It’s not rocket science. It’s the steamer wand in your espresso coffee maker.
Your espresso coffee maker is equipped with a steamer wand which passes steam generated in the boiler or thermoblock. This steam can be used to heat up any liquid simply by inserting the wand into the liquid and revving up the steamer. One major advantage of having an espresso maker with a steaming wand is that you can steam up the milk for that frothy texture externally. There is no milk running inside the machine and that saves a lot of cleaning after every use. Milk running in the machine is tricky; if not cleaned up properly, it can cause a build-up of bacteria.
The best way to froth milk is to use cold milk at about 10 to 12°C. Milk straight from the refrigerator is just fine and even skimmed milk is good for frothing. The steam passes hot air into the milk and heats it up. It also injects air into the milk to denature the milk proteins. In simple terms, this means it changes the shape of the proteins which gives the milk a thicker shinier look and the microbubbles that form the froth.
The only way to get it right is to practice it a bit. It’s not difficult to learn. But there is a knack to it. Once you’ve held your cup of milk in your hand and inserted your espresso machine’s steam wand in it, you’re on your way to becoming a Barista.