Water Chemistry for Better Coffee

Water is a solvent, not an ingredient

Brewing is extraction: water dissolves the soluble part of a ground coffee and carries it into the cup. A cup is roughly 98 to 99 percent water by weight, and the remaining one or two percent is everything you taste.

That makes the water's own composition part of the recipe. Water that arrives already carrying a load of dissolved minerals has less room to take on coffee. Water carrying almost nothing extracts aggressively and unevenly, pulling the harsh compounds along with the sweet ones.

Three numbers that matter

Magnesium is the one that does the work. It binds readily to the aromatic compounds in coffee and pulls them into solution, which is why water with some magnesium tastes fruitier and more vivid.

Calcium extracts too, but less selectively, and it leans towards a fuller, rounder mouthfeel. It is also what builds limescale in your kettle.

Bicarbonate — alkalinity — is the buffer. It neutralises acids as they are extracted. A little keeps a bright coffee from turning sharp; too much flattens the cup into something dull and chalky, and it is the single most common fault in hard tap water.

What to aim for

The widely used specialty target is around 150 mg/L of total dissolved solids, with alkalinity near 40 mg/L. You do not need to hit that precisely. What matters far more is avoiding the two extremes: water so hard it mutes everything, and water so soft it has no buffer at all.

Distilled or fully de-ionised water is not the answer. With nothing dissolved in it, extraction runs fast and thin, and the result tastes hollow and unpleasantly sour. Coffee needs some mineral content to taste like coffee.

Practical steps, in order of effort

Start with a simple carbon filter — a jug filter or a tap-mounted one. Carbon removes chlorine, which otherwise shows up as a faint swimming-pool note over everything, and it costs almost nothing.

If your tap water is hard, and in much of Iran it is, a filter that reduces hardness as well as chlorine is the meaningful upgrade. That is where a flat, chalky cup usually comes from.

At the far end, some people mix their own brewing water from distilled water and a mineral concentrate. It is precise and repeatable, and it is more trouble than most people want. Filtered tap water gets you most of the way.

Tasting the difference

Brew the same coffee twice, same dose, same grind, same temperature — once with tap water and once with filtered. Taste them side by side while both are warm.

If the filtered cup is brighter and the fruit is easier to name, your water was buffering the acidity away. If the tap cup tastes rounder and the filtered one thin or sour, your filter has stripped too much and you want less filtration, not more.