A few days ago, Dutch barista Zeeshan Malik posted a question to my YES or NO Instagram Q&A experiment. (Early YES or NO results:  most questions submitted haven't been well-suited to a yes-or-no answer.  Unless it gets a little easier, YES or NO will probably be short-lived.) Zeeshan wrote:

Can I use the hot water dispenser on my 3-group commercial espresso machine for regular tea/filter coffee brewing (assuming correct temperature) without any negative impact on flavour?

I answered "NO" because the typical commercial-espresso machine's steam-boiler water is alkaline and packed with minerals.  When used to brew tea or coffee it results in a chalky taste.  When added to an already-brewed espresso to make a drink such as an americano or long black, the effect is less pronounced, but the steam-boiler water is still usually not the best choice.

Why is the water so bad?

Every time a barista steams milk, the steam is pure water vapor, free of minerals.*  Any minerals that had been dissolved in the vaporized water remain in the liquid in the boiler, increasing the water's mineral concentration.  After each steaming, the boiler refills with a bit of mineral-laden water, continuing the process of removing pure water (vapor) from the boiler and replacing it with mineralized water.  (I prefer not to complicate this simple explanation with a discussion of scaling, corrosion, or milk being sucked back up a steam wand.)

*I'm neither a chemist nor a physicist.  Perhaps a reader who is one can tell us if there can be, in fact,  trivial amounts of minerals in the steam.

See for yourself

If you'd like to test this idea, find an espresso machine that has been used to steam tens of thousands of milk beverages since the last time its steam boiler had been drained.  The taste defect will be most obvious when brewing tea, less obvious when using the water to brew coffee, and least obvious in an americano.

Of course, if the water feeding an espresso machine is extremely low in mineral content, it will require a greater number of milk-steaming episodes for the taste defect to become apparent.  As well, machines with a hot-water tap fed by a mix of "fresh" water and steam-boiler water will produce noticeably tastier tea or coffee, perhaps with little or no noticeable taste defect, depending in part on the mineral content of the feed water and the boiler's past volume of milk steaming.

Regardless of the quality of steam-boiler water, overuse of an espresso machine's hot-water tap may decrease steam-boiler pressure too much during busy service times.  If the steam boiler refills with cold water too fast for it to maintain effective boiler temperature (and therefore pressure), steaming power will be inadequate.

An easy problem to solve

I cannot count the number of times baristas have served me an undrinkable, chalky cup of tea brewed with steam-boiler water.  This is a particularly common problem in countries where cafes typically serve espresso but not filter coffee, as cafes with batch brewers or pourover kettles typically use those for tea and americano water. I hope this post helps decreases the use of steam-boiler water to make beverages.  I recommend quality-oriented cafes, even in espresso-dominated markets, keep a temperature-controlled kettle on hand for tea and americanos.  It's a wortwhile, small investment.

Thoughts?  Experiences you'd like to share?

*The skull and crossbones is just for fun, not meant to imply your steam-boiler water is poisonous. 

** I wish we didn't have to have such disclaimers in modern society.