Water Chemistry

Water is not a neutral carrier. Its mineral content, pH, and ion composition directly influence every stage of coffee extraction: which compounds dissolve, how quickly, in what concentration, and how they taste in the final cup. Poor water produces poor coffee regardless of coffee quality, brew method, or technique. Understanding water chemistry is one of the highest-leverage improvements available to any coffee drinker.

What "Good" Water Contains

The Specialty Coffee Association (SCA) publishes a water quality standard for brewing:

• TDS (Total Dissolved Solids): 75–250 mg/L, with 150 mg/L as the target.
• Hardness: 50–175 mg/L as CaCO3.
• Alkalinity (bicarbonate): 40–70 mg/L as CaCO3.
• pH: 6.5–7.5.
• Sodium: Under 30 mg/L.
• Chlorine: 0 mg/L. Chlorine and chloramines added to municipal water for sanitation are detrimental to coffee flavour and must be removed.

Water that falls outside these ranges will produce coffee that is noticeably different — often worse — than water within them.

The Role of Individual Minerals

Magnesium (Mg²⁺): The most extraction-active mineral in water. Magnesium ions bind specifically to many of the aromatic compounds responsible for coffee's flavour and carry them into solution more effectively than other ions. Brewing water with higher magnesium content extracts more of these aromatic compounds, producing a brighter, more flavourful cup. Research from University College London (2020) confirmed magnesium as the key ion for coffee extraction quality.

Calcium (Ca²⁺): Also contributes to extraction but less selectively than magnesium. Very high calcium causes scaling on equipment (boiler scale) and can produce a flat, chalky cup quality. Some calcium is beneficial; excess is not.

Bicarbonate (HCO3⁻): Functions as a buffer — it neutralises acids in solution, including the organic acids in coffee that contribute brightness. High bicarbonate water produces coffee that tastes flat, muted, and lacking acidity. Low bicarbonate (very soft or distilled water) produces coffee that is sharply acidic and sometimes sour, because there is no buffer to moderate the naturally acidic extraction. The SCA target range of 40–70 mg/L represents the optimal buffering effect.

Sodium (Na⁺): In small quantities (under 30 mg/L), sodium can enhance sweetness perception. Above this threshold, it begins to impart a salty or flat character. Very high sodium water from some softened water systems is actively harmful to coffee flavour.

Chloride (Cl⁻): In moderate amounts, chloride can enhance sweetness and body. It is distinct from chlorine; the concern is chlorine/chloramine disinfectants, not the chloride ion in mineral water.

Hard Water vs. Soft Water

Hard water has high calcium and magnesium content. While this can benefit extraction (more minerals available), very hard water (above 250 mg/L TDS) causes scaling in espresso machines, group heads, and boilers. Scale reduces heat transfer efficiency, restricts water flow, and eventually damages equipment. Regular descaling is essential with hard water.

Soft water has very low mineral content. In extreme cases (distilled or RO water at near 0 TDS), it extracts poorly — the lack of minerals means fewer binding sites for coffee compounds — and the resulting cup tastes flat and over-acidic due to absent buffering. Never use distilled or deionised water for brewing coffee.

Reverse osmosis (RO) water removes essentially all minerals. Many specialty cafés use RO systems paired with remineralisation — adding precise amounts of magnesium and calcium back to produce water with an ideal mineral profile for extraction.

Practical Water Choices

Filtered tap water: A carbon filter (Brita-style) removes chlorine and chloramines, which is the most impactful single improvement available to most home brewers. If your tap water falls within the SCA hardness range, filtered tap water is excellent.

Bottled mineral water: Check the label. Many European mineral waters (Volvic, for example) are within the SCA ideal range. Some are very hard and scale machines; some are very soft and produce acidic cups. The mineral analysis is printed on the label.

Third Wave Water: Commercially available mineral concentrate packets designed to be mixed with distilled water to produce water at the SCA ideal. Used by competition baristas for consistency and by home brewers who want precise control.

RO + remineralisation: The professional approach for cafés. Expensive to set up but produces water at exactly the desired specification regardless of what comes out of the municipal supply.

Water and Equipment Longevity

Beyond flavour, water chemistry affects equipment life. Hard water scales boilers, solenoids, and group heads. Many espresso machine failures are caused by untreated hard water. For any machine with a boiler, understanding local water hardness and implementing appropriate filtration or softening is not optional — it is maintenance.