Coffee: Water Chemistry — SCA Standards and Mineral Targets

Water constitutes approximately 98–99% of brewed coffee by mass. It is not a neutral solvent — its mineral content actively shapes extraction chemistry, flavor balance, and equipment longevity. The Specialty Coffee Association’s Water Quality Standards provide the most widely adopted targets for brewing water composition.

Why Water Minerals Matter

Water minerals interact with coffee compounds through specific chemical mechanisms, not just general “hardness” or “softness”:

Magnesium (Mg²⁺): Research by Hendon et al. (2014) demonstrated that magnesium ions selectively enhance the extraction of flavor-active compounds — particularly those responsible for brighter, fruitier notes. Magnesium appears to interact with aromatic compounds in ways that increase their solubility and transfer from grounds to water.

Calcium (Ca²⁺): Calcium also enhances extraction but shows preference for different compound classes than magnesium — more associated with extraction of body-contributing compounds. High calcium with high bicarbonate creates scale (calcium carbonate) in boilers and groupheads.

Bicarbonate (HCO₃⁻ — alkalinity): Acts as a buffer, neutralizing acids in brewed coffee. Moderate alkalinity (40–70 ppm) prevents the brewed cup from becoming too sharply acidic. Very high alkalinity (>100 ppm) suppresses desirable acidity, producing flat, hollow-tasting coffee. Very low alkalinity allows acids to dominate.

Chloride (Cl⁻): Low concentrations of chloride ions enhance perceived sweetness in brewed coffee, without adding any flavor of their own. This is the same phenomenon exploited in baking (small amounts of salt enhance sweetness).

Sodium (Na⁺): At low concentrations (10–30 ppm), sodium can enhance sweetness. Above ~150 ppm it produces detectably salty or sharp flavors.

SCA Water Quality Targets

Parameter	SCA Ideal	SCA Acceptable Range	Effect if Too Low	Effect if Too High
TDS	150 ppm	75–250 ppm	Flat extraction, hollow	Mineral flavors, over-extraction
Hardness (as CaCO₃)	68 mg/L (4 grains)	17–85 mg/L	Under-extraction, flat	Scale, astringency
Bicarbonate	40 ppm	40–70 ppm	Over-acidic, sharp	Flat, muted acidity
pH	7.0	6.5–7.5	May increase equipment corrosion	Same; off-flavors above 8.0
Sodium	10 ppm	0–30 ppm	No issue	Salty, sharp
Chloride	~25 ppm	Not defined	Possible sweetness reduction	No studied threshold

Common Water Sources and Their Coffee Performance

Water Source	Typical TDS	Hardness	Alkalinity	Suitability
Distilled / deionized	0–5 ppm	0	0	Poor — flat extraction
Reverse osmosis (pure)	5–20 ppm	~0	~0	Poor unless remineralized
Filtered tap (carbon)	80–200 ppm	Variable	Variable	Good to excellent
Typical UK tap	200–400 ppm	High	High	Often too hard; scale risk
Typical US soft-water tap	50–100 ppm	Low	Low	Acceptable; may need minerals
Bottled spring water	50–300 ppm	Variable	Variable	Check label; varies widely
SCA-recipe water (DIY)	~150 ppm	68 mg/L	40 ppm	Ideal

Building Your Own Brewing Water

For specialty applications, brewers blend remineralizing salts into RO or distilled water to hit SCA targets precisely. Common approach (per 1L starting from RO water):

• Epsom salt (MgSO₄·7H₂O): ~0.37g → adds ~36 mg/L Mg²⁺, ~136 mg/L SO₄²⁻
• Potassium bicarbonate (KHCO₃): ~0.05g → adds ~40 mg/L alkalinity
• Sodium chloride (NaCl): small addition for chloride and sodium targets

This approach is common in competition barista preparation and among home brewing enthusiasts who want complete control over extraction variables.

Equipment Longevity Considerations

Water with high hardness (calcium above 150 mg/L or ~8 grains) poses a scaling risk for espresso machines, stovetop moka pots, and electric kettles. Scale (calcium carbonate deposits) builds on heating elements and groupheads, reducing heating efficiency and potentially damaging equipment. Descaling frequency depends directly on water hardness — a practical reason beyond flavor to manage mineral content.