Building your own brew water

If you have read Water 101, you know the three numbers that matter: TDS, hardness, and alkalinity. Building your own brew water is the natural next step. Instead of hunting for a bottled water that happens to land in a good zone, you start from a blank slate and add minerals back on purpose. You control extraction power and buffer independently, you get the same cup in any city, and you stop feeding limescale to your kettle. This lesson is the practical, advanced version: the chemistry, two ways to make it, and the safety lines you should not cross.

Start from a blank slate

Custom water starts from water with almost nothing in it, so the only minerals present are the ones you put there. Two sources work.

Distilled water is boiled and recondensed, so TDS sits near 0. It is sold cheaply by the gallon at most supermarkets and pharmacies. It is the most reliable starting point because it is genuinely empty.

Reverse-osmosis (RO) water is forced through a membrane that strips most dissolved solids, usually leaving 5 to 30 mg/L behind. Close enough to blank for our purposes, and if you have a home RO system it is the cheapest source by far. Check it with a TDS pen; if it reads under about 20 to 30 mg/L, treat it as a blank slate.

Do not use de-ionized water sold for car batteries or steam irons unless the label confirms it is safe for consumption. And never brew with pure distilled or RO water on its own: with no minerals to grab flavor, it under-extracts badly and the cup comes out thin, hollow, and oddly sour. The point of starting blank is to build back up, not to brew empty.

The two minerals that do the work

You are rebuilding two of the three numbers from Water 101. You do not rebuild the third (TDS) directly; it is just the sum of what you add.

Magnesium drives extraction. It is the active part of hardness for flavor. Magnesium binds to coffee’s flavor compounds and helps pull them into the water, and many tasters find it gives a brighter, juicier, more aromatic cup than calcium. Most DIY recipes lean on magnesium for exactly this reason, and it has the bonus of not forming scale the way calcium does. You add it as Epsom salt (magnesium sulfate, MgSO4 heptahydrate), the same stuff sold for baths. Buy plain Epsom salt with no added fragrance.

Bicarbonate is the buffer. This is alkalinity (KH), and it sets how much of coffee’s natural acidity survives. Too little buffer and bright coffees tip into harsh and sour; too much and everything goes flat and chalky. You add it as baking soda (sodium bicarbonate, NaHCO3), the ordinary kitchen kind.

That split is the whole trick: magnesium and bicarbonate are separate knobs. Crank magnesium for more extraction power, ease off bicarbonate to let acidity shine, and so on. Calcium (from calcium chloride or gypsum) shows up in more advanced recipes for body and mouthfeel, but you can make excellent water with magnesium and bicarbonate alone.

DIY concentrates: the two-bottle method

You never dose tiny amounts of powder into a single liter; the measurements are too small to be accurate. Instead you make strong concentrates, then add a few milliliters of each to a jug of distilled water. A jeweler’s scale that reads to 0.01 g is worth owning here.

A simple, widely used starting pair:

• Hardness concentrate: dissolve about 12.3 g Epsom salt in 1 liter of distilled water.
• Buffer concentrate: dissolve about 6.7 g baking soda in 1 liter of distilled water.

To brew, add 10 mL of the hardness concentrate and 10 mL of the buffer concentrate to 1 liter of distilled water. That lands you in a sensible filter zone, around 50 to 60 mg/L of contributed hardness with a modest buffer near 40 mg/L as CaCO3, which is close to the SCA-style target. Shake the concentrates before each use, and refrigerate them; with no chlorine present they can grow biofilm over weeks.

The most famous published recipe is Rao and Perger’s “magnesium water,” which uses magnesium and bicarbonate concentrates dosed into distilled water and is built around the same logic. There are several variants floating around at slightly different strengths; treat the exact grams as a starting point and adjust to taste rather than a sacred number. From there you can run your own experiments: brew the same coffee with more buffer and less buffer, or more magnesium and less, and taste the difference side by side.

Third-party packs and when to skip DIY

If grams and concentrates are not your idea of fun, ready-made mineral packs do the same job. You tear a sachet into distilled or RO water and you are done. They are precise, repeatable, and great for travel or for dialing espresso where consistency matters. The tradeoff is cost per liter and that you are locked into someone else’s profile. Several reputable brands exist; pick one rather than trusting any single claim, and remember the packs assume you start from a blank slate, so they will not fix hard tap water.

Safety and taste

A few lines to respect:

• Use food-grade ingredients. Plain Epsom salt and kitchen baking soda are fine. Skip anything scented or “bath” branded with additives.
• Keep sodium modest. Baking soda adds sodium. The doses above are small and fine for most people, but if you are on a sodium-restricted diet, account for it.
• Watch your machine. Magnesium-led water is far gentler on espresso boilers than calcium-heavy tap water, but bicarbonate still contributes some scale over time. Very low buffer can let coffee’s acidity nudge metal parts, so keep some bicarbonate in the mix rather than running pure magnesium water through a machine.
• Taste, do not just measure. A TDS pen reads total solids but cannot tell magnesium from bicarbonate, so it will not tell you whether your buffer is right. Your palate and an aquarium GH/KH test kit together beat the pen.

Next: pick one coffee you know well, brew it with a basic concentrate recipe, and compare it against your usual tap or filtered water. Once you can taste what magnesium and bicarbonate each do, you can build water that flatters a specific bean instead of chasing a generic target.