Underextraction: too little dissolved — coffee tastes sour, thin, salty. Overextraction: too much dissolved — coffee tastes bitter, woody, dry in the mouth. The goal is the balance in between, where sweetness, acidity, and bitterness harmonize.

What is underextraction and overextraction?

What happens during extraction

When brewing, hot water dissolves substances from the coffee grounds — acids, sugars, bitter compounds, oils. These substances do not all dissolve at the same time: First come the acids (fast dissolving), then the sugars and caramel notes, finally the heavy bitter compounds. Extraction is therefore a temporal process with a clear order.

Ideal is 18–22% extraction — meaning 18–22% of the dry mass of the coffee grounds end up in the cup. Below that is too little, above is too much.

Recognizing underextraction

Underextracted coffee tastes sour (piercing, not pleasantly fruity), thin, sometimes salty or grassy. It lacks sweetness and body. The finish is short and unsatisfying.

Causes:

Grind too coarse (water flows through too quickly)
Brew time too short
Water temperature too low (under 90 °C)
Too little water relative to coffee

Solution: Grind finer, brew longer, or use hotter water — changing one variable at a time.

Recognizing overextraction

Overextracted coffee tastes bitter, woody, ashy, or astringent (dry mouthfeel, like over-steeped black tea). The bitterness masks everything else.

Causes:

Grind too fine (water takes too long)
Brew time too long
Water temperature too high (over 96 °C)
Too much water relative to coffee

Solution: Grind coarser, brew shorter, or lower the temperature.

The sweet spot

The ideal extraction lies in between: acidity and bitterness balance each other out, sweetness emerges, the body feels round. With espresso, you recognize this by the color of the crema — golden brown with tiger stripes. With filter coffee, by the balanced taste without sour or bitter dominance.

In practice at Green Wall Coffee

At Sophienstraße 27 in Berlin-Lichtenberg, I adjust the grind size several times a day — because humidity, bean degassing, and temperature influence extraction. If an espresso tastes sour, I go one click finer. If it gets bitter, one click coarser. This loop — taste, adjust, repeat — is the core of good coffee preparation.

You can find more depth on this topic in the article How to make perfect espresso. Or drop by Sophienstraße 27 — Mon–Fri 8am–5pm, Sat 10am–5pm.