Water Chemistry for Coffee

Coffee is around 98.5 % water, and the minerals in that water are not passive. They change how compounds extract, how acids taste, and whether your kettle and espresso machine survive the year.

The two numbers that matter

General hardness (GH) is the concentration of calcium and magnesium ions. These ions bind to flavour compounds and help carry them into the cup; magnesium in particular is associated with brighter, fruitier extraction, calcium with body and sweetness. Expressed in ppm as CaCO₃.

Alkalinity (KH) is the water's buffering capacity — mostly bicarbonate. It neutralises acids. Too much and a bright Kenyan tastes flat and chalky; too little and the same coffee is sharp and unbalanced.

Neither is captured by a cheap "TDS pen", which measures total conductivity and can't tell you which minerals are present. Use test strips, a titration kit, or your utility's water report.

Reference targets

	SCA target	Practical range
Total hardness (GH)	50–175 ppm CaCO₃	60–100 ppm
Alkalinity (KH)	40–75 ppm CaCO₃	30–50 ppm
pH	6.5–7.5	—
Sodium	< 30 ppm	as low as possible
Chlorine	0	0 — always filter

The general pattern of modern specialty water: moderate hardness, lowish alkalinity, magnesium-forward.

What goes wrong

• Distilled / RO water alone — nothing to carry flavour; brews taste hollow and thin, and pure water is aggressive toward metal boilers.
• Very hard tap water — heavy scale, muted acidity, often a metallic edge. Kills espresso machines.
• High alkalinity — flattens acidity and makes light roasts taste dull and "baked".
• Softened water (ion exchange) — sodium replaces calcium; the coffee tastes salty and flat.

Building your own water

Start from distilled or RO water and add minerals. Two well-known recipes:

Rao / Perger style (per litre of distilled water)

• 0.1 g magnesium sulphate heptahydrate (Epsom salt) contributes ~40 ppm hardness
• 0.05 g sodium bicarbonate contributes ~35 ppm alkalinity

Concentrate approach

Dissolve 24 g Epsom salt in 500 g water for a magnesium concentrate and 17 g baking soda in 500 g water for a buffer concentrate. A few grams of each into a litre of distilled water lets you tune GH and KH independently.

Commercial mineral packets exist for people who'd rather not weigh milligrams.

Temperature and gas

Freshly boiled water has driven off dissolved oxygen and CO₂; that's fine. What you want to avoid is water that has been kept simmering for hours — it concentrates minerals through evaporation and tastes flat. Fresh water, brought to temperature once.

For espresso machines

Scale is calcium carbonate precipitating out of hard water at high temperature. Aim for the lower end of hardness (60–80 ppm) and alkalinity (~40 ppm), or use a magnesium-forward recipe — magnesium carbonate is far more soluble and scales far less than calcium carbonate. Check the manufacturer's water spec: some warn explicitly against very low alkalinity because it becomes corrosive to brass and copper.