How to recognize over-extracted coffee?
An over-extracted coffee is one where too many soluble compounds — notably bitter and astringent ones — have been dissolved beyond the optimal point. In the cup, this produces a dry, lingering bitterness, a hollow mid-palate, and an aftertaste that grows progressively harsher as the coffee cools. Unlike under-extracted coffee (sour), over-extracted coffee is bitter and often hollow.
Over-extraction results from contact time too long, grind too fine, or water temperature too high — all of which dissolve an excessive proportion of late-stage extraction compounds. The compounds extracted last — degraded chlorogenates, tannins, certain phenolic acids — are precisely those that generate the dry bitterness characteristic of over-extraction. This bitterness is qualitatively different from the natural bitterness of a well-pulled espresso: natural espresso bitterness is intense but short and integrated, while over-extraction bitterness is dry, lingering, and progressively invasive.
In espresso, visual signs of over-extraction include a very long shot (over 35-40 seconds for a standard dose), a crema that is very dark and rapidly turns black at the centre, and excessive output volume. With filter methods (V60, Chemex), over-extraction shows as a total brew time exceeding 4-5 minutes, a dense cup without perceptible sweetness, and bitterness that intensifies on cooling. A less-known fact: over-extraction can also result from water that is too hard (highly mineralised), particularly high in magnesium and calcium ions, which can solubilise undesirable compounds more aggressively than soft water. The optimal TDS for espresso is 1.8-2.2 %; above 2.5 %, you enter over-extraction territory.
The basic correction is the inverse of under-extraction: coarser grind to slow extraction, slightly lower water temperature (drop to 90-92 °C), shorter contact time or lower coffee-to-water ratio. In espresso: aim for 25-30 ml in 25-30 seconds maximum. Over-extraction is generally considered harder to correct than under-extraction, because an over-extracted coffee also loses primary aromas — fruity and floral notes disappear behind the burnt bitterness.