Many people use the terms “freeze dried” and “dehydrated” interchangeably when referring to dried food. While both freeze dried and dehydrated result in a very similar end product (eg, dried shelf stable food), the process is very different and the results vary as well!
Dried food is inclusive of both dehydrated food and freeze dried food; simply put, the term dried food implies that the water content of the food item has been reduced either by old fashioned methods such as air and sun drying or by modern appliances such as dehydrators or freezers and vacuums. It's important to note that while all dehydrated or freeze dried food is “dry food”, not all “dry food” is dehydrated or freeze dried. In order to be dehydrated, food has had between 90-95% of the moisture removed; for freeze dried, this percentage is higher, usually around 98 or 99%.
So what exactly makes these two processes so different? I'm going to focus on the three big points: equipment, texture, and nutrition. But first, let's define what the two processes are.
Dehydrating Food: Accomplished either by 1) sun dried or air dried, placing food outside in the sun on days where there is low humidity and high temperatures or 2) utilizing an electrical device with trays that has a fan either at the bottom forcing air up vertically or in the back forcing air through horizontally. In dehydrator devices, a temperature dial gives the operator control over how hot the air is, and the air temperature generally ranges from 95 F to 165 F depending on the food group. To dehydrate food you need both airflow and heat. To have the longest shelf life, food should be dried until 95% or more of the moisture is removed; food can be dried to a higher moisture content (lower moisture removed percentage), but the food will have a short shelf life. Sun dried food is the oldest form of food preservation and can be found throughout many ancient cultures.
Freeze Drying Food: Compared to dehydration which has been around since antiquity, freeze drying is relatively new to mankind being invented in 1906, though it wasn't widely used until World War II when the USA and Europe needed a way to distribute shelf stable blood serum (which until then needed refrigeration) to treat the wounded. Freeze drying is a much more complex process and requires specialized equipment that the average household wouldn't need or be able to afford. Food is subjected to extremely low temperatures (-40F or colder) until frozen. Once frozen, a vacuum is formed around the food, and then the temperature is slowly raised. The heat energy causes the frozen water to sublime (sublimation: a process in which frozen liquids are converted to vapor without going through a liquid phase.) Freeze drying is a more complex process compare to dehydrating and is utilized most typically in commercial kitchens, not home kitchens.
The biggest difference between freeze drying food and dehydrating food is the tools used to achieve the end product. Dehydrated food can easily be made at home while freeze dried is more suited to commercial kitchens.
To better illustrate this, let's visually compare the two machines.
Here is a personal use Excalibur dehydrator, this one costing around $300-$350. I consider Excalibur a high end manufacturer and thus they also have a higher price tag than other companies. For comparison, I also have this Presto dehydrator which I bought on sale for around $60, regularly $100. Both of these are kitchen countertop friendly (more-so the Presto) and plug directly into your outlet.
Majority of freeze dryers are not manufactured for home use; however, Harvest Right does sell models for that purpose. These machines are much bigger and have a much higher price tag. Your entry level machine here will run you close to $3,000.
Yep! Both dehydrated food and freeze dried food retain a high nutrition profile. Because freeze dried removes more moisture than dehydrated, freeze dried does have a longer shelf life. However, with proper use of high barrier pouches and oxygen absorbers, both dried foods can have a shelf life over a year.
While most claim that freeze dried food retains a higher nutrition profile than dehydrated, I have never found a study proving this (though I don't doubt it is true.) There are simply no conclusive reports that state the exact nutrition difference between the two methods.
Freeze dried food rehydrates slightly better and faster than dehydrated food due to the more porous nature of freeze dried food.
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