Freeze drying is drying of an already deep frozen product in a vacuum below the triple point. Here water can only be present in two phases: ice and vapor. The ice will in a sublimation process pass through the water phase directly into vapor. This ensures that the product retains most of its original shape, color, taste and nutritients. If energy is applied the sublimation process will proceed faster.
The basic idea of freeze-drying is to completely remove water from some material, such as food, while leaving the basic structure and composition of the material intact. There are two reasons someone might want to do this with food:
- 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.
How Freeze-Drying Works?
Freeze-drying, or lyophilization, is the sublimation/removal of water content from frozen food. The dehydration occurs under a vacuum, with the plant/animal product solidly frozen during the process. Shrinkage is eliminated or minimized, and a near-perfect preservation results. Freeze-dried food lasts longer than other preserved food and is very light, which makes it perfect for space travel.
The purpose of freeze-drying is to remove a solvent (usually water) from dissolved or dispersed solids.
Freeze-drying is method for preserving materials, which are unstable in solution. In addition, freeze-drying can be used to separate and recover volatile substances, and to purify materials. The fundamental process steps are:
- Freezing: The product is frozen. This provides a necessary condition for low temperature drying.
- Vacuum: After freezing, the product is placed under vacuum. This enables the frozen solvent in the product to vaporize without passing through the liquid phase, a process known as sublimation.
- Heat: Heat is applied to the frozen product to accelerate sublimation.
- Condensation: Low-temperature condenser plates remove the vaporized solvent from the vacuum chamber by converting it back to a solid. This completes the seperation process.
In freeze-drying, moisture sublimes directly from the solid state to vapor, thus producing a product with controllable moisture, no need for cooking or refrigeration, and natural flavor and color.