Hi everyone,
My La Marzocco water test shows minimal hardness, chlorine, etc, yet a white residue has been accumulating on the walls of my plastic water tube. Can anyone explain why this is?

For the first year, I used the stock tube and it never developed the white residue, although it did start to accumulate mildew. I then replaced the tube with a beverage / food-grade tube.

Now, it is a year later and, while minimal signs of mildew are in the tube, there is a white reside that goes all the way up the tube from the reservoir into the machine.

Our tap water is fairly soft, and gets filtered through a 3M Aqua-Pure C-Cyst-FF under the kitchen counter. I just did a water test today with the La Marzocco kit, and here are the results.

It's hard to say what that whitish material is just by looking at it. Where does the tube get the water from? If it's from the vacuum braker valve it could be a small amount of limescale, as that would be water discharged from the steam boiler.

Even if your testing shows "0" hardness, there may be a very small amount of Mg++ or Ca++ dissolved in the water, which may be undetectable at the time of the test but over the course of a year could show up as some limescale. Furthermore, your water supply could have variations over time, especially seasonally, in mineral content.

Hardness refers to hardness minerals, not everything in the water. There can still be plenty of dissolved solids in soft water. For example, if you run hard water through an ion exchange softener, then evaporate the water, you'll get deposits resembling those in your tube. These deposits are easy to remove, compared to hardness scale. A TDS meter can show quite high numbers in "soft" water. The only water that won't leave something behind on evaporation is pure distilled water.

Thanks for the info regarding hardness and variation.

What is interesting is that, during the first year of use, the tube did not have any white sediment buildup - just mildew.
This new tube, used for a year, has the white stuff.

This tube is from the water reservoir of the Linea Mini, going up to the pump/boiler.

The one new thing I did add between year 1 and year 2 is a quick-release right at the reservoir to the tube.
Could that somehow be contributing to the white stuff?

If the water in this tube just comes from the reservoir and not from the boiler, then I doubt it is due to mineral precipitates from hard water. More likely some other dissolved solids or something that reacts with the tubing itself.

This is Tygon S3 beverage tubing.

https://www.usplastic.com/catalog/item. ... unt-orders

I still can't find the answers to what the white stuff is. Could it be plasticizer residue that is leeching?

I will try a different tube or finish my plumb-in...