What is the difference between Freeze Drying & Dehydrating?

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First, a basic overview of both processes:

Dehydrating is a centuries-old method for preserving food. Drying meat, fish, and plants in the sun was a simple way to preserve food before electricity. 

Dehydrators use a form of energy (usually heat carried by hot air) to reduce the moisture level in foods, which then increases the storage life of the food.

Because of the heat, about half of the nutritional value is lost, and the texture, taste, and appearance changes.