Water is the ingredient you forget
A brewed coffee is about 98 percent water, so it is strange how long I ignored it. The thing that made me notice was brewing the same beans at a friend’s flat across town and getting a visibly different, duller cup. Same grinder setting, same ratio, same kettle. Different tap.
Very soft water tends to give a flat, hollow cup — there isn’t enough mineral content for the flavours to bind to. Very hard water goes chalky and mutes the brighter notes, and furs up the kettle besides. There is a comfortable middle, and most tap water sits somewhere you can nudge.
I don’t chase a lab number. I just stopped using the softened kitchen tap.
Our kitchen tap runs through the building’s softener, which is good for the dishwasher and bad for coffee. The bathroom tap is unsoftened and, it turns out, makes a better cup. That was a free fix. When I travel I carry a small bottle of still mineral water with a middling mineral content for the first brew in a new place, which takes one variable out of an unfamiliar kitchen.
If you want to go further there are remineralisation sachets you add to distilled water, and they do work, but for most home brewing “use the tap that isn’t softened” gets you most of the way for nothing.