The terms ‘dehydrated’ and ‘freeze-dried’ are pretty often used interchangeably, but there’s more difference to them than you might think. They’re both great ways of preserving food, but that’s where the similarity ends.
The Dehydration Process
Dehydration is the simpler of the two processes. It has been used for millennia as a method of food preservation – as early as 12,000 BC people were drying meat and fruits in the sun. These days sun drying is only used for fruit (which has to be pasteurized later.) [Source: Virginia Cooperative Extension]
Industrial dehydrators are essentially large ovens. Hot air kept around 100°F circulates throughout the chamber, slowly ridding the food of all its moisture. Foods are kept in the dehydrator for different amounts of time depending on the amount of moisture in the food. [Source: EPA]
In the end, dehydrated foods will lose about 90-95% of their moisture content. Shelf life will depend on manufacturing, but for dehydrated foods (according to the website The ReadyBlog) their shelf life averages 15-20 years.
The Freeze-Drying Process
What we think of as freeze-drying today actually dates back as far as World War II. The technology was developed to help get medicines onto the battlefield without their spoiling. [Source: How Products Are Made] Food storage actually came later.
The process starts as you might think it would, with freezing. Once that’s done the food will go through two stages of drying. The first, primary drying, utilizes something known as sublimation.
Food is placed inside a sealed dryer and the pressure is lowered, which creates a partial vacuum. Inside that vacuum the solid water (ice crystals) in the food turn directly into water vapor. The water vapor is filtered out of the freeze-drying chamber and re-freezes onto the refrigeration coils. [Source: SPScientific] (This keeps food from reabsorbing the moisture it lost.)
To remove the remaining water in the food, the drying chamber is heated up, which helps to force out any liquid water still left in the food.
At the end of this process, foods have lost between 95-99% of their moisture and (stored properly), they can last 25-30 years. [Source: The ReadyBlog]
The Nutritional Side
It’s impossible for fresh food not to lose some nutrients, whether it’s to cooking, preservation or simply time. With dehydration, contact with heat damages vitamins A & C, which are very heat sensitive. (Vitamin A is also light sensitive, which makes it important to store dehydrated foods in a dark place.)
According to the EPA, some foods can be treated with sulfur compounds such as sulfur dioxide to prevent the loss of antioxidants in the dehydration process. (Sulfur dioxide also helps with color retention.)
Freeze-dried foods minimize the loss of nutrients – primarily restricted to vitamin C.
That was the long version, but to sum-up, a chart for comparison:
Whether you choose freeze-dried or dehydrated, the benefits far outweigh any potential nutrient loss. The cooking is gentle (or non-existent) and shelf-life is extended without any additives.
Pet Food Safety Advocate (In Training)