Freeze Dried vs. Dehydrated Foods

While many of us are familiar with the terms "Dehydrated" (example: Beef Jerky) and "Freeze Dried" (example: Instant Coffee), you may not be aware of the actual processing differences between these two methods. While both provide the advantage of a long and stable shelf life by removing the water from a product, that is where the similarity ends.

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 food in this way 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.

Freeze drying, on the other hand, is a bit 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 slightly 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.

So what exactly is "Freeze Drying"?

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.

A simplified explanation of the freeze-drying machine process:

Simplified "Freeze Drying Machine from "How Stuff Works"

With most Freeze Drying machines, you place the material to be preserved onto the shelves when it is still unfrozen. When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber. The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.

Next, the machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day.

This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure. Additionally, accelerating the sublimation process could produce more water vapor in a period of time then the pumping system can remove from the chamber. This could re-hydrate the material somewhat, degrading its quality.

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!

Reference:

HowStuffWorks.com