Two quite different causes, and they are easy to tell apart.
Severe under-extraction
Coffee contains salts that dissolve very early in an extraction, before the acids and well before the sugars. If a brew stops extremely early — a badly gushing espresso, a filter brew that drained in ninety seconds — those salts dominate.
Salty is therefore a signal of more severe under-extraction than sour. If you are tasting salt, the grind is far too coarse rather than slightly.
Grind finer, and check the dose while you are there.
Softened water
The other cause, and the one nobody suspects. Domestic ion-exchange softeners do not remove hardness — they exchange calcium and magnesium for sodium. Sodium is perceptible at low concentrations and reads as salty.
You also lose the minerals that help extraction, so the cup is flat as well as salty.
Fill the machine from an unsoftened tap — most installations leave the kitchen cold tap on the hard supply — through a filter.
Telling them apart
Under-extraction salt comes with thinness and sourness, and it changes when you change the grind. Water salt is consistent across every brew method and every setting.
Last reviewed 8 August 2026