Espresso has only two ingredients: coffee beans and water. We spend a lot of time thinking about the coffee bean part- where are the beans from, blend or single origin, roast level... the list goes on.
But the major component of espresso is the water used- it extracts the coffee oils, aromatics, and solids that give our espresso body and make it taste and smell great. Regular coffee is 98% water, and its high powered cousin, espresso, runs at about 90% - so good tasting water means delicious espresso.
Additionally, the water you use affects the health and longevity of your machine. This post will delve into the wet and wild(?) world of water.
Note that water composition is one of the deepest rabbit holes you can drop into when it comes to brewing espresso. It has produced endless discussion, arguments, and at least one academic paper, which I actually read for this.
TL;DR: you should use clean, filtered, fairly soft water for your espresso machine (and all your coffee, for that matter). If you are remotely curious why, keep reading.
There are two factors to consider with brewing water:
1) Minerals, specifically calcium and magnesium. Calcium and magnesium help extract the flavor compounds in coffee.
Low or no minerals = under extraction, acidic and sour espresso
Too many minerals = over extraction, bitterness, astringency
2) Alkalinity, or buffer, in your water. Because coffee is acidic (to varying degrees), more alkaline water will act as a neutralizing agent- this is also affected by the type of coffee you are using and the roast.
Too little buffer, and your coffee is sour and acidic
Too much buffer, and your espresso can be chalky or bland
The first step is to test the hardness, or mineral content, of your tap water. Sometimes water test strips come with your machine, or they are also available at your local hardware or pool supply store, or aquarium supply store, if you want to get granular. You can also often look up water composition from your local municipality, most cities do annual public reports on water quality.
All water absorbs minerals as it flows from its source (mountain springs, wells, rivers, or lakes) to you. Hard water is a result of groundwater running through areas with limestone, chalk, or gypsum, and absorbing more calcium and magnesium.
Why does this matter? The high mineral levels in hard water can make bitter, astringent flavors in your espresso. More importantly, it will damage your espresso machine.
When hard water is heated, it pulls out some of those minerals as limescale, a chalky substance that builds up over time into a rock-like deposit. Excessive scale in your machine can break off and lead to clogged lines, failed heating elements, and expensive repairs- not to mention you won't be able to make espresso while you wait for that machine to get fixed.
In the opposite direction is distilled water, which is the cooled, condensed steam that results from boiling water. The vapor rises, leaving all the heavy solids behind, and gets collected to create 99.9% pure water. So using distilled water won’t leave limescale in your machine, because there are no minerals to leave behind. Seems like a great solution.
However, many espresso machines use an electrical charge to detect the presence of water in the machine. Distilled water doesn't conduct electricity, so your machine will read as out of water- and either won't activate the pump, or conversely, will not recognize water in the steam boiler, and continue to fill it even when at capacity.
Additionally, many people think distilled water feels and tastes flat, because those minerals contribute, however subtly, to the flavor of water. Since your espresso is roughly 90% water... well, that’s a lot of bad.
I wanted to know if I could really taste the difference, so I pulled two shots with exactly the same parameters, using distilled water for one, and regular filtered tap water for the other shot (we have soft tap water in Seattle).
The espresso made from the distilled water tasted very harsh, metallic and bitter, and the espresso made with filtered water tasted sweeter, with some brightness and a smooth finish.
Granted, I have made a living by tasting things, but I was still surprised. If you don’t want to empty out your boiler for the sake of experimentation, try comparing distilled water to filtered tap water for a pour over or French press sometime. It’s surprisingly obvious which makes superior coffee.
WHAT IS THE BEST WATER FOR ESPRESSO?
The Specialty Coffee Association of America (now just the Specialty Coffee Association, or SCA) set guidelines for water for the ideal cup of coffee- and rather than pure water, they recommend 150ml/L of Total Dissolved Solids (TDS). TDS is all the stuff in water that isn't water- minerals, salts or metals. However, SCA doesn't specify what those solids should be.
Research conducted by Christopher Hendon and Maxwell Colonna-Dashwood at the University of Bath studied water composition and its effect on espresso, eventually leading to them writing Water for Coffee (currently out of print). They found that it wasn’t the overall total of TDS, but the actual minerals dissolved.
Mr. Hendon explains, “Hard water is generally considered to be bad for coffee, but we found it was the type of hardness that mattered – while high bicarbonate levels are bad, high magnesium levels increase the extraction of coffee into water and improve the taste.”
Hendon added: “There is no one particular perfect composition of water that produces consistently flavoursome extractions from all roasted coffee. But magnesium-rich water is better at extracting coffee compounds and the resultant flavour depends on the balance between both the ions in the water and the quantity of bicarbonate present.”
So your end goal is clean, filtered water that has just enough of the right mineral content to taste good. There are a few options:
1) If you are a lucky person that lives in a place with naturally occurring soft water, you can get away with just filtering your water and you’ll have pretty good results. However, there aren't many places with the right water for this method, so don't count on it.
2) Buying bottled water forever (it works, but it's pretty awful for the environment and you really shouldn’t do this).
2) Water softening packets, which can be put into your reservoir and last for months at a time (relatively inexpensive and work pretty well).
4) Installing hard water reverse osmosis (RO) and re-mineralization systems (WOW! THIS CAN GET VERY EXPENSIVE).
And if you've read this far, you should also know that you can also
5) Make your own espresso and coffee water by purchasing distilled water and re-mineralizing it to exact specifications
You can purchase pre-measured packets of minerals, and more recently, pre-measured concentrates in dropper bottles for you to add to your water. For the DIY inclined there are recipes that use inexpensive ingredients from the grocery store.
If all this water talk feels like drinking from a firehose (I can’t help it, it’s right there), just know that using softened water is preventative maintenance for your espresso machine, and prevents the production of scale. If reading this just makes you thirsty (you knew it was going to happen) for more information, you can check out these resources.