Water Chemistry for Coffee Brewing: The Complete Guide

clear glass of water beside coffee cup on wooden surface

Photo: nameless 3791 / Unsplash, Unsplash License.

Summary

Water makes up most of a cup of coffee, and what is dissolved in it affects how coffee extracts and tastes. The Specialty Coffee Association (SCA) publishes a water standard for brewed coffee, with targets for chlorine, total dissolved solids, calcium hardness, alkalinity, pH and sodium. This guide sets out that standard, explains what each measure means, reports what research says about magnesium and calcium, and describes the common ways of treating water.

Why Water Matters

Depending on the drink, water makes up about 94 to 98 percent of a coffee beverage, a figure the SCA takes from the textbook Espresso Coffee: The Science of Quality. Brewing dissolves part of the ground coffee into the water; the share of the coffee that dissolves is called the extraction yield. The SCA's former Golden Cup Standard gave an extraction yield of 18 to 22 percent as its target for filter coffee; the SCA has since withdrawn that standard and published a newer one. The minerals and other substances already in the water before brewing can change how the coffee extracts and how it tastes.

The SCA Water Standard

The SCA set out its water standard for brewed coffee in its Water Quality Handbook. The SCA's coffee science manager summarised it as follows:

Odour: none.
Colour: clear.
Total chlorine: 0 mg/L.
Total dissolved solids (TDS): 75 to 250 mg/L, with a target of 150 mg/L.
Calcium hardness: 17 to 85 mg/L (1 to 5 grains per gallon), with a target of 51–68 mg/L (3 to 4 grains per gallon).
Total alkalinity: at or near 40 mg/L.
pH: 6.5 to 7.5, with a target of 7.
Sodium: less than 30 mg/L, with a target of 10 mg/L.

What Each Measure Means

Chlorine. Water systems add chlorine to prevent contamination by micro-organisms. The World Health Organization reports that most disinfected water contains chlorine at 0.2–1 mg/L. The SCA's standard requires all chlorine to be removed because of its large potential effect on the cup.

Total dissolved solids. TDS measures the total amount of dissolved material in water. For comparison, the US Environmental Protection Agency recommends less than 500 mg/L for drinking water, in its non-mandatory guidelines. A TDS reading does not say which minerals make up the total.

A handheld TDS meter for measuring dissolved solids in water

Photo: Dqwyy / Wikimedia Commons, CC0.

Hardness. Hardness comes mainly from dissolved calcium and magnesium. The SCA notes that hardness is not bad for health but that too much leaves mineral deposits, called scale, on brewing equipment, which can cause it to malfunction.

Alkalinity. Alkalinity measures compounds such as bicarbonates and carbonates that neutralise acid. It is reported as milligrams per litre of calcium carbonate. According to the SCA, too much alkalinity can affect extraction and the flavour of the coffee.

A laboratory dropper and glassware

Photo: David McClenaghan, CSIRO / Wikimedia Commons, CC BY 3.0.

pH. Pure water is neutral, at pH 7. The SCA notes that coffee brewed with alkaline (high-pH) water typically tastes flat.

Sodium. Water softeners can add sodium to water. The SCA advises avoiding high sodium in brewing water because salts change how sweetness and sourness are perceived.

Magnesium and Calcium: What the Research Says

In 2014 the chemist Christopher Hendon, with Lesley and Maxwell Colonna-Dashwood, published a study in the Journal of Agricultural and Food Chemistry on how dissolved sodium, magnesium and calcium interact with compounds in coffee. The study used density functional theory, a computer-modelling method, to calculate how strongly these ions bind to several coffee acids, caffeine and a flavour compound. Its results suggested that magnesium and calcium help extract flavour compounds.

A later laboratory study at Umeå University tested this directly by adding magnesium and calcium salts to brewing water and measuring organic acids in the coffee. It found that normal levels of these ions had only small effects on the acid content, and that the extraction of acids appeared to proceed independently of them. The two studies used different methods, and the question remains open.

Watch: The Best Water for Coffee - An Introduction — James Hoffmann

Treating Water

The American Water Works Association describes the common home treatments:

Activated carbon filters, such as pitcher, faucet and under-sink filters, are particularly effective at reducing the taste and odour of chlorine. They need to be replaced often.
Reverse osmosis pushes water through a membrane that removes most dissolved solids. Because it removes minerals, water treated this way may taste flat.
Water softeners reduce hardness but add sodium to the water.

The same association warns that a poorly maintained filter can make water quality worse, and that home treatment devices can be certified against NSF/ANSI drinking-water standards.

Scale and Equipment

When hard water is heated again and again, calcium and magnesium deposit scale on heating elements and inside boilers. The SCA notes that excessive scale can cause brewing equipment to stop working properly.

An electric heating element pulled from a boiler, thickly encrusted with limescale beside a corroded magnesium anode - the calcium and magnesium scale that untreated water deposits inside espresso machine boilers

Photo: Zbigniew.czernik / Wikimedia Commons, CC BY 3.0.

Key Facts

SCA chlorine target: 0 mg/L
SCA TDS: 75 to 250 mg/L, target 150
SCA calcium hardness: 17 to 85 mg/L, target 51–68
SCA total alkalinity: at or near 40 mg/L
SCA pH: 6.5 to 7.5, target 7
SCA sodium: below 30 mg/L, target 10
Extraction yield target in the SCA's former Golden Cup Standard: 18 to 22 percent

Frequently Asked Questions

What does the SCA water standard recommend? No chlorine; total dissolved solids of 75 to 250 mg/L (target 150); calcium hardness of 17 to 85 mg/L (target 51–68); alkalinity at or near 40 mg/L; pH 6.5 to 7.5; and sodium below 30 mg/L (target 10).

Can a TDS meter tell me whether my water is good for coffee? Only partly. It measures total dissolved solids but not which minerals make up the total, so hardness and alkalinity need separate tests.

Should I use softened water for coffee? Water softeners add sodium, and the SCA advises avoiding high sodium in brewing water.

Does magnesium improve coffee? A 2014 computer-modelling study suggested that magnesium and calcium help extract flavour compounds; a later laboratory study found only small effects at normal levels. The question remains open.

Sources

Sage, Emma (SCAA Coffee Science Manager), "Dissecting SCAA's Water Quality Standard", SCA News, 8 July 2013, summarising the SCAA Water Quality Handbook, 2nd ed. (Beeman, Songer and Lingle, 2011). Supports: every figure in the standard; the notes on chlorine (with the WHO figure), TDS (with the EPA guideline), hardness and scale, alkalinity, pH and sodium; water as 94 to 98 percent of the beverage (citing Illy and Viani, 2005).
Specialty Coffee Association, Golden Cup Standard (archived copy of sca.coffee/research/coffee-standards, 2021), since withdrawn. Supports: the 18 to 22 percent extraction yield target.
Hendon, C. H., Colonna-Dashwood, L. and Colonna-Dashwood, M., "The Role of Dissolved Cations in Coffee Extraction", Journal of Agricultural and Food Chemistry 62 (21): 4947–4950 (2014). Supports: the density functional theory study of sodium, magnesium and calcium binding.
Bratthäll, T., Figueira, J. and Nording, M. L. (Umeå University), "Influence of divalent cations on the extraction of organic acids in coffee determined by GC-MS and NMR", Heliyon (2024), open access via PubMed Central (PMC10907646). Supports: the limited effect of normal magnesium and calcium levels on acid content; acid extraction independent of the water composition.
American Water Works Association, DrinkTap.org, "Water Filters and Home Treatment Devices". Supports: activated carbon and chlorine; reverse osmosis; filter maintenance; NSF/ANSI certification.
Dakota County, Minnesota, "Summary of Home Water Treatment Options". Supports: reverse osmosis removing dissolved solids, and its water possibly tasting flat.
SCA News (Sage, 2013), as above. Supports: water softeners adding sodium.

Claims this encyclopedia could not source

The following statements appear in published material about coffee water. This encyclopedia could not trace them to a reliable source:

That the same coffee would extract at about 16 percent with reverse-osmosis water and about 24 percent with hard tap water.
That each milligram of bicarbonate cancels about one milligram of extracted acid.
That the SCA standard dates from 2009, sets ideal hardness at 50 ppm, or calls for sodium at or near zero. (The SCA's own summary gives a calcium hardness target of 51–68 mg/L and a sodium target of 10 mg/L.)
That most natural waters fall between pH 7.0 and 8.5, and that pH matters much less than alkalinity.
That commercial espresso machines without water treatment fail within two or three years and last 15 to 20 years on treated water.
That distilled water leaches minerals from espresso machine boilers.
That the book Water for Coffee was published in 2014.
Prices for TDS meters, mineral packets and reverse-osmosis systems.
That Puerto Rico's coffee farms sit among some of the cleanest natural water in the Caribbean, and that volcanic soil gives the island balanced, mineral-rich groundwater.

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