What is Freeze Drying?
Freeze Drying (Lyophilization) is accomplished by sublimation of ice in a vacuum. Sublimation refers to a solid (ice) changing directly to the vapor state (water vapor). The basic elements of this process have been known for nearly 100 years. However, it was not until World War II that much emphasis was placed on the refinement of the process. During this time, a lot of attention was given to the development of equipment and techniques for the purpose of supplying large quantities of dried blood plasma and penicillin to the armed forces. By the end of the war, freeze drying by a Lyophilizer had become known as a near perfect method of preservation. Today, freeze drying (Lyophilization) is used in many industries, such as, pharmaceutical manufacturers, food processors, museums, taxidermy and floral.
3 Stages of Freeze Drying
Stage 1 – Freezing
The goal of the freezing process is to turn liquid water into a solid without bursting cell walls. This is accomplished by changing cooling speeds and temperatures in such a way that ensures ice crystals form at the right size and shape to preserve biological materials without damaging them. This freezing process occurs slowly.
Stage 2 – Sublimation
During the next phase, pressure within the freeze dryer is reduced to create a vacuum. The temperature is marginally increased so the ice will begin to sublime into a gas. About 95% of the moisture removal occurs during this primary drying period.
Stage 3 – Desorption
The final phase of freeze drying involves breaking down the ionic bonds that are holding onto the last remnants of water molecules. In order to break these bonds, the temperature is raised higher than it was during the sublimation phase.