A sour cup tastes sharp, thin and a little salty, with a finish that disappears fast. In almost every case the coffee is under-extracted, meaning the water didn't pull enough out of the grounds. The sweet, rounded flavours come out after the bright acids, so a short or weak extraction stops before it gets there.
The quickest fix: grind finer
Finer grounds have more surface, so water extracts more in the same time. Move one or two steps finer and brew again with everything else the same. On espresso the shot should also run a little slower, and on pour-over the drawdown should take a little longer.
If grinding finer isn't enough
- Use more water for the same dose. On espresso, let the shot run longer (for example 18 g in, 40 to 45 g out instead of 36 g). On pour-over, a longer ratio like 1:16 to 1:17 extracts more.
- Use hotter water. Light roasts in particular want water just off the boil, around 94 to 98 °C.
- Brew for longer. A slower pour, an extra pour or a longer steep all give the water more time.
- Check the beans. Very fresh light roasts can taste sour for the first week after roasting, and a few more days of rest often helps.
Sour and bitter at the same time?
That usually means uneven extraction, where some grounds over-extract while others under-extract. On espresso, look at puck prep (break up clumps, tamp level). On pour-over, make sure every pour wets the whole bed, and swirl to settle it flat.
Change one thing at a time
Move the grind, brew, taste, and write down what changed. Once a setting tastes right, note it down with the coffee, grinder and brew method, so the next time you brew it you start from there instead of from scratch.
Find your grinder's typical settings in the grind size chart.
Notchbook is a free app that remembers the grind setting that worked for each coffee, grinder, brew method and filter paper, and keeps a journal of every brew. Get it free on Google Play. The iPhone app is coming soon. More about Notchbook.