What is over-extraction in coffee brewing?

Over-extraction happens when too much material is dissolved from the coffee grounds during brewing - pulling out the desirable compounds but continuing past them into the bitter, astringent, and hollow-tasting ones that develop with prolonged contact. The cup tastes harsh, dry, and unpleasantly bitter.

The main causes are grind size too fine (slowing water flow and increasing contact time), water temperature too high (increasing extraction rate), brew time too long, or too little coffee relative to water. Any of these alone, or in combination, can push extraction past the ideal window.

For home roasters, over-extraction is worth understanding because roast level affects how quickly coffee extracts. A lighter roast is denser and less porous than a darker one, which means it resists extraction - you typically need a finer grind or higher temperature to hit the same extraction level. A darker roast extracts faster and is more susceptible to over-extraction at the same brew parameters. If a freshly roasted light-to-medium coffee is tasting bitter rather than sweet and bright, under-development is more likely the culprit than over-extraction - though the symptoms can feel similar.