Alkalinity — mostly bicarbonate — is the water’s capacity to neutralize acid, and it is the parameter that most often ruins coffee.
What it does
Coffee’s pleasant flavors include citric, malic, phosphoric and quinic acids. Those are the brightness, the fruit and the juiciness.
Bicarbonate neutralizes them chemically in the cup.
The effect
High-alkalinity water gives coffee that tastes flat, dull, chalky and lifeless, regardless of the beans.
A bright Kenyan brewed in high-alkalinity water tastes like a mediocre Brazilian. No grind, dose or temperature change recovers it.
The target
Around 40 ppm as CaCO₃, and below 70.
Many municipal supplies run 100 to 200 ppm, particularly in limestone regions. That is well above where coffee wants it.
Why hardness gets more attention
Hardness causes limescale, which destroys machines. That is visible, expensive and urgent.
Alkalinity only ruins the taste, which is quieter.
For flavor, alkalinity matters more.
Reducing it
Reverse osmosis removes it, and then you remineralize.
Ion exchange filters reduce it, partially.
Recipe water controls it exactly.
Boiling precipitates some temporary hardness and does relatively little for alkalinity overall.
Testing
Aquarium KH test kits measure carbonate hardness directly and cost very little. They are the practical home tool.
Last reviewed 30 October 2026