6 September 2026 · 4 min read
a finished cup is about 98% water and only about 2% dissolved compounds from the bean. which gives a paradox: we agonise over the coffee, the grinder and the brew method, then take the single biggest ingredient straight from the tap without a thought. a mistake — the same lot on different water can taste like two different coffees.
water is a solvent
extraction is water pulling flavour out of the ground coffee. how well it does that depends on its makeup. soft minerals (magnesium and calcium above all) help «grab» the aromatic compounds — they act like magnets for flavour. carbonates (alkalinity, or «carbonate hardness») do the opposite: they mute acidity and can flatten the cup into something dull. so what matters isn't just «purity» but balance: a little mineral yes, excess alkalinity no.
what's wrong with tap water
the problem usually isn't that it's «dirty». it's chlorine, which gives a chemical/swimming-pool taste and kills aroma, and excess hardness or alkalinity, which dulls the flavour. in many cities the water is exactly «hard» — coffee on it comes out flat and «muffled» even when the bean is excellent. distilled water is the other extreme: with no minerals at all, extraction goes poorly and the coffee comes out flat and «nothing». the truth is in the middle.
targets (without obsessing)
specialists talk about roughly 50–150 mg/l of total dissolved solids (tds), moderate total hardness and low alkalinity. there are even official sca recommendations with numbers. but you don't need to memorise tables — just hold the principle: a little mineral for body and extraction, minimal alkalinity and zero chlorine.
simple fixes without a lab
you don't need reagents and meters to noticeably improve the cup:
- a filter jug with a carbon cartridge removes chlorine and some hardness — the cheapest noticeable step
- bottled water with a low mineral content (check the label: ~50–150 mg/l) — convenient and consistent
- dedicated coffee mineral sachets/drops added to pure (osmosis/distilled) water — for those who want full repeatability
- reverse osmosis with remineralisation — for the most demanding and for cafes.
about scale and equipment
hard water hurts more than taste: carbonates settle as scale in the kettle, the coffee maker and the espresso machine, and slowly kill them. softer, balanced water also means a longer life for your equipment and less descaling.
test it on yourself
the most convincing test is free: take your favourite coffee and brew it twice, side by side — once with tap water, once with filtered or bottled. the difference is usually obvious from the first sip: cleaner acidity, more sweetness, less «dullness» and chemical taste. it's the cheapest flavour upgrade there is — and, oddly, the one most often ignored, even though it costs less than a new bag of beans.