Why is my espresso sour?

Sour espresso is under-extracted. The acids in coffee dissolve first and the sugars need more contact time to follow, so a shot that finishes too early gives you the acids alone — sharp, thin, sometimes salty, like a lemon with no sweetness behind it.

Three things cause it, in order of how often they are the culprit.

Your grind is too coarse. This is the answer nine times out of ten. Water passes through a coarse bed too quickly to extract properly. Grind finer, one step at a time, and re-pull. If your shot is hitting 36 g in under 22 seconds, this is almost certainly it.

Your dose is too low. A basket rated for 18 g with 15 g in it leaves headroom, so the puck swells unevenly and water finds the edges. Weigh the dose and fill the basket to its rating.

Your water is too cool. Cool water extracts sugars poorly, which is exactly what a sour shot lacks. On a machine without temperature control, flush a little water through the group and wait ten seconds before pulling — this is the routine every single-boiler owner learns.

The one that looks the same but is not

If the shot tastes sour and bitter at once, stop adjusting the grind. That combination is channelling: water found a path around part of the puck, so some coffee is over-extracted and some barely touched. No grind setting fixes it. Stir the bed with a WDT tool before tamping.

What to do next

Fix the dose and the yield, then change only the grind until the shot lands between 25 and 32 seconds. Taste it then, and not before — time is a far easier signal to read than flavour when you are starting out.

If a single click takes you from gushing to choking, the method is fine and the grinder is not.