Every time you steam milk, you release pressure from the boiler through a valve to the steam wand, forcing air into the pitcher of milk. As you close off the valve, the pressure change pulls a tiny bit of that milk back into the wand.
That little bit of residue can build up and eventually turn rancid, decreasing your steam power and making your drinks taste bad. The solution: wipe and purge the steam wand after every use!
Purging just means sending a blast of steam through the wand as soon as you’re done steaming milk. Additionally, right after use, wipe down the exterior of the wand with a damp or wet towel and you’ll get that layer of milk off the outside as well.
If milk scalds and does adhere, wrapping it in a damp towel for a few minutes, then running the wand to heat it up will allow you to clean it off.
If you forget to purge (but you won’t, now that you know), sometimes the holes in the wand tip will get clogged. Fortunately, the end of the wand can be unscrewed, and you can clear the holes with a pin or even the end of a straightened paper clip.
Taking off the end also allows you to get to the inside of the wand with a brush to clear out any stubborn residue inside.
The less frequent part of maintenance is to soak the wand in a detergent especially formulated to break down milk proteins (Rinza, Full Circle, or another similar cleaner).
You can also briefly run the wand in the cleaner- this cleans out any milk residue that gets pulled up, which can happen even with your diligent purging. For a home machine, this can be done every month or two.