When is a Freeze Dryer not a Dehydrator?

These “Freeze Dryers” (which can be quite expensive) that people are buying are really a class of dehydrators that do not use heat to dehydrate.

A good example is from my younger days when we used to help Mum with the laundry in the winter. Back then, we used a clothesline to dry our clothes. In very cold weather, the laundry would freeze to the clothesline, and we sometimes needed to pry the clothes off because they were as rigid as cardboard. However, they were dry.

As an engineer, I know a lot about what was happening.

The term that applies is “relative humidity.” We know that the atmosphere can contain moisture, and at each temperature, the amount of moisture is different. If the amount of moisture in the air rises, we feel that it is humid. When the air contains too much moisture (close to 90-100%), it rains!

Below freezing, it can’t rain; instead, the water (moisture) changes “phase” from solid to gas (“steam”) without first changing to liquid. Freeze “dehydrating” occurs quite fast. The speed will be determined by the moisture content relative to the outside temperature. If something is “wet” (100% moisture) and humidity is low (15% of what it could be), the moisture content will drop drastically and quickly.

A good example of how fast humidity can change is trying to run an air conditioner with the window open. Humidity is really “vapor pressure.” The relative vapor pressures (between inside and outside the window) reach equilibrium almost instantaneously!

In theory, you could put your dehydrator outdoors, put frozen vegetables into it, and run the dehydrator fan (without heat), and the vegetables should dehydrate. Just be careful about the raccoons! LOL!

This process may even work for meats, although we all know about “freezer burn.”

If I conduct this experiment and use store-bought frozen vegetables, all should be okay. However, if I freeze my own vegetables, I will blanch them before putting them into the freezer (in summer) or dehydrator (in winter), otherwise, the results will not be positive!