Under-extraction

Extraction below 18% EY, producing an acidic, salty, hollow cup. Causes: too coarse grind, too cool water, too short time. Correction: finer grind or higher temperature.

Background & Context

Under-extraction occurs when an insufficient proportion of the coffee's soluble compounds has been dissolved during brewing, the extraction yield is below 18% on the SCA Brewing Control Chart. The flavour signature is characteristic: sharp, piercing sourness (undissolved sugars leave unbalanced organic acids dominant), lack of sweetness or body, sometimes a salty or grassy quality, and a very short, empty finish. The mechanism: coffee compounds dissolve in a fixed sequential order as extraction progresses. At low EY, only the first soluble compounds, acids and small aromatic molecules, have dissolved, with the sugars and body-giving compounds (melanoidins, colloids) not yet fully extracted. This is why under-extracted coffee tastes acidic and hollow simultaneously. In espresso, under-extraction typically results from: too coarse a grind (water flows too fast through the puck), too low a temperature (below 90°C for most coffees), too short an extraction time (below 20 seconds), or insufficient pre-infusion. In filter coffee: too coarse a grind, too short a brew time, or too cold water. Under-extraction can also occur with fresh beans whose excessive CO₂ degassing prevented proper water-coffee contact, the bloom step exists precisely to solve this.

Practical Use

Diagnosing under-extraction from over-extraction: under-extraction = sour + hollow + lacking sweetness; over-extraction = bitter + astringent + drying. To fix under-extraction in espresso: grind finer (most direct lever), increase temperature to 93-95°C, or extend brew time by 3-5 seconds. In filter: grind finer, increase water temperature to 95-96°C (for light roast, which is denser). The salt test: adding a pinch of salt makes over-extracted coffee taste much better but has minimal effect on under-extracted coffee.

Related Terms

Related terms: Over-extractionthe opposite defect with different causes and flavour. Extraction yieldunder-extraction = EY below 18%. Aciditydominant flavour signal of under-extracted coffee. Grind sizetoo coarse is the primary cause of under-extraction.