Hardness and alkalinity, in plain words
Hardness (GH) is mostly calcium and magnesium. They help pull flavor out of the coffee. Magnesium is "believed to extract slightly different flavors," Jonathan Gagné writes, though he knows of no lab tests that back this up yet. Too little and coffee tastes flat and thin; far too much and it turns chalky and scales machines.
Alkalinity (KH) is mostly bicarbonate. It buffers the coffee's acids: too little and the cup can taste sharp; too much and it tastes flat and dull. The map's dashed lines mark hardness two and three times the alkalinity; they're guides for reading the map, not a rule, and the recipes sit on both sides of them.
Distilled and reverse-osmosis water have almost no minerals, which is why coffee brewed with them tastes hollow. Very hard tap water has the opposite problem.
Well-known coffee water recipes
| Recipe | GH | KH | Minerals | Good for |
|---|---|---|---|---|
| SCA standard | 68 | 40 | The SCA's own target | Cupping, a neutral reference |
| Melbourne | 24 | 12 | Very soft, very low alkalinity | Light-roast filter |
| Holy Water (original not traced) | 62 | 23 | Soft, low alkalinity | Light-roast pour-over |
| Barista Hustle #4 | 80 | 40 | Moderate hardness and alkalinity | Light-to-medium filter |
| Rao/Perger (simplified) | 76 | 50 | Moderate hardness, higher alkalinity | All-purpose, filter and espresso |
| Hendon | 100 | 31 | Harder, moderate alkalinity | Origin-focused pour-over |
| Third Wave Water Classic (GH/KH only) | 150 (Builder's estimate) | 40 | Hard, moderate alkalinity | Light roasts |
| Third Wave Water Espresso (GH/KH only) | 135 (Builder's estimate) | 80 | Hard, high alkalinity | Espresso |
Hardness and alkalinity in ppm (mg/L) as calcium carbonate. These are each recipe's GH and KH targets; branded products such as Third Wave Water also contain other minerals that two salts don't copy. We couldn't trace Holy Water to a published original, so its 62 GH and 23 KH are the Builder's target, not a credited recipe. Third Wave Water publishes alkalinity (about 40) but not hardness, so GH 150 is the Builder's estimate, not a maker figure. Third Wave Water publishes alkalinity (about 80) but not hardness, so GH 135 is the Builder's estimate, not a maker figure.
The two-bottle method
Barista Hustle's method keeps it simple. Dissolve 2.45 g of Epsom salt in a liter of distilled water (bottle A) and 1.68 g of baking soda in another (bottle B). Each milliliter of A in a liter of finished water adds about 1 ppm of hardness; each milliliter of B adds about 1 ppm of alkalinity. So the SCA's 68 GH / 40 KH is roughly 68 mL of A and 40 mL of B in a jug, filled with distilled water to the one-liter line. The builder above works out the exact amounts, including how much of your own water to use if you start from tap.
For espresso machines
- Check your machine maker's water specification first. Harder water scales boilers faster, steam boilers most of all.
- Chloride corrodes stainless steel and copper under heat. Epsom salt (a sulfate) and baking soda add none; avoid calcium or magnesium chloride in machines.
- Softened water is a poor base: a softener swaps calcium and magnesium for sodium, removing the hardness that helps extraction.
Sources
- Barista Hustle, Water recipes and the two-bottle concentrate method.
- Specialty Coffee Association, Water quality standard for brewing.
- Hendon, Colonna-Dashwood and Colonna-Dashwood, The role of dissolved cations in coffee extraction (Journal of Agricultural and Food Chemistry) (2014).
- Jonathan Gagné, Water for Coffee Extraction (Coffee ad Astra) (2018).
- Scott Rao, The Professional Barista's Handbook; Espresso Extraction.
- JayArr Coffee, Coffee Knowledge Synthesis (seventeen sources).