How does water chemistry affect coffee taste?

Because a cup of coffee is almost entirely water, the quality of that water is just as important as the quality of the beans. Water is a solvent, and its ability to dissolve the various acids, sugars, and oils in coffee depends on the minerals dissolved within it. This is why water chemistry is a major focus for specialty coffee professionals.

Magnesium and calcium are the two most important minerals for extraction. Magnesium, in particular, is excellent at pulling out the fruity and floral notes that many coffee lovers prize. Calcium also aids in extraction but can sometimes lead to a heavier, more muted flavor profile. If your water lacks these minerals—such as with distilled or reverse osmosis water—the coffee will often taste thin, hollow, or unpleasantly acidic, as there is nothing to balance the extraction.

On the other end of the spectrum, 'hard' water contains high levels of minerals, particularly bicarbonates. While some alkalinity is necessary to buffer the acids in coffee and prevent it from tasting overly sour, too much alkalinity will neutralize the coffee's natural acidity entirely. This results in a flat, dull, or even chalky-tasting cup. Furthermore, extremely hard water can lead to scale buildup in your brewing equipment, which can eventually affect the temperature stability of your machine.

In practice, the goal is to find a 'sweet spot' for your water. Many home brewers use filtered water to remove chlorine and other off-flavors, but if your local tap water is very soft or very hard, you might consider using a water treatment system or adding mineral packets designed for coffee. If you are unsure about your water, try a simple test: brew the same coffee with bottled spring water and your tap water. The difference in clarity, sweetness, and acidity will likely surprise you.