- Education and Science
Uses of Freeze Drying
Lyophilization in Industry
What is Freeze Drying?
Freeze-drying, or lyophilization, is the removal of water from a substance using controlled temperature cycles and a vacuum. The product is frozen, and slowly warmed under vacuum: the frozen water molecules in the item are then sublimed – they pass directly from the solid “ice” phase directly to the gas “vapor” phase, completely bypassing the liquid phase. The vacuum pulls the water vapor away from the product and onto a frozen condenser coil, where the vapor re-freezes. This part of the process is termed “primary drying.”
After the primary drying process is complete, a secondary drying process is initiated. Secondary drying involves the removal of any residual, non-frozen water molecules that remain in the product. The temperature is increased – a different lyophilization cycle is used for different products (the adsorption isotherms, or temperature at which the water molecules remain bound to a material, will vary by the product being dried). The amount of vacuum is increased (pressure reduced) to further drive water off the product.
Once the freeze-drying process is complete, the freeze-dried item may be sealed under vacuum or in the presence of an inert gas (usually Nitrogen) to prevent moisture from contaminating the final product.
Freeze-Dried Food for Backpacking
Freeze Dried Food
Many food products are lyophilized as a preservation method. Once a food product has been freeze-dried, the shelf life is extended for an extremely long time. Some food products may have a shelf life of up to 25 years! Freeze-dried food may be used for:
- Emergency preparedness kits
- Backpacking and hiking
- Long term food storage
Since the water has been removed, lyophilized food is extremely lightweight. This makes it extremely useful for camping and hiking, as it is easier to carrier a greater amount of food when it is freeze-dried. The long shelf life is great for storage in an emergency kit or food pantry. The military uses freeze-dried food for MRE’s (“Meals- Ready to Eat”) and NASA uses it for expeditions to space. Water is added when it is time to eat.
Document Restoration and Preservation
Flood and fire damage are terrible tragedies that can occur to any homeowner or business. Often, the water damage from fire sprinklers causes significant damage to property, including documents. Freeze-drying is one method to rescue water-logged books, documents, or even X-rays from damaged hospital labs.
When water-logged paper is lyophilized, the water is removed from the fibers without causing any warping or fading of the document. Rare books and legal documents are usually restored in this manner, as the process causes no damage to the delicate paper.
As a hypothetical example of this use, imagine that several saturated books and wooden toys were found in a 19th century time capsule. A freeze-drier could be used to return the books to their original state, along with the wooden toys.
Preserving floral arrangements is another commercial application of lyophilization. Many brides wish to preserve their bouquets indefinitely, and will contact a freeze-drying company to perform this service. While expensive, freeze-dried flowers retain their color better and last longer than conventionally dried arrangements.
Another popular modern wedding tradition is to obtain freeze-dried rose petals to toss as the bride and groom leave the ceremony. Unlike fresh rose petals, the freeze dried version will not rot or stain. They are also completely biodegradable, which makes them an environmentally friendly option.
Freeze Drying Pets
Hunters often want to preserve a trophy of the game they catch, and freeze-drying is one of the best ways to create a lasting mount. Fishers sometimes have their catches processed and mounted. Smaller animals and fish are easier to process than larger specimens: most deer and bear are preserved according to more traditional methods (placing the tanned skin over a mount).
As an alternative to burial or cremation, some pet owners have their pets freeze-dried. This is an expensive option for preserving a pet that has passed on, but the animal’s body will be preserved for a very long time. In essence, freeze drying an animal is a form of mummification – the water molecules are removed, preventing any decay.
Freeze-drying cycle time increases with time, so lyophilizing a pet will require a significantly longer period of time than freeze drying a floral bouquet. A small cat can take as long as 3 months, and a larger dog may take six full months to freeze dry!
Pharmaceutical Freeze Drying
Pharmaceutical and Laboratory Use
Lyophilization is used to preserve many different drugs and laboratory reagents. Bacterial strains may be freeze-dried to preserve the strain for future use, or a drug may be lyophilized to improve its stability and shelf life.
Freeze drying pharmaceutical products reduces shipping weight, improves shelf life, and often removes the requirement for refrigeration. As an example, blood plasma is not particularly stable in liquid form, as clotting factors and other blood enzymes will deteriorate rapidly, despite cold storage. Lyophilized blood plasma is extremely stable, lightweight, and will remain stable for a longer period of time than liquid plasma.
While some freeze-dried vaccines and biological will still require refrigeration, the storage at 2-8°C is preferable to storage of a liquid at -20°C. The ability to lyophilize vaccines and other drugs makes shipment to developing countries possible.
Lyophilizers are not only used by large pharmaceutical companies. Many small businesses thrive by selling lyophilized flowers, food products,and performing restoration work. Freeze drying equipment varies in size and may be purchased according to the specific application in mind.
All freeze dryers have the following components:
- A vacuum chamber
- Heat-controlled shelves
- An ice condenser
- A vacuum pump
- Vacuum gauge
- Temperature gauge
Additional useful equipment includes the use of product temperature monitoring probes, and trays to place the product on while it is in the lyophilizer. Lyophilization trays must remain unbent and be made of a conductive material, such as stainless steel. The product must maintain good contact with the shelf during the lyophilization process, as the shelf temperature controls the product temperature.