What is Freeze-Drying?
It's kind of nerdy, and it's much more fun than all the messy canning dad hated as a kid.
Freeze-Drying is Actually Called Lyphilisation
Skipping the Liquid Phase of Water
For context, in a normal drying process, you remove water from food by applying heat. As the food heats, the heated water changes phase from a liquid to a gas (vapor), which allows the water vapor to escape the food.
The basic idea behind freeze-drying is controlling the phase change of water. The first thing to understand is that water cannot exist as a liquid at extremely low pressures (i.e., in a vacuum).
You don't have to heat water to boil it!
You can drop the pressure. When you put liquid water into a vacuum, it boils rapidly. Check it out. Not something you see every day.
Back to freeze-drying.
The first step in freeze-drying is to freeze all the water (and the food it is in). Freeze-drying happens in an air-tight sealed chamber. The next step is to drop the temperature and the pressure inside the chamber. The food (and the water it contains) drops to around -50 ° F. Then, by dropping the pressure you create a vacuum, and liquid water cannot exist in a vacuum. If the pressure drops low enough, the ice turns straight to a gas (sublimation). So instead of boiling water by dropping the pressure, we are skipping the liquid phase and the ice converts directly from a solid (ice) to a gas (water vapor) and escapes from the food. Even crazier, at these extremely low pressures, you can raise the temperature up to 100 ° F or more and water still skips the liquid phase.