What extraction actually is

Extraction is the process of dissolving soluble compounds out of ground coffee into water. Three variables control it: grind size (how much surface area is exposed), water temperature, and contact time. Under-extraction leaves mostly acids in the cup and tastes sour or sharp; over-extraction pulls more bitter compounds and tastes harsh, hollow or astringent.

The SCA's widely used gold-cup reference targets an extraction yield (EY) of 18–22% — meaning roughly a fifth of the dry coffee's mass ends up dissolved in the final drink.