistry — Hardness and Alkalinity |
You don't "fully extract" coffee — you stop somewhere along a curve. Pull too little and you get under-extraction; push too far and you get over-extraction. The goal is to land in the tasty middle.
#A Common Myth
Half-true at best. Sourness and harsh astringency can both come from the same brew if extraction is uneven — some particles over-extracted while others under-extracted via channeling. Average extraction can look fine while the cup tastes wrong. Read average and local extraction as different things.
The numbers that describe the outcome — how much dissolved and how strong the cup is — are covered in Extraction Yield and Strength, and the order in which compounds come out is in Solubility and What Dissolves.
#Continue Reading
- Solubility and What Dissolves — which compounds leave, and when
- Extraction Yield and Strength — the two numbers that score a brew
- Grind Size and Surface Area — the master rate lever
- The Role of Temperature in Extraction — heat and dissolution
- Science and Extraction — back to the domain hub