Coffee is 98.5% water. Most of us think carefully about the beans. Almost none of us think about the other 98.5%.
There's a moment that every serious coffee drinker has had. You buy an exceptional bag, brew it carefully, and the cup is Flat, Dull, Nothing like what the tasting notes promised. You adjust your grind. You change your temperature. You try again. Still wrong.
The variable you likely never touched was your water.
Most overlook water as a passive carrier. But in reality it's a chemically active solvent, and its mineral composition is one of the primary determinants of what your final cup turns out to be. This is not new, infact it has been reiterated over and over. A landmark 2014 paper by chemist Christopher Hendon and barista Maxwell Colonna-Dashwood in the Journal of Agricultural and Food Chemistry established that dissolved cations, specifically magnesium and calcium, are the main drivers of how flavor compounds are pulled from coffee grounds into solution. What's come since has only refined that picture.
Here's what the science actually says.
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The minerals doing the work
When water contacts ground coffee, dissolved ions interact with the organic molecules in the grounds mainly, acids, sugars, aromatic compounds, and pull them into solution. Magnesium (Mg²⁺) and calcium (Ca²⁺) are the two ions that do this most effectively. Whereas, Sodium (Na⁺), while present in most water, contributes comparatively little to extraction.
Computational modeling by Hendon's team showed that divalent cations (those carrying a double positive charge, like Mg²⁺ and Ca²⁺) bind more strongly to coffee's organic acids than monovalent ions like Na⁺. The practical result: water with adequate magnesium and calcium extracts more of the compounds that make coffee taste good.
- Magnesium (Mg²⁺)
Particularly effective at extracting smaller, polar molecules. Associated with brighter, fruitier, more acidic profiles. Higher charge density makes it an efficient extractor.
- Calcium (Ca²⁺)
Contributes to body, mouthfeel, and creaminess. At high concentrations, it's also the primary cause of limescale when bicarbonates are present — a real equipment consideration.
- Sodium (Na⁺)
Present in softened water where Ca/Mg have been exchanged out. Poor extractor. Softened water consistently produces flat, under-extracted coffee despite having high TDS.
- Bicarbonate (HCO₃⁻)
Primarily a buffer that controls how acidic the final cup tastes. Too much buffer mutes brightness and Too little leaves the acids sharp and unbalanced.
The key distinction that most people miss: General Hardness (GH) measures the cation concentration - the extractors. Alkalinity (KH) measures the bicarbonate concentration - the buffer. They often correlate in tap water because both come from dissolved limestone, but they are independent variables that can be tuned separately.
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The buffering system, and why acidity is actually delicate
Brewed Arabica coffee has a pH of roughly 4.85 to 5.10. Those acids - citric, malic, lactic, quinic, chlorogenic, and others are the brightness, the fruit, the complexity that distinguishes a well-grown specialty coffee from a flat commercial one.
Bicarbonate ions in water react with these acids when they enter solution:
The buffering reaction
H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂
In plain terms: bicarbonate neutralises hydrogen ions. That's useful in small amounts as it prevents the cup from tasting aggressively sour. But going past a certain threshold, it also neutralises the very acids that make the coffee interesting. The result is a cup that tastes flat, dull, and muddy.
"Bicarbonate can turn on or off acids depending on its concentration. Coffee is an acidic beverage,if you have high levels of bicarbonate, you won't taste the acids." ~ Christopher Hendon
The SCA currently recommends an alkalinity between 40 and 70 ppm as CaCO₃, as the target window for most specialty brewing. Going below 40 ppm, acidity can become sharp and less integrated, while above 70ppm, the cup begins to flatten.
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The SCA water standards, what they actually say
The Specialty Coffee Association has set water quality parameters since 2009. Here are the current figures, which incorporate updates to the original SCAA Heritage Standard:
| Parameter | Target | Acceptable Range |
|---|---|---|
| TDS (Total Dissolved Solids) | 150 mg/L | 75–250 mg/L |
| Total Hardness (Ca/Mg) | ~68 mg/L as CaCO₃ | 50–175 mg/L as CaCO₃ |
| Total Alkalinity | 40 mg/L | 40–70 mg/L as CaCO₃ |
| pH | 7.0 | 6.5–7.5 |
| Sodium | 10 mg/L | At or near 10 mg/L |
| Total Chlorine | 0 mg/L | 0 mg/L — none detectable |
A few things worth noting. The pH range (6.5–7.5) reflects the initial water pH and not the brewed coffee, coffee's own acids shift the system pH immediately upon brewing, so this parameter matters less than most people assume. Hardness and alkalinity are the variables that actually move the cup.
Softened water, wherein Ca²⁺ and Mg²⁺ have been swapped for Na⁺ via ion exchange, typically sits within acceptable TDS and alkalinity ranges but produces poor extraction.
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The physics: what actually happens when water meets grounds
Extraction is two processes running simultaneously at different speeds.
The first is erosion : the rapid dissolution of soluble compounds from the surface of ground particles. It's nearly instantaneous. This is why the first few seconds of a pour-over bloom release such a large amount of CO₂ and surface solubles.
The second is diffusion : the slower migration of compounds from inside the coffee particles outward through the cellulose matrix. Diffusion is the rate-limiting step. It is governed by the concentration gradient as compounds move from the high-concentration environment inside the particle toward the lower-concentration environment of the surrounding water.
Grind size determines which process dominates. Very fine particles (typically under 100 microns) are largely broken cell fragments with no intact interior to diffuse from. They over-extract rapidly. Larger particles rely almost entirely on diffusion and under-extract if brew time is insufficient. A non-uniform grind produces both simultaneously, which is why a bad grinder can produce a cup that is both sour and bitter at once.
In 2025, a team at the University of Pennsylvania published new research in Physics of Fluids that visualised pour-over dynamics for the first time, replacing opaque coffee grounds with transparent silica gel particles in a glass cone. Using a laser sheet and high-speed camera, they observed that water poured from a controlled height creates "miniature avalanches" in the coffee bed, local turbulence that improves mixing and increases contact between water and grounds. They found that higher pour heights (within a controlled stream) increased extraction efficiency, and that using a gooseneck kettle was key to maintaining the controlled, directed pour that produced the avalanche effect without breaking the stream into droplets. Breaking the stream into droplets carries air into the bed and decreases extraction efficiency.
"We tried finding ways to use as little coffee as possible and just take advantage of the fluid dynamics of the pour." — Ernest Park, Mathijssen Lab, University of Pennsylvania
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Engineering your water, and when it matters
For most home brewers, the goal is simpler than water chemistry makes it sound: remove chlorine, check that your tap water isn't extremely hard or extremely soft, and brew. A carbon filter handles the first. A basic TDS meter or water hardness test strip handles the second.
For those working with RO (reverse osmosis) water or distilled water, which strips nearly all dissolved solids, remineralisation is necessary. Distilled water alone produces flat, under-extracted coffee. The two variables to target are hardness (Mg²⁺ and/or Ca²⁺ source) and alkalinity (bicarbonate source). Common salts used:
| Salt | Formula | What it contributes |
|---|---|---|
| Magnesium sulfate | MgSO₄·7H₂O | Hardness (GH) : brightness, fruitiness |
| Calcium chloride | CaCl₂ | Hardness (GH) : body, mouthfeel |
| Sodium bicarbonate | NaHCO₃ | Alkalinity (KH) : acid buffer |
| Potassium bicarbonate | KHCO₃ | Alkalinity (KH) : acid buffer, no sodium |
A practical note on chloride: while calcium chloride is an effective hardness source, chloride ions (Cl⁻) are corrosive to stainless steel and should be used with care in espresso machines. Magnesium sulfate is safer for equipment, though high sulfate concentrations can add a harsh edge at very elevated levels.
Practical starting point
The SCA-aligned recipe targets 40 ppm alkalinity and ~68 ppm total hardness as CaCO₃. Most published specialty water recipes (Barista Hustle, Rao Water, Hendon Water) are variations on this baseline, tuned toward brightness, body, or specific roast levels. All quantities are calibrated per litre of base water, verify the volume before following any recipe.
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What roast level asks of water
Light roasts keep more of the coffee’s original acids and delicate flavours from the green bean. Because of this, they usually work best with water that has enough magnesium to help bring out those bright, fruity, and floral notes. Lower alkalinity also helps preserve clarity and liveliness, but if it’s too low, the coffee can taste overly sharp or sour. In those cases, slightly increasing the alkalinity can smooth out the acidity without flattening the flavour.
Dark roasts are different because the roasting process has already broken down many of the original acids and created deeper, roast-heavy flavours. They’re generally less sensitive to water chemistry, but softer water with lower hardness often helps avoid pulling out too much bitterness. A bit more alkalinity can also help soften harsh roasted flavours and make the cup taste smoother.
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New frontiers : what 2025 and 2026 science is saying
The field has moved quickly in the last two years. Two pieces of research are worth knowing about.
The University of Pennsylvania pour-over study (covered in section 04) is one. The other is Hendon's electrochemical work, published in Nature Communications in April 2026.
The research repurposes a potentiostat, a tool normally used to test batteries and fuel cells, to analyse brewed coffee (bunch of nerds). When a platinum electrode is immersed in coffee and a controlled voltage is applied, coffee molecules bind to the electrode surface and suppress the electrical current in a measurable way. The technique simultaneously measures two independent variables that standard refractometers cannot separate: brew strength (concentration) and roast colour (which correlates with the ensemble chemical composition of the coffee).
In a validation test, the team was given four samples of the same coffee from a roaster in England, all visually identical. Their electrochemical method accurately identified the one sample that had failed the roaster's quality control, later confirmed to have been under-roasted and too acidic. The goal isn't to replace sensory evaluation. It's to give baristas and roasters a fast, objective way to confirm they've hit the mark, and to understand precisely why a cup tastes the way it does.
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The short version
If none of the chemistry above interests you, here is what it distills to:
What to actually do
① Filter out chlorine, a carbon filter is enough, and it's the single highest-impact change most people can make.
② Check your water isn't extremely hard or soft, a basic TDS meter tells you. The SCA target is 75–250 ppm total dissolved solids, with 150 ppm as the ideal.
③ Alkalinity in the 40–70 ppm range keeps your coffee's acidity alive without making it sharp.
④ If you're using RO or distilled water, you need to remineralise. Distilled alone produces flat coffee.
⑤ Pour with intention. the physics of how water contacts the coffee bed is real. A gooseneck kettle and a controlled pour height improve extraction measurably.
Water is not a passive ingredient. It's the medium through which everything you've chosen about a coffee, the origin, the process, the roast, either reaches the cup or doesn't.
Most of what's wrong with a flat cup isn't the coffee.