Image: Freeze dried bananas on the left, dehydrated bananas on the right
What is Freeze Drying?
Freeze drying is a low temperature dehydration process that involves freezing the product, lowering pressure, then removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat. Freeze drying results in a high quality product because of the low temperature used in processing.The original shape of the product is maintained and quality of the re-hydrated product is excellent. Freeze drying also maintains 97% nutritional value when compared to the dehydration process.
How was it initially used?
Freeze-dried foods became a major component of astronaut and military rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate, were transformed into warm hot meals in space by improving the process of re-hydrating freeze-dried meals with water. As technology and food processing improved, NASA looked for ways to provide a complete nutrient profile while reducing crumbs, disease-producing bacteria, and toxins. The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids. Polyunsaturated fatty acids are beneficial in mental and vision development, and as it remains stable during space travel, can provide astronauts with its added benefits. The crumb problem was solved with the addition of a gelatin coating on the foods to lock in and prevent crumbs.Disease-producing bacteria and toxins were reduced by quality control and the development of the Hazard Analysis Critical Control Point (HACCP) plan, which is widely used today to evaluate food material before, during and after processing. With the combination of these three innovations, NASA could provide safe and wholesome foods to their crews from freeze-dried meals.
The primary purpose of freeze drying within the food industry is to extend the shelf-life of the food while maintaining the quality. Freeze-drying is known to result in the highest quality of foods amongst all drying techniques because structural integrity is maintained along with preservation of flavors. Because freeze drying is expensive, it is used mainly with high-value products. Examples of high-value freeze-dried products are seasonal fruits and vegetables because of their limited availability, coffee, and foods used for military rations, astronauts/cosmonauts, and/or hikers.
A few points about freeze drying:
- Removing water keeps food from spoiling for a long period of time. Food spoils when microorganisms, such as bacteria, feed on the matter and decompose it. Bacteria may release chemicals that cause disease, or they may just release chemicals that make food taste bad. Additionally, naturally occurring enzymes in food can react with oxygen to cause spoiling and ripening. Like people, microorganisms need water to survive, so if you remove water from food, it won't spoil. Enzymes also need water to react with food, so dehydrating food will also stop ripening.
- Freeze-drying significantly reduces the total weight of the food. Most food is largely made up of water (many fruits are more than 80 to 90 percent water, in fact). Removing this water makes the food a lot lighter, which means it's easier to transport. The military and camping supply companies freeze-dry foods to make them easier for one person to carry. NASA has also freeze-dried foods for the cramped quarters onboard spacecraft.