Why does my coffee taste too sour?

Coffee tastes too sour when acidity shifts from a quality trait (lively, clean, fruit-like) to an aggressive, piercing, vinegar-sharp or astringent sensation. In eight cases out of ten, this signals under-extraction: water did not have enough time or surface to pull the soft, balancing part of the coffee.

Acidity and sourness are not synonyms. Quality acidity is prized in specialty coffee: it brings vibration, fruit character and geographic identity — an Ethiopian coffee must be acidic, a Kenyan SL28 is structurally so. Chemically, it rests on citric acid (citrus), malic acid (apple-pear), quinic acid (clean, dry), phosphoric acid (bright, mineral) and lactic acid (yoghurt, soft). Sourness, on the other hand, is the signal of a brew that stopped too early, leaving bright acids dominant because sugars and sweetness compounds have not been released.

Four practical causes to diagnose. First — and most common by far — grind too coarse: on a V60 or Chemex, if the brew drains in 2 min instead of 3-4, particles are too large and water lacks contact surface. Second, ratio too long (too much water): a 1:18 instead of 1:15 on filter thins out the cup and sharpens acidity. Third, water too cold: under 90 °C, sugars and melanoidins extract slowly and acids take over. Fourth, very light roast on a short-contact method: a Nordic light roast needs more contact time than a medium roast to balance out.

Corrections in order, all methods. 1. Grind one step finer — that solves roughly 70 % of cases. 2. Lift temperature to 94-96 °C on filter, 92-94 °C on espresso. 3. Tighten ratio to 1:15 (from a diluted 1:17-1:18). 4. Lengthen contact time (on French press, 4 min instead of 3). On espresso, a shot that pours in under 20 s at 1:2 is too coarse — finer grind. A key technical fact: SCA Golden Cup targets an extraction yield of 18-22 %; below 18 %, acidity dominates; above 22 %, bitterness does. Defect-acidity therefore marks extraction below 18 %.

In Belgium, many drinkers used to traditional filter coffee find Nordic specialty cups 'too sour' on first encounter. Two adaptation routes: favour Central American origins (Honduras, Guatemala) over African origins (Ethiopia, Kenya), and pick a medium roast over a light roast. At a specialty roaster in Brussels or Ghent, asking for 'a round, gentle cup' will usually route you toward a Brazil or Honduras medium — a natural bridge from brasserie filter to the third wave without an acidity shock.

Under-extraction: diagnostic and fix