Alkalinity (KH/buffer)
also: KH, carbonate hardness, buffer
Water's buffering capacity, mostly bicarbonate, which tames acidity; too much flattens the cup.
Alkalinity, often labeled KH or “buffer,” is water’s capacity to neutralize acids. It comes mainly from dissolved bicarbonate, and it works by absorbing the acidic compounds coffee releases during brewing, holding the water’s pH steady.
This is the second of the two key water numbers, alongside general hardness (GH). Where GH controls how much flavor the water extracts, alkalinity controls how much of the coffee’s acidity survives into the cup. Some buffering is good: it smooths out harsh sourness and keeps a brew from tasting sharp. But too much acts like a blanket, muting the bright, lively notes that make a light-roasted single origin sing, leaving the cup flat and chalky.
This is why high-alkalinity tap water can make even great beans taste dull, and why water-recipes aim for low buffer with adequate hardness. A common SCA-style target for alkalinity is around 40 ppm (as CaCO3), within a roughly 40 to 70 ppm range. Confusingly, alkalinity often tracks with hardness in nature even though it is a separate property.