Coffee is 98 % water, and across much of Spain that water is hard. What almost nobody checks is not hardness but alkalinity — which is what actually flattens the cup.
The ingredient nobody looks at
An espresso is around 90 % water; a filter coffee, 98 %. It is by far the majority ingredient, and yet it gets chosen by default: whatever comes out of the tap.
Water is not a neutral solvent. The minerals dissolved in it take part in extraction, binding to particular coffee compounds and helping them into the cup. With too few minerals, extraction falls short; with too many, it goes out of balance.
Hardness: neither much nor little
Hardness is essentially the dissolved calcium and magnesium. Magnesium is especially good at extracting the fruity compounds; calcium contributes body.
The SCA reference sits at around 150 ppm of total dissolved solids, with a reasonable range between 75 and 250.
- Very soft water (below 50 ppm) — a flat cup, no body, sour and short. There aren't enough minerals to carry the flavour.
- Very hard water (above 300 ppm) — mutes acidity, leaves a chalky or metallic impression, and fills your machine with scale.
What almost nobody checks: alkalinity
And it matters more than hardness for flavour.
Alkalinity measures bicarbonate above all, and it works as a buffer that neutralises acids. Since much of what's interesting in specialty coffee is precisely its acids — citric, malic, the fruit character — highly alkaline water eats them.
If your specialty coffee tastes correct but muted, without the acidity the bag promised, the problem is quite likely the water rather than the coffee or the machine.
A sensible target is around 40-70 ppm of alkalinity. Much of the Spanish Mediterranean coast and the south sits well above that.
What to look for on a bottle
Spanish mineral water labels state the dry residue at 180 °C, in mg/l. That's the useful figure:
| Dry residue | For coffee |
|---|---|
| Under 50 mg/l | Too soft: weak cup |
| 100-200 mg/l | The recommended range |
| Over 500 mg/l | Too heavily mineralised |
Check the bicarbonate figure too, usually on the same label: the lower it is, the better you'll preserve the coffee's acidity. And avoid sparkling or high-sodium waters.
If your tap water is hard
Which is the case across much of the country, and especially on the Mediterranean coast:
- A filter jug. The most practical option. It reduces hardness and chlorine, improves things noticeably and costs little. Change the cartridge on schedule: an exhausted one filters nothing.
- Low-mineral bottled water. Works well, with the cost and the plastic as drawbacks.
- Reverse osmosis. Careful here: it strips the water almost entirely of minerals, and that gives flat coffee. If you have RO, you want to remineralise.
Never use distilled or demineralised water. Without minerals there's no decent extraction, and it's corrosive to a machine's circuits besides.
Scale, the other problem
Beyond flavour, hard water deposits calcium carbonate on the heating element and in the pipes. That makes the machine heat less well — and temperature is one of the variables that matters most — until one day it stops working.
Filtering your water extends the life of a coffee machine considerably more than descaling it afterwards. That's the practical reason it's worth doing even if you don't care about the flavour nuance.
Temperature, the other half
The SCA puts extraction between 90 and 96 °C. Within that range, the adjustment that shows most is the one almost nobody makes: dark roasts want cooler water (88-91 °C) because they're more soluble and over-extract readily, and light roasts want it hotter (94-96 °C).
Without a thermometer, boil and wait: about a minute in an open jug brings the water down into the right zone.
With water sorted, the next bottleneck is usually the grind — that's in the grinder guide. And to apply all this to espresso, the home espresso guide. Specialty coffee to notice the difference is in the shop.
