Freeze Dried vs. Dehydrated "THE FACTS"
Freeze drying is a process which is suitable for a wide variety of industrial products. These include agrochemicals, pharmaceutical intermediates, biological products.
What is "Dehydration"?
Dehydration is basically the process of removing water by using heat and moving air. The warm heated air moves over the item you are dehydrating and after a few hours, the majority of the water is removed.
The heat gives the water molecules enough energy to "break free" of the liquid and become gas particles. Then you seal it in a container, and it stays dry. This is how manufacturers make dehydrated meals like powdered soup and baking mixes.
That's really all their is to the process. If you have an oven or a solar dehydrator, you can make your own dehydrated foods.
There are two big problems with this approach. First, it's difficult to remove water completely using evaporation because most of the water isn't directly exposed to air. Generally, dehydrating only removes 90 to 95 percent of the water, which will certainly slow down bacteria and enzyme activity, but won't stop it completely.
Secondly, the heat involved in the evaporation process significantly changes the shape, texture and composition of the material, in the same way that heat in an oven changes food. Heat energy facilitates chemical reactions in the food that change its overall form, taste, smell or appearance. This is the fundamental purpose of cooking.
These changes can be good, if they make the food taste better (or taste good in a different way), but if you're drying something so you can revitalize it later, the process compromises quality somewhat.
What is "Freeze Drying"?
Freeze drying, on the other hand, is more complicated. The plus side is it has the advantages of retaining all the original nutrients, enzymes & amino acids of the original product. The down side to freeze drying is that freeze dried products are more expensive than a dehydrated cousin. Given the health benefits of freeze drying, that difference is truly minimal in the nutritional benefits remaining with the freeze dried product.
The basic idea of freeze-drying is to "lock in" the composition and structure of the material by drying it without applying the heat necessary for the evaporation process. Instead, the freeze-drying process converts solid water -- ice -- directly into water vapor, skipping the liquid phase entirely.
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.