You Asked: How can I make my own powdered milk? --- Trinity, Kampala, Uganda
Marshall Answered: Powdered milk is one of those things that is not so easy to make at home. Here's why.
Let's say that you want to make salt from salt water. You can simply put salt water on the stove and boil it. The salt is completely unaffected by heat, so it doesn't matter how fast you boil it. Eventually all the water is gone and you have dry salt powder.
Now let's say that you want to make sugar from sugar water. You can get it part of the way by boiling the water. This is exactly how they make maple syrup. But to turn the syrup into dry sugar you have to be a lot more careful. If you heat it too much you may caramelize the sugar or carbonize (burn) the sugar. Or you might turn the sugar into hard candy.
Milk is even more complex. To dry milk into powder, you cannot raise the temperature too much or you change the flavor. So a factory would first evaporate off half the water in a vacuum chamber. This is similar to the idea used in freeze drying. The milk only has to be heated to 135 degrees F or so in a vacuum chamber to get it boiling, and the milk isn't damaged or scorched at that temperature.
This evaporated milk is then sprayed through very fine nozzles to mist it in air at high temperature. The tiny droplets of milk dry almost instantly. The flavor definitely changes in the process, but not drastically. For more info see: How Is Powdered Milk Made?