The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form.
There are two major factors that determine what phase (solid, liquid or gas) a substance will take: heat and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a certain range. Without these conditions, that phase of the substance can't exist. The chart below shows the necessary pressure and temperature values of different phases of water.
So, how do we do that at home?
First, you get a heavy duty freezer (the Harvest Right units drop to -30°F (-34°C) or colder).
Second, you pair this up with a completely airtight chamber that can hold a vacuum (no oxygen) every single time you use it.
Third, you tie in a high end vacuum pump strong enough to suck the stripes off a zebra.
Fourth, you add a heater and thermostat, so you can cycle the temps up and down, repeating the sublimation process for hours on end.
Fifth, tie in a humidity sensor to make sure the water is out, triggering the cycle completion.
Once the material is dried sufficiently, it's sealed in a moisture-free package, often with an oxygen-absorbing material. As long as the package is secure, the material can sit on a shelf for years and years without degrading, until it's restored to its original form with a bit of water (a very small amount of moisture remains, so the material will eventually spoil). If everything works correctly, the material will go through the entire process almost completely unscathed!
For more information on freeze-drying, including its history and applications, check out the links Freeze Dry Machine or https://www.gf-machine.com/