Tap water and your filter coffee

Last spring a customer wrote to say our Huila tasted flat on her machine, although she had loved it at the roastery. We asked what water she used. Straight from the tap, she said, and the tap was in a part of the country with very hard water. That was the whole story.

Hard water carries a lot of dissolved calcium and magnesium. Some of that is useful, because those minerals help pull flavour out of the coffee. But if the carbonate hardness is high, it neutralises the acids that give a light roast its brightness. You end up with a cup that tastes heavy and dull.

What we do at the roastery

Our own tap water is on the harder side. For cupping and for the filter coffee we brew in the shop, we mix tap water with an equal amount of soft bottled water that has a low mineral content. It is not a perfect recipe, but it is cheap and gets close.

At home you can do the same. Try a filter jug first, since it removes some of the carbonate hardness. If you want to experiment, buy a bottle of low-mineral water and compare two brews side by side, same coffee, same ratio, same grind. Taste them while they are still warm and again once they have cooled. The difference is usually obvious.

What not to do

Do not use distilled water. Coffee needs some minerals to extract properly, and a cup made with distilled water tastes thin and strangely sour. And do not worry too much. If your coffee tastes good, your water is good enough.