Water for Coffee: What Should You Brew With?
People fuss over beans and grinders and then use whatever comes out of the tap. Yet more than 98% of the cup is water, and water is also the solvent doing the extracting. If a coffee tastes different at home than it did in the cafe, water is a real suspect.
Water does two jobs
Water is both the solvent that carries compounds out of the grounds and an ingredient that contributes flavor of its own.
As a solvent, its performance depends on minerals. Completely pure water actually extracts poorly: magnesium and calcium ions bind to coffee compounds and help pull them into solution. Too few minerals gives a flat, hollow cup.
Too many creates the opposite problem. The ions are already saturated and cannot carry much more, and high bicarbonate content in particular neutralizes coffee acidity, leaving it dull and blunted.
Hardness and alkalinity
Two numbers come up in water discussions. Hardness is the calcium and magnesium content, which governs extraction power. Alkalinity is the bicarbonate content, which measures buffering capacity against acid.
They do different things. Low hardness gives weak, hollow extraction; high hardness over-extracts and scales your equipment. Low alkalinity lets acidity spike sharply; high alkalinity flattens acidity and removes the character.
The SCA figures below are the usual reference. Few people measure these at home, but the mineral content printed on a bottle of water gives you a rough idea.
Choosing your water
In practice the options are tap, filtered and bottled, and each has a clear trade-off.
Tap water in many places is reasonably soft and perfectly usable, but residual chlorine masks aroma. Boiling drives most of it off, so for filter brewing it is often not a serious issue.
Reverse-osmosis filters strip out the minerals along with everything else, which can leave extraction underpowered. If the water tastes clean but the coffee tastes flat, this is worth suspecting. At the other extreme, mineral-rich European bottled waters carry enough alkalinity to flatten acidity entirely.
Testing it at home
You do not need a meter to find out whether water is your problem. Brew the same coffee at the same grind and recipe with two different waters and taste them side by side.
Compare whatever you normally use against a soft bottled water in the 50–150 hardness range. If there is a clear difference, water is a variable worth managing. If there is barely any, you can move it down the list.
White scale building up inside your kettle is a sign you are on hard water. Scale shortens the life of equipment too, so in that case changing water helps on more than one front.
Record the water too
Water is easy to leave out of notes because it rarely changes. Then you move house, replace a filter cartridge or switch bottled brands, and the coffee quietly changes with it. If water is not on your list of suspects you can spend a long time looking elsewhere.
A single line in your recipe notes is enough. Recording the water and the temperature alongside the rest means you can line up the date the water changed against the date the coffee did.