Sour coffee is under-extracted coffee. The acids come out of the bean first; the sugars and the compounds that balance them come out later. Stop too early and you get only the acids.
The distinction that matters
Sour is sharp, thin and puckering, and it fades quickly. Acidic is bright, juicy and pleasant, like citrus or apple, and it has sweetness behind it.
The first is a fault. The second is what good coffee is supposed to taste like.
The fixes, in order
Grind finer. More surface area, more extraction. This is the biggest lever and the first thing to change.
Brew hotter. 94 to 96 °C. Under 90 °C, extraction slows enough to leave the cup thin and sour.
Brew longer. More contact time, more extraction.
Use more coffee. Sometimes what reads as sour is simply weak.
The roast factor
Light roasts are dense and hard to extract. If you switched to a light roast and everything went sour, that is why. Grind noticeably finer and use water at the top of the range.
The freshness factor
Coffee under four days from roast is still degassing heavily, and the gas interferes with extraction. Sourness that resolves after a few days was freshness.
If it is still sour
Check the water. Very soft or distilled water extracts poorly, and no grind adjustment fixes it.
Last reviewed 8 August 2026