Under-Extraction

Under-extraction occurs when the brewing process fails to dissolve enough soluble material from the coffee grounds — generally below 18% extraction yield — leaving the pleasant sweet and balancing compounds trapped in the grounds while only the quickly dissolving acids and volatile aromatics make it into the cup. The result is coffee that tastes sour, thin, salty, sharp, and lacking the sweetness and body that a properly extracted cup should have. Under-extraction is the most common cause of sour coffee.

The sourness in under-extracted coffee comes from the early-dissolving organic acids — primarily citric, malic, and chlorogenic acids — that make it into the cup without the later-dissolving sugars and caramelization compounds to balance them. This creates a cup dominated by raw acidity with no sweetness or body to round it out. The flavor might remind you of biting into an unripe fruit — acidic and sharp rather than sweet and complex.

The most common causes of under-extraction are: grinding too coarse (which reduces surface area and slows extraction), brewing for too short a time, using water that is not hot enough, and using a coffee-to-water ratio with too much coffee for the amount of water. Light-roasted beans, which are denser and less soluble than darker roasts, are especially prone to under-extraction if brewed with parameters better suited to darker coffees.

Fixing under-extraction involves increasing one or more extraction variables. Grind finer, extend brew time, raise water temperature, or reduce your dose. For pour-over, pouring more slowly and agitating the bed (gentle swirling or stirring) can also increase extraction. As with over-extraction, change one variable at a time and taste the result. If the coffee tastes sour and thin, grinding two or three settings finer is usually the most impactful single adjustment.