A filter jug
A carbon and ion-exchange jug removes chlorine and softens moderately hard water. Change the cartridge on schedule; an exhausted one does nothing.
Chapter 04
The ingredient nobody weighs up
A cup of filter coffee is roughly 98.5% water. If the same beans taste lively in one kitchen and dull in another, the tap is the first suspect.
What a cup is made of
Brewed filter coffee is surprisingly dilute. Only a little over one percent of what is in the cup came out of the beans.
The bars show the approximate share of water in the finished drink. Typical strength for filter coffee is 1.15–1.45% dissolved coffee solids; espresso sits around 8–12%.
Hardness
Hardness is mostly dissolved calcium and magnesium. These minerals help pull flavour compounds out of the grounds, so a little is welcome: completely pure distilled water makes flat, oddly sharp coffee.
Too much, and the cup turns chalky and heavy while limescale builds up in kettles and machines.
Alkalinity
Alkalinity, mainly bicarbonate, is the water’s ability to neutralise acid. It matters more to taste than hardness does.
High alkalinity swallows the pleasant acidity of the coffee and leaves it dull and flat. Very low alkalinity lets acidity run unchecked, which can taste sour and thin.
Ranges to aim for
| Property | Good range | Too low | Too high |
|---|---|---|---|
| Total hardness | 50–175 mg/L as CaCO₃ | Flat, weak extraction | Chalky taste, limescale |
| Alkalinity | about 40–75 mg/L | Sharp, sour | Dull, muted acidity |
| Total dissolved solids | 75–250 mg/L | Thin, empty | Heavy, muddy |
| pH | 6.5–7.5 | Corrosive to machines | Usually signals high alkalinity |
| Chlorine | none | — | Chemical, swimming-pool notes |
Three practical fixes
Bottled water
Check the label for dry residue or total dissolved solids between roughly 75 and 150 mg/L and low bicarbonate. Many mineral waters are far too hard.
Build your own
Add measured amounts of food-grade magnesium sulphate and sodium bicarbonate to distilled water. Fiddly, but it is perfectly repeatable and very cheap per litre.
Temperature
Hotter water extracts faster. For light and medium roasts use water just off the boil, around 93–96 °C; these coffees are dense and need the energy. For dark roasts, which are brittle and very soluble, 88–91 °C keeps harshness down.
Without a thermometer: bring the kettle to a full boil and wait about thirty seconds with the lid open. That lands close enough for everyday brewing.
Altitude matters: water boils at a lower temperature the higher you live — about 95 °C at 1,500 metres. At elevation, pour straight off the boil.
Next chapterGear worth owning →