Water is the ingredient nobody tests

Two numbers that matter

General hardness pulls flavour out of the coffee; alkalinity buffers the acids once it is in the cup. Get the ratio wrong in either direction and the same beans, grinder and machine produce a noticeably different drink.

Those two numbers do almost all of the work, and almost nobody measures either of them. A bag of coffee is two per cent of what ends up in the cup. The other ninety-eight per cent comes out of a tap, and most people treat it as though it were a constant.

It is not a constant. It varies by town, by season, and by whether the supply was blended from a different source that week.

Hardness is the solvent

General hardness, usually written GH, is the concentration of calcium and magnesium. Those ions are not passengers. They actively bind to the flavour compounds in coffee and carry them into solution, which means hardness is part of the extraction mechanism rather than a contaminant in it.

Magnesium is the more effective of the two, and it is associated with the fruit and the sweetness. Calcium pulls more of the heavier, body-forming compounds and is also the ion that becomes scale. Most tap water has considerably more calcium than magnesium, which is one reason bottled or remineralised water can taste livelier through the same recipe.

Take hardness to zero and you can see the mechanism directly. Distilled or reverse-osmosis water with nothing added brews a cup that is thin, flat and oddly hollow, no matter how fine you grind. There is nothing in the water to grab the coffee with.

There is a second reason not to run a machine on it: many espresso machines sense their boiler level through the conductivity of the water. Fill one with water that conducts almost nothing and the probe reads an empty boiler permanently.

Alkalinity is the brake

Alkalinity, written KH, measures carbonates. It is not a measure of how hard the water is; it is a measure of how strongly the water resists a change in pH.

Coffee is acidic. Those acids are most of what people enjoy in a good light roast: the citrus, the stone fruit, the sensation of brightness. Alkalinity neutralises them on contact. A little is necessary, because water with no buffering at all lets the sharper acids through undamped and the cup reads as sour and aggressive. Too much and the acids are flattened before they reach you.

This is the mechanism behind a complaint that comes up constantly and is almost always blamed on the roaster. A coffee described as bright and floral arrives in the cup as vaguely sweet and characterless. The beans are fine. The water erased the part of them that was worth buying, which is why light roasts suffer from high alkalinity far more than dark ones do.

Targets worth aiming at

The Specialty Coffee Association publishes a water standard, and the useful part of it is narrower than the full document suggests.

Measure	Sensible range	What goes wrong outside it
Total dissolved solids	75–250 ppm	Below: thin and hollow. Above: muddy, and scale
General hardness	50–175 ppm as CaCO₃	Below: nothing to extract with. Above: scale
Alkalinity	around 40 ppm as CaCO₃	Below: sour and sharp. Above: flat and dull
pH	6.5–7.5	Outside: corrosion risk at both ends

The relationship between the two middle rows is the thing to hold onto. Hardness noticeably above alkalinity is the combination most people find best: enough calcium and magnesium to extract with, not enough carbonate to erase the acidity afterwards.

How to find out, and what to do

Your water utility publishes hardness, usually in French degrees or in milligrams of calcium carbonate per litre, and a search for the supplier plus your commune will normally turn it up. Aquarium test strips measure GH and KH separately for a few euros, which is more useful than a single total-hardness number because it gives you the ratio rather than the sum.

Three routes from there, in ascending order of effort.

A softening filter jug reduces hardness and alkalinity together. It is the cheapest option and it has one failure mode that matters: an exhausted cartridge stops working while you continue to believe it is working. Diarise the changes.

Cutting tap water with bottled suits very hard supplies and gives you control over the ratio. Look for a dry residue between 50 and 150 mg/l on the label, and read it, because waters marketed as mineral-rich will scale a machine faster than the tap did.

Mixing your own from distilled water and a measured sachet of minerals is what people with a refractometer and strong opinions end up doing. It is genuinely the most controlled option and it is more work than most drinkers want.

It is the cheapest upgrade left

Two last points, because they reframe the priority.

Water is the only variable that changes every other one. Adjust your ratio or your temperature and you are tuning an extraction that the water has already constrained. Fix the water and every recipe you have tuned around it shifts, usually in the direction you were trying to push it.

And it is the same decision as descaling. Hardness is what forms scale, so treating the water going in is simultaneously a flavour fix and the thing that keeps a machine out of the bin. The yearly maintenance jobs get considerably shorter on soft water, and a shot that tastes sour for no obvious reason is worth testing the tap for before buying anything.