(Spoiler: Don't use distilled water.)
The science: To make quality coffee, optimally water needs to actually not be perfect (i.e. perfectly chemically pure water). A small amount of total dissolved solids (TDS) is actually beneficial to the chemical processes of extraction. The Specialty Coffee Association of America has listed best-practise specifications for coffee for its cupping. They specify that water needs to be odour-free, clear, free of chlorine, have a mineral TDS of 150mg/L, have a calcium hardness of 68mg/L, an akalinity of of ~40mg/L, a neutral pH and ~10mg/L of sodium. All of these specifications are derived from the SCAA Water Quality Handbook (available in print only from the SCAA), and based on achievable standards from US water supplies and iterative taste testing. In reality, many pro brewers shoot for 'somewhere around' these figures to get something good enough.
The problem: Tap water rarely meets these expectations. It's usually too low, and sometimes way Remembering we're looking for 150mg/L, have a look at the following list of some of the coffee capitals of the world (oh man, don't get jealous! I know there are others):
- New York, NY: 53 (source: nyc.gov, 2014)
- San Francisco, CA: 71 (source: sfwater.org 2013
- Wellington, NZ: 48 (Te Marua, 50% of supply, source: gw.gov.nz)
- Melbourne, Australia: 39 (largest reservoir with data, source: melbournewater.com.au 2014)
- Hong Kong, China: 64.6 (including Ca, Mg, Cl, Sulphate and SiO2, source: wsd.gov.hk Sep 2014)
- London, UK: 320-400 (community sources including ukaps.org, jimseven.com, comingsooncoffee)
Plastic bottled water introduces further problems - very little of it is true mineral spring water, mostly being some form of either cleaned or distilled water - not optimal for coffee. So what is one to do?
The solution: Live somewhere amazing, optimise your water source or buy better water for coffee. This right now is just starter advice - with more research, I'll post more. I recognise it's an incomplete list, but it's three introduction to approaches that can yield good results.
- Live somewhere amazing! Difficult to do. Most of us don't put tap water TDS high on the priorities list for choosing what city to live in. Sure, water being drinkable is important... but anywhere from 50-150 will taste fine (assuming it's odour free, clear etc.). To see if you're in luck, check your local water department's annual data poll, or buy a TDS meter (it might have other uses) to really see what's coming out of your tap. Apart from TDS, check the other parameters to see if your water meets the requirements. I've heard (but haven't checked) some Nordic countries hit the nail on the head, but haven't found supporting data (some claim as high as 110ppm, some as low as 40, making it similar to many other cities int he world). Out of luck and live in the rest of the world? See below...
- Second best: Filter your water and add minerals back in: Use a solid block carbon filter, or a reverse osmosis type filter, to remove all mineral content. This is already a very expensive step, and it's only step one! These high end filter machines can set you back many hundreds of USD. The only time this would be practical (unless you're just willing to throw money at this hobby based on one blog) is if the water is disgusting and effectively undrinkable. Much has been written on the science and rationales for each kind of filter, and there are advantages and disadvantages as well as cost/benefit trade-offs on each type. Suffice to say that a good installation of either kind of filter will outperform a normal carbon filter (such as a Brita jug, or the kind that sits in-line with your faucet and performs superficial improvement only). Once all TDS is removed, your filtration system should add minerals back in using a special device. Obviously, this is starting to add up in cost and complexity, but it's good quality.
- Buy good water. Oh, the environment! Minimise packaging at least and buy it in bulk. Note that you can't just go and buy any old mineral water off the shelf. Many mineral waters, as their names suggest, actually have far too many minerals in them. Read the labels of waters available in your local supermarket, check particularly for TDS which can either be near zero or through the roof, and try to buy something that approximates what's above.
In all cases, do controlled double blind taste tests between different water sources/types to see what works best. Be scientific, it is worth it!