Water quality in coffee brewing: TDS, minerals, and why your espresso tastes off

Coffee is roughly 98% water. Baristas will spend months dialling grind size and dose, then brew through water they have never once tested. In my training sessions, water is the variable people skip, and it is often the one that explains the cup.

Why water decides so much of the cup

Extraction is chemistry, and water is the solvent doing the work. The minerals dissolved in it change what gets pulled from the coffee and how it tastes once it is in the cup.

The two minerals that matter most are magnesium and calcium. Magnesium tends to pull the bright, acidic, fruit-forward compounds. Calcium extracts toward sweetness and body, a fuller, rounder cup. Neither is wrong. The balance between them shapes the style of the coffee you serve, which is why two cafés with identical machines and identical beans can taste completely different.

The three numbers to know

1. TDS: total dissolved solids

TDS measures everything dissolved in your water, in parts per million. The workable range for brewing sits between 75 and 250 ppm, with the sweet spot for most setups around 150 ppm.

• Too low (under 75 ppm): water strips flavor aggressively and unevenly. Cups taste thin, sharp, hollow.
• Too high (over 250 to 300 ppm): the water is already saturated, extraction turns muddy and bitter, and your machine scales up fast.

A basic TDS meter costs less than a bag of specialty coffee. If you run a bar and do not own one, that is the first fix.

2. Hardness: the scale question

Hardness is the calcium and magnesium specifically. Beyond taste, this is an equipment decision. Hard water builds limescale inside boilers and group heads, and I have seen more machine breakdowns caused by scale than by any other single cause. Soft water protects the machine but can underdeliver on flavor if it is stripped of everything.

3. pH: the quiet one

Aim for water between 6.5 and 7.5, close to neutral. Water outside that range pushes the cup sour or flat before the coffee has a chance to speak. Most municipal water sits in range, but it is worth confirming once rather than assuming forever.

The two most common problems I find in cafés

Chlorine and chloramine. Added to municipal water for good sanitary reasons, terrible for coffee. Even small amounts flatten aromatics and add a papery, chemical edge. An activated carbon filter removes chlorine easily. Chloramine is more stubborn and needs catalytic carbon, so check which one your supply uses.

Unmanaged hard water. The taste drifts, the machine scales, service costs climb. The fix that works reliably: filtration matched to your local water, or in harder-water regions, reverse osmosis with a remineralization cartridge so you are not brewing with empty water.

What to actually do

1. Test before you buy anything. A TDS meter and pH strips give you 80% of the picture in five minutes. A professional water report gives you the rest, including the chlorine question.
2. Filter to match your water, not by default. A carbon filter fixes chlorine but does nothing about hardness. RO plus remineralization fixes hardness but is overkill on already-soft water. Diagnose first.
3. Descale on a schedule, not on failure. By the time scale affects shot quality, it has been building for months.
4. Retest occasionally. Municipal water changes with season and source. The number you measured last year is not a permanent fact.

The takeaway

You cannot taste your way around bad water with a better grinder. If your coffee tastes different from one week to the next and nothing else changed, test the water first. It is the cheapest diagnosis in coffee and the one most often skipped.

Originally published on LinkedIn, expanded for this site.