Water is the ingredient nobody adjusts

The theory of water and extraction is well covered elsewhere. This is about the version you meet in practice.

A cup is almost entirely water

By weight, an espresso is overwhelmingly water and a filter coffee is even more so, with only a small fraction of dissolved coffee solids. That means the water is not a neutral carrier but the largest single ingredient, and its composition changes the result more than most people expect. Two cafés using the same beans, the same grinder and the same machine can produce noticeably different cups purely because their supply differs.

This is why competition brewers control water composition tightly, while ordinary cafés almost never think about it at all. Once you know it, the mystery of why coffee tastes different in different cities becomes considerably less mysterious.

Minerals do the pulling

Dissolved magnesium and calcium bind to flavour compounds in the grounds and carry them into solution, which is the physical basis of extraction. Water with almost nothing dissolved in it extracts poorly and tends to produce a thin, hollow, slightly sour cup regardless of technique. Water with too much dissolved in it extracts unevenly and can taste flat, chalky or dull, muting the acidity that gives a coffee definition.

There is a broad middle range where things work well, which is why coffee is drinkable in most places without anybody intervening. Bicarbonate matters separately, because it neutralises acids in the coffee and can flatten a bright bean into something characterless.

Scale is the other half

The same dissolved minerals that help extraction precipitate out when water is heated, forming scale inside boilers, pipes and heat exchangers. Scale insulates the heating element, so a machine gradually loses its ability to hold a stable temperature and the shots drift without anyone changing anything. It also narrows internal passages, which changes flow and pressure in ways that are easy to blame on the grinder or the beans.

Put simply, a machine that has never been descaled in a hard-water area can fail expensively, which is why water treatment is sold as protection rather than as flavour. The awkward truth is that the treatment which best protects the machine is not always the treatment that makes the best coffee.

What a cafe actually does

Most cafés with a serious machine run a filter cartridge on the incoming supply, chosen largely to keep the machine alive. Softening filters exchange the scale-forming minerals for something harmless, which protects the boiler but can strip out what the extraction needed. Better systems blend treated and untreated water to hit a target, but they cost more and require someone to monitor and replace cartridges on schedule.

A neglected cartridge is worse than none, because it stops working silently and nobody notices until the cups change.

If a café you know suddenly tastes different with no other change, the water treatment is a genuinely likely culprit.

Why your home cup differs

Domestic kettles and filter machines use whatever comes from the tap, which varies enormously between cities and even between neighbourhoods. A jug filter softens water and removes chlorine, which usually improves the taste but can push the mineral content below the useful range. Bottled water is not automatically better, since some mineral waters are far too heavily mineralised for brewing and will taste flat.

Boiling water repeatedly concentrates what is dissolved in it, so a kettle topped up all morning is not brewing with the same water it started with. The cheap experiment is to brew the same coffee with tap, filtered and one low-mineral bottled water on the same morning and taste them side by side.

Adjust the size of it until it is something you would actually do tired.

What you can change

At a café counter, nothing, which is worth accepting rather than agonising over, because the water is a house decision. At home, you control it completely, and it is the one variable that costs almost nothing to test properly. Start by descaling your kettle and using fresh cold water each time, since both are free and both have an audible effect.

For most people, if your area has very hard water, a jug filter is the simplest improvement; if it is very soft, adding a mineral formulation is the equivalent move. Chasing beans and grinders while ignoring water is the most common way home brewers spend money without improving the cup.

The takeaway

Fix the water before you buy another bag of beans. It is the cheapest change available.

Pick the one that costs you least, and let the rest wait.