An excerpt from Chapter 17 of the book Mastering the Bean.
Coffee is 98.5% water. Yet water is the variable most often overlooked by the people making it.
Of all the variables in coffee preparation — grind size, temperature, brew ratio, contact time — water chemistry is arguably the most fundamentally impactful and the least understood by most coffee professionals. This is partly because water's effects are invisible to the eye and subtle to the untrained palate, and partly because the science of water-coffee interaction is genuinely complex. But ignorance of water chemistry does not make it go away. It simply means you are building on a foundation you do not understand.
Coffee is approximately 98.5% water by weight. Every flavor compound, every aromatic molecule, every acid and sugar and protein that defines the cup's character must first be dissolved by water and then interact with it throughout the preparation process. The mineral composition of that water determines how efficiently different compounds are extracted, how they taste once dissolved, how stable the flavors are in the cup, and even how the espresso machine performs over time.
The key parameters of water chemistry
Total Dissolved Solids (TDS)
Total Dissolved Solids (TDS) is the cumulative measure of all mineral ions dissolved in water, expressed in milligrams per liter (mg/L) or parts per million (ppm), where 1 mg/L equals approximately 1 ppm for dilute solutions. TDS is measured with a conductivity meter, which estimates ion concentration from electrical conductivity, or more precisely with gravimetric analysis (evaporating a sample and weighing the residue).
The SCA recommends a target TDS of 150 ppm (range: 75–250 ppm) for brewing water. Water below 75 ppm is 'soft' and extracts coffee aggressively and unevenly, often producing sharp, acidic, astringent cups — the ions that ordinarily moderate extraction are absent. Water above 250 ppm is 'hard' and can suppress extraction of delicate aromatics, produce flat, chalky cups, and cause severe scale buildup in espresso machines.
However, TDS alone is insufficient as a water quality descriptor. Two water samples with identical TDS can behave very differently in coffee extraction depending on which ions dominate the composition. A water with 150 ppm dominated by magnesium ions behaves differently from one with 150 ppm dominated by calcium ions, and differently still from one dominated by sodium bicarbonate (alkalinity). Breaking down the TDS into its component parts is the more informative approach.
Calcium and magnesium: the extraction ions
Calcium (Ca2+) and magnesium (Mg2+) are divalent cations — positively charged ions carrying a charge of +2. Research by Christopher Hendon and colleagues at the University of Bath (published in the Journal of Agricultural and Food Chemistry in 2014) demonstrated that these ions interact differently with coffee compounds during extraction. According to their computational models, magnesium ions bind particularly effectively to aromatic organic compounds and acids — the compounds associated with coffee's most desirable flavor attributes. Calcium, while also important, is less selective and is associated more with body and perceived richness.
This means that, all else being equal, water with a higher magnesium-to-calcium ratio will tend to produce brighter, more aromatic cups with greater complexity. Water with a higher calcium-to-magnesium ratio will produce rounder, fuller cups with somewhat less aromatic intensity. Neither is universally superior — it depends on the coffee and the desired cup profile. But understanding this distinction allows you to deliberately engineer your water recipe toward a specific outcome.
Alkalinity: the acidity buffer
Alkalinity — the concentration of bicarbonate (HCO3−) and carbonate (CO32−) ions in water — is one of the most practically impactful water parameters for coffee, and one of the least intuitively understood. Alkalinity acts as a pH buffer: it resists changes in pH by neutralizing acids added to the water. In the context of coffee, this means that alkalinity literally neutralizes the organic acids extracted from coffee during brewing.
High alkalinity water suppresses perceived acidity in the cup — the organic acids extracted from coffee are partially neutralized by the bicarbonates, producing a flatter, duller profile. For a bright Ethiopian Yirgacheffe, high alkalinity is devastating: it strips out the citric and malic acids that define the coffee's character, leaving a muted, lifeless cup. For a dark-roasted espresso blend where excess acidity is a concern, moderate alkalinity provides a useful buffering effect.
The SCA recommends alkalinity (as CaCO3 equivalent) at or near 40 mg/L. Well below this, the water is poorly buffered and pH may fluctuate significantly during brewing. Well above it, acid suppression begins to noticeably impact cup quality. Many specialty baristas target even lower alkalinity — 20–40 mg/L — to maximize acidity expression in light-roast filter coffee.
pH
pH measures the concentration of hydrogen ions in solution — the fundamental measure of acidity and alkalinity. A pH of 7.0 is neutral; values below 7.0 are acidic; values above 7.0 are alkaline (basic). The SCA recommends a brewing water pH between 6.5 and 7.5.
Very acidic water (pH below 6.0) can cause metallic or sour notes in extraction and may accelerate corrosion of metal components in brewing equipment. Very alkaline water (pH above 8.0) suppresses acidity perception and produces flat, soapy-tasting cups. Within the recommended range, small pH variations have relatively minor direct effects on cup quality — alkalinity and hardness matter more.
The SCA standard at a glance
| Parameter | SCA Target | Practical Specialty Range | Effect If Too High | Effect If Too Low |
|---|---|---|---|---|
| TDS (ppm) | 150 (75–250) | 100–200 | Chalky, flat, scale formation | Sharp, acidic, aggressive extraction |
| Magnesium (mg/L) | — (not specified) | 5–30 | Metallic, bitter notes | Loss of aromatic complexity |
| Calcium hardness (mg/L as CaCO3) | 68 (17–85) | 30–85 | Hard water scale, flat cup | Loss of body and richness |
| Alkalinity (mg/L as CaCO3) | ≈ 40 | 20–60 | Acid suppression, flat, dull | Unstable pH, sharp |
| pH | 6.5–7.5 | 6.5–7.5 | Soapy, flat | Metallic, corrosive to equipment |
In practice, tap water in most cities falls outside this ideal range — either too hard (high TDS and bicarbonate), too soft (low minerals, producing flat and metallic-tasting coffee), or contaminated with chlorine (which produces unpleasant medicinal or chemical off-notes). Understanding your local water chemistry is the first step to addressing it.
Water and your espresso machine
Scale is calcium carbonate (CaCO3) precipitated from hard water during heating. It accumulates in boilers, heat exchangers, shower screens, solenoid valves, and flow meters — progressively degrading temperature stability, pressure consistency, and ultimately, espresso quality. A machine with heavily scaled components cannot produce consistent results regardless of how well the coffee is prepared.
The solution is twofold: use appropriately soft water with low alkalinity to reduce scale formation rate, and implement a regular descaling and maintenance schedule. Most commercial espresso machine manufacturers recommend descaling every 3–6 months for machines using properly treated water, and every 1–2 months for machines using hard, untreated municipal water.
Ion exchange water softeners — the cartridge-based systems installed inline before espresso machines — exchange calcium and magnesium ions for sodium ions, reducing scale formation. However, pure ion exchange softening does not reduce TDS and can produce very high sodium content water, which gives a slightly salty character to espresso. More sophisticated systems (reverse osmosis with remineralization, or blending filtered and softened water) allow precise control of both hardness and TDS.
