How are powdered instant sauces/soups manufactured?
We make the sauce and then remove the water. This is done through freeze drying or low heat drying. Usually freeze drying is used more often as heat can make for a environment for bacteria to form.
At one plant where I designed most of the processes they made instant gravy. The gravy was made in large batch kettles then poured slowly onto a slow conveyor belt. The gravy was spread to about 4mm think and the flat belt carried it through a nitrogen freeze tunnel for about 5 minutes. The gravy was a powder by then and scrapped off the belt and packaged into 50lb bags or sent to a packaging line for smaller containers.
Another company made freeze dried instant coffee. Same process but with a lot higher volume and different drying techniques using refrigerant systems to freeze dry.
This would be hard to do at home unless you maybe used dry ice. You would need to keep the area ventilated so the moisture driven off can escape and need to make sure your product is in a thin layer to speed the process so extra flavors from other sources do not mess with it.
Doing it in a freezer will take too long, it can't go down far enough and minimal air circulation. Plus all the smells it would pick up.
If I were going to try it at home I would use a food dehydrator. These have the proper ventilation to help remove the moisture from the air around your product and low heat so as not to cook it.
Keep in mind that once water is removed the powder will become hydroscopic and will attract moisture. So you will need to seal it in something as soon as possible. Check a package every couple weeks for mold growth and any changes in smell. This will let you know if you dried it enough and the rest of the batch is still safe.
I am sure there are a bunch of sites and YouTube videos for drying foods.