What's this, there is something called 'instant coffee?' That's funny, because all coffee is technically instant. Sure. You know, 'just add water.' Seriously though, coffee is such a beautifully basic craft. The two ingredients are coffee and water. We spend a lot of time talking about the bean and what it means. Let's spend some time contemplating water.
Water Throughout the Coffee Narrative
From the farm to the kitchen, the way water interacts with coffee at each stage is measured and critically considered. Water is an active aspect of coffee craft in so many ways. Annual rainfall will determine rate of coffee fruit maturity. Too much rain and fruit can develop too fast and lack complexity. Water is also a main tool used in processing the fruit off of the bean (in washed coffees). In the drying phase, internal moisture content of the raw bean is monitored painstakingly as a determinant for the stage's completion. At the roastery, a popcorn-like hydraulic crack (also called first crack) can be heard as the remaining moisture escapes from the bean in the form of steam. This 'crack' is an important event in the roasting process. Finally, we reintroduce water in the brewing phase. We then consume our coffee and rest from all this work.
Water in Brewing
The three critical aspects of water used in brewing are temperature, ratio to coffee, and purity. Water that is not hot enough will leave your cup tasting pretty drab. Water that is too hot can extract a bit too much of the astringent and bitter characteristics from the bean, overpowering sweetness. Regarding ratio, most Americans will find a favorable cup somewhere in the 15:1 and 17:1 range (water:coffee). That leaves us with the final consideration—water purity. This can be a great big rabbit hole if you aren't careful. Purity of water is critical, obviously, as coffee is mostly water and bad water will make bad coffee. But just how good does it need to be?
Unless it is distilled, all water including bottled drinking water will have something called 'dissolved solids' which contribute to what it tastes like. It is an abundance of these dissolved solids (called hard water) that wreak havoc on plumbing, contribute to 'off' tastes in your cup, and can make your water under-solvent—a fancy word that means you get less out of your coffee grounds. Conversely, water that is too low in dissolved solids (called soft water) will have an almost 'chemical' quality to it that our tastebuds reject as unnatural.
Make It Yourself
Try not to get too hung up on which solids your water contains. Instead, find ways to dial your water in to an acceptable range of TDS (total dissolved solids). If you don't know what your water is out of the tap, you can buy a TDS measurer online or bring a sample to your local roastery and ask them to test it for you. The range you should aim for is between 75 and 200 PPM of TDS. More often than not, municipal water will be higher than this. Don't fret, all you need is distilled water (usually under a dollar per gallon at the grocery store) and a calculator. Distilled water is always 0 PPM and so you can make your desired TDS by combining it with your tap water in proper ratio before brewing.
It may seem nit-picky, but you will notice a difference. Also, it isn't just about the taste of the coffee. You will also see that your kettle will have considerably less scale build up after prolonged use.
Home coffee craft should be pure and simple. Keep it pure and keep it simple.