Water for coffee: what each mineral does

Pure, distilled water extracts poorly: with nothing dissolved in it, coffee brewed with it tends to taste thin and flat. Tap water fixes that, but its minerals vary from town to town and season to season, so a recipe that works in one kitchen doesn't travel.

Mineral concentrates solve both problems. You start from distilled or reverse-osmosis water and add a measured dose, so you know exactly what's in the cup and can repeat it. The dose is usually given in ppm (parts per million) of dissolved minerals — but ppm only says how much, not what kind. 80 ppm of a magnesium salt and 80 ppm of a bicarbonate brew very different coffee.

Calcium and magnesium carry a double positive charge, which lets them hold on to flavour compounds in the coffee and help draw them into the water. More hardness extracts more: too little and the cup tastes thin and sour-edged; too much and it reads heavy, chalky or dry.

Magnesium is usually credited with brighter, fruitier, more complex cups. A computational study by Hendon, Colonna-Dashwood and Colonna-Dashwood (Journal of Agricultural and Food Chemistry, 2014) found it binds more strongly than calcium to most of the flavour compounds it modelled, which fits — though tasting evidence is thinner than the claim is popular.

Calcium tends to read rounder, sweeter and heavier. It's also what builds scale in kettles and espresso machines, which matters for equipment rather than for a pour over. Sources don't agree on which of the two gives more clarity, so treat "magnesium for brightness, calcium for body" as a starting point to taste, not a rule.

Bicarbonate is an alkali: it neutralises some of the coffee's acids before you taste them. It's the single biggest lever on how acidic a cup seems. Too little buffer and acidity turns sharp and sour; too much and the cup goes flat, dull and chalky.

Buffer is usually quoted as alkalinity, or KH, in mg/L of calcium carbonate. The Specialty Coffee Association's water standard aims for about 40, but many light-roast recipes go lower to let the acidity through — and most mineral products deliberately carry less buffer than hardness.

In most products bicarbonate arrives paired with sodium or potassium. Third Wave Water buffers with citrate instead.

• Sodium rounds a cup off in small amounts and tastes salty in larger ones. The SCA standard aims for about 10 mg/L and treats 30 as the ceiling. If your coffee tastes salty or metallic, sodium in the water is a likely cause.
• Potassium is mostly there as bicarbonate's partner. At brewing concentrations it isn't usually tasted on its own, which makes it the gentlest way to add buffer.
• Chloride and sulphate are the other halves of the salts that deliver calcium and magnesium. Brewers often describe sulphate as drier and crisper and chloride as rounder — a rule of thumb borrowed from beer, and not well tested for coffee.
• Silica appears in a few products, sold for mouthfeel. There's little published evidence either way.

A good place to start for filter coffee is around 80 ppm, with hardness near 50 and buffer near 20 (both as mg/L of calcium carbonate). That's at the soft end of the SCA's range, which runs from 50 to 175 for hardness. Most recipes land between 80 and 150 ppm; light roasts tend to do better at the lower end, and harder water can push a delicate coffee dry.

If a cup tastes thin, add a little more of the same water. If it tastes dry, chalky or heavy, use less. Change the amount before changing the product — it's one variable instead of several. The water drops calculator turns a target ppm into drops.

These come from each maker's own ingredient list and description. Most don't publish amounts, so this says what's in each one rather than how much.

• APAX Tonik — magnesium and calcium chloride, with sodium and potassium bicarbonate and sodium chloride. Magnesium-led and lightly buffered. APAX pairs it with washed, high-acidity coffees.
• APAX Jamm — magnesium and calcium chloride, potassium chloride, potassium and sodium bicarbonate, described as built on calcium and potassium bicarbonate. Calcium-led and the most buffered of the range, for naturals and other process-forward coffees.
• APAX Lylac — magnesium sulphate and chloride, potassium chloride, potassium and sodium bicarbonate, and sodium chloride. No calcium at all. Sold for delicate, floral coffees.
• APAX Konflux — calcium chloride, potassium and sodium chloride, silica, and potassium and sodium bicarbonate. The one APAX water with no magnesium, sold for depth and mouthfeel.
• Third Wave Water Classic — magnesium sulphate, calcium citrate and sodium chloride. The maker targets about 150 ppm with an alkalinity of 36–40, and independent tests show it's mostly magnesium.
• Third Wave Water Medium — magnesium sulphate, calcium citrate and potassium bicarbonate. No sodium, and about twice Classic's buffer, for medium and darker roasts.
• Aquacode — minerals concentrated from deep sea water, with calcium and magnesium at a stated 1:2.7. An independent test found almost no sodium and a moderate buffer.
• Lotus drops — four single salts sold as a kit: calcium chloride, magnesium chloride, sodium bicarbonate and potassium bicarbonate. Hardness and buffer come in separate bottles, so you build the water yourself.

Minerals add up exactly. Sixty parts of one water to forty of another gives you precisely 60:40 of their calcium, magnesium and bicarbonate, so blending two products is a reliable way to land between them — a little Jamm under Tonik to soften its edge, say, or a buffer drop under a hardness drop to build a water from scratch.

Flavour doesn't add up quite so neatly as the minerals do, so taste a blend rather than assuming it sits halfway between its parts.

Water is the last thing to change when dialling in, not the first. A different water moves several flavours at once, which makes it hard to tell what did what. The exception is a salty or mineral taste: that's the water distorting everything else, so fix it before judging the coffee. The dial-in guide covers the order to change things in.

• Water drops calculatorHow many drops of mineral concentrate for a target ppm.
• Salty or metallic coffeeWhen the water is the problem.
• How to dial in coffeeThe order to change things in.