Minerals in coffee means two different things, and mixing them up is why so much advice on this topic goes nowhere. One is the mineral content of the bean itself, which is small and nutritionally marginal. The other is the mineral content of your brew water, which decides how much of the coffee you paid for actually ends up in the cup.
This guide covers both, with the numbers. Indonesia Specialty Coffee grades and exports green beans, and we cup every lot before it ships, so the water side of this is not theory for us.
Last updated: 23 September 2026
What Minerals Are in Coffee?
Green coffee beans contain potassium, calcium, magnesium, phosphorus, and trace amounts of iron, zinc, manganese, and copper. Potassium dominates by a wide margin.
| Mineral | In the bean | In one brewed cup | Share of daily intake |
|---|---|---|---|
| Potassium | approx. 6,179 mg/kg | approx. 49 mg | approx. 2% |
| Calcium | approx. 2,554 mg/kg | approx. 2 mg | under 0.3% |
| Magnesium | approx. 1,651 mg/kg | approx. 3 mg | approx. 1.5% |
| Phosphorus | lower | approx. 3 mg | approx. 0.4% |
Read the third column before anyone sells you coffee as a mineral supplement. A cup delivers around 2 percent of daily potassium and less than 2 percent of magnesium. That is not nothing, but it is not a nutritional reason to drink coffee either. Mineral levels do vary by species, cultivar, altitude, and soil: one comparative study found Excelsa carrying significantly higher magnesium, calcium, sodium, zinc, and copper than Robusta from the same analysis. Our guide to the chemical composition of coffee beans places these alongside the lipids, sugars, and alkaloids.
The minerals that change how your coffee tastes are the ones in the water.
Why Brew Water Minerals Decide the Cup
Brewed coffee is roughly 98 percent water, and that water is not a passive carrier. Calcium and magnesium are divalent cations, meaning they carry a double positive charge. During extraction they bond with negatively charged coffee compounds, carboxylic acids, polyphenols, and aromatic molecules, and pull them into solution.
The two do different jobs:
- Magnesium is the more effective extractor of aromatic and fruit-forward compounds
- Calcium pulls body-building organic acids and contributes perceived sweetness and mouthfeel
Together they make up what the industry calls hardness.
Alkalinity is a separate measurement and a separate problem. Alkalinity is the water’s buffering capacity against pH change, driven mostly by bicarbonate. When coffee acids enter the water during extraction, bicarbonate neutralizes them. A little of that rounds off harshness. Too much of it buffers the bright acids out of the cup entirely, which is why high-alkalinity water produces coffee described as muddy, chalky, or lifeless no matter how good the beans were. Above roughly 100 ppm, that neutralization also generates enough carbon dioxide to slow flow and push a brew toward over-extraction. Very soft water fails the opposite way. With too few cations available to bond with, extraction runs thin and papery.
The SCA Water Standard for Brewing Coffee
The Specialty Coffee Association published its brewing water standard in 2009, and it remains the reference most roasters and cafes specify against.
| Characteristic | Target | Acceptable range |
|---|---|---|
| Odor | Clean, fresh, odor free | Clean, fresh, odor free |
| Total chlorine | 0 mg/L | 0 mg/L |
| TDS | 150 mg/L | 75 to 250 mg/L |
| Calcium hardness | 68 mg/L (4 grains) | 17 to 85 mg/L |
| Total alkalinity | 40 mg/L | at or near 40 mg/L |
| pH | 7.0 | 6.5 to 7.5 |
| Sodium | 10 mg/L | at or near 10 mg/L |
Two honest caveats on those figures. Working practice has narrowed since 2009: the SCAE water chart report records optimum hardness recommendations clustering between 51 and 75 ppm CaCO3, and most roasters today target 50 to 80 ppm hardness against 30 to 50 ppm alkalinity. And several widely shared versions of the standard circulate with different numbers. If you are writing a water spec into a supplier contract, work from the published document, not a blog table.
Sourcing green coffee and want lots cupped on controlled water before you commit? See our current wholesale pricelist or contact our team for 1 kg samples.
Hardness or Alkalinity: Which One Is Flattening Your Cup?
Most water problems present as one of four symptoms. Diagnose before you re-mineralize.
| What the cup does | Likely cause | Fix |
|---|---|---|
| Thin, papery, hollow | Hardness too low | Raise hardness toward 50 to 80 ppm |
| Muddy, chalky, lifeless despite good beans | Alkalinity too high | Cut alkalinity toward 30 to 50 ppm |
| Flat and faintly salty | Sodium-softened water | Sodium is inert for extraction; use RO instead of an ion-exchange softener |
| Bitter and astringent, fast flow | Hardness too low, over-extraction | Coarsen grind and raise hardness |
Alkalinity is the first number to check when the beans are good and the cup still reads dull. Bringing alkalinity down into range often makes acidity and sweetness reappear with no change to roast or grind at all. Our guide to acids in coffee covers which acids you are protecting when you do this.
Hardness also carries an equipment cost that flavor discussions tend to skip. Above roughly 90 ppm, additional hardness stops improving the cup and starts building limescale in boilers and group heads. Our guide to coffee machine maintenance covers descaling intervals by water hardness.
Water for Indonesian Wet-Hulled Coffee
Water spec is not one-size-fits-all across origins, and Indonesian coffee is the clearest case of that.
Most Sumatran and Sulawesi lots are semi-washed at the cherry stage and wet-hulled (Giling Basah) at 35 to 40 percent moisture. That combination produces the low acidity, heavy body, and earthy depth buyers know Gayo Arabica, Mandheling, and Lintong for. It also means there is less acidity in the cup to begin with.
Run a wet-hulled Sumatran lot on high-alkalinity water and you buffer away the small amount of brightness that was holding the cup together. What is left is body and earth with nothing lifting it. The same water does less visible damage to a washed East African coffee carrying three times the acidity.
This is the part roasters new to Indonesian coffee get wrong most often. They dial in water for a bright washed program, then blame the Sumatra lot when it cups flat. Cup the lot on water at or near 40 ppm alkalinity before you judge it. Our notes on single origin Java coffee cover the same diagnostic on a cleaner, drier-hulled profile.
Re-mineralizing and Testing Your Water
If your tap water swings, stop guessing and build water instead.
- Step 1: Start from reverse osmosis or distilled water. You need a blank base with a known starting point of zero.
- Step 2: Dose minerals into cold water. Minerals precipitate at high temperature. Mix cold, then heat.
- Step 3: Use a documented recipe. Per 1 litre of RO water, 0.17 g of Epsom salt (MgSO4 7H2O) plus 0.10 g of sodium bicarbonate gives roughly 70 ppm hardness and 50 ppm alkalinity, a serviceable general-purpose water for both filter and espresso.
- Step 4: Test weekly. Total hardness and alkalinity test strips read in seconds at roughly plus or minus 10 ppm. A titration kit gives laboratory precision for monthly calibration.
- Step 5: Remove chlorine. Even 0.5 mg/L produces medicinal off-flavors that no grind adjustment will fix.
Two mistakes worth naming. Do not use an ion-exchange water softener as your coffee water: it swaps calcium and magnesium for sodium, and sodium does almost nothing for extraction. And do not re-mineralize before you have tested, because half the cafes we talk to are correcting a problem they do not have.
Frequently Asked Questions
What minerals are in coffee?
Coffee beans contain potassium, calcium, magnesium, and phosphorus, plus trace iron, zinc, manganese, and copper. Potassium is by far the most abundant, at roughly 6,179 mg per kilogram of bean. Levels vary by species, cultivar, altitude, and soil management.
How much potassium is in a cup of coffee?
A brewed cup delivers roughly 49 mg of potassium, about 2 percent of typical daily intake. Magnesium comes in around 3 mg, calcium around 2 mg. Coffee contributes meaningfully to daily antioxidant intake but is not a significant dietary mineral source.
What is the SCA water standard for brewing coffee?
The Specialty Coffee Association standard targets TDS of 150 mg/L within a 75 to 250 range, calcium hardness of 68 mg/L within 17 to 85, total alkalinity at or near 40 mg/L, pH between 6.5 and 7.5, and zero total chlorine.
Is hard water bad for coffee?
Not inherently. Moderate hardness carries sweetness and body, and water that is too soft extracts thin and papery. Problems appear at the extremes. Above roughly 90 ppm, extra hardness stops helping flavor and starts building limescale in your equipment.
Does roasting change the mineral content of coffee beans?
Roasting does not add or remove minerals. It removes water and mass, which concentrates the minerals slightly per gram of roasted coffee. What roasting does change is the acids and aromatics that mineral-rich water interacts with during extraction.
Buy Indonesian Green Coffee Beans from Indonesia Specialty Coffee
Indonesia Specialty Coffee exports Gayo Arabica, Mandheling, Lintong, Bali Kintamani, Toraja, Flores, Java, Robusta, and Liberica green beans from Medan, North Sumatra. We ship Grade 1 Specialty only, graded against SNI 01-2907-2008 at 82 to 88 SCA points, maximum 12.5 percent moisture, FOB Belawan. All lots are Halal certified, with Organic and Rainforest Alliance available on request. Current pricing sits at [confirm current price with ISC pricelist].
Order tiers run 1 kg for a cupping sample, 60 kg microlot, 350 kg standard wholesale, and container loads from 9 MT quoted per metric tonne.
Evaluating an Indonesian lot for the first time? Fix your water before you cup it, then judge the coffee. See our green coffee beans or contact our team for a wholesale quote.
