Why is my coffee sour?
Sour usually means under-extraction: you did not pull enough out of the grounds. Grind finer, use hotter water, or brew longer.
A sharp, sour, lemony tang (the unpleasant kind, like an unripe taste rather than a pleasant brightness) almost always means under-extraction. You pulled too little flavour out of the grounds, so the first, most acidic compounds dominate and the sweeter, rounder ones never arrive.
Fix it by extracting more
Change one thing at a time:
- Grind finer. This is the biggest lever. Smaller particles expose more surface area, so water pulls out more. If your coffee is sour, finer is usually the answer.
- Use hotter water. Aim for around 92 to 96°C (just off the boil). Cool water extracts slowly and leaves you short.
- Brew a little longer, or pour more slowly, so water stays in contact with the grounds.
- Check your ratio. Wildly too much coffee for the water can also read as sour and weak at once.
The opposite problem
If your coffee is instead harsh and bitter, you have gone too far the other way (over-extraction): grind coarser, cool the water slightly, or brew faster. Sour and bitter are the two ends of one dial, which is why the sour vs bitter axis is the single most useful troubleshooting idea in brewing.
One caution: make sure it is genuinely sour and not just bright. A good light-roast Kenyan is supposed to taste juicy and acidic. Sourness that feels thin and empty is under-extraction; acidity that feels sweet and lively is the coffee doing its job.