Water Changes the Taste: The Science of Coffee and Water

About 98% of the coffee in your cup is water. And yet the conversation is almost always about which beans to choose, while the water is all but ignored. In fact, with the very same beans, just changing the water can make acidity bloom or bitterness vanish. Here we lay out, with numbers, how water quality changes the taste.

Why minerals decide the taste

Brew coffee with pure water (distilled or RO water) and it actually tastes poor. The magnesium and calcium dissolved in the water act as the "hands" that grab and draw out coffee's aromatic compounds. With too few minerals the taste is thin and flat; with too many, the water hoards the compounds and the cup turns muddy.

Hardness (GH) and alkalinity (KH) are different things

This is where many people get confused. Total hardness (GH) is the amount of calcium plus magnesium, and it sets the "strength of extraction." Alkalinity (KH, carbonate hardness), on the other hand, is the "power to neutralize acid — the buffer." When KH is too high, it cancels out the bright acidity you were after and the cup goes flat. To show off a fruity light roast, the ideal is moderate GH and low KH.

• Total hardness (GH) at or below 50 ppm (soft water): acidity and delicacy stand out; too low and the flavor turns thin
• Total hardness (GH) 50–150 ppm (medium-hard): well balanced — the SCA-recommended zone
• Total hardness (GH) above 150 ppm (hard water): thick body, but acidity is suppressed and bitterness tends to dominate
• High-alkalinity (KH) water: acid is neutralized and the coffee blurs (aim for KH around 40 ppm)

The SCA water standard (worth memorizing by the numbers)

The Specialty Coffee Association (SCA) has clear recommended targets for brewing water. Keep them in mind and you have a yardstick for choosing bottled water.

• TDS (total dissolved solids): target 150 mg/L (acceptable 75–250)
• Calcium hardness: target about 68 mg/L (as CaCO₃)
• Alkalinity: target around 40 mg/L
• pH: target 7.0 (acceptable 6.5–7.5)
• Sodium: about 10 mg/L; ideally zero chlorine and no off-odors

Is Japanese tap water good enough?

Tap water from the Kanto to Kansai regions runs around 60–100 ppm hardness, almost squarely within the SCA range. Japan leans soft by world standards — a country blessed with water well suited to coffee. The only real concern is the chlorine smell; running it through an activated-carbon filter, or leaving it to stand so the chlorine escapes, is enough. In areas with harder water, such as parts of Okinawa and Chiba, it is worth trying a soft bottled water.

TWW: Third Wave Water

In the Western specialty scene, a mineral-additive powder called "Third Wave Water" has become a staple. Dissolve one stick into RO water (ultra-soft) and the GH/KH settle into an ideal ratio. Because it reproduces exactly the same water every time, it suits competitors and anyone who wants to compare beans strictly. It can be ordered online in Japan too.

Try the experiment

With the same beans and the same recipe, brew three cups side by side: tap water, soft water (Volvic, etc.) and hard water (Evian, etc.). With a light-roast Ethiopia you should be able to tell that the soft water opens up the floral aromatics while the hard water muddies them. The difference will be far larger than anything you would get from changing beans.