February 2004
Take an organic substance. Now, remove the water (or other solvent). Remove enough to preserve the substance and in such a way that the cell structures and volatile components are undamaged. Sound tricky? This process has become commonplace in the production of pharmaceuticals, biological products, food products and flavorings. The process is commonly known as freeze drying. The benefits of freeze drying are longer shelf life, lower storage costs, less weight, and, in the case of foods, fresh taste.
Here?s how it works. When a substance is freeze dried, the water in it is separated out as ice crystals. Using a pump, a vacuum is applied, producing a controlled level of heat in the environment. The ice crystals evaporate off the substance without any melting. The water vapor is immediately drawn off to a colder condensing surface, usually at minus 50 degrees Celsius or colder, where it is collected as ice and not drawn into the vacuum pump. The key to keeping biological and cell structure components intact is to only remove the substance from the process after it has reached the same vapor pressure as that of the collector.
Sublimation time can vary due to vapor pressure differentials between the substance being frozen, the collector, and the eutectic temperature of the freezing substance. For example: milk dries in approximately 10 hours; influenza vaccine dries in 24 hours; human tissue dries in 48 hours.
Freeze drying systems have been developed to accommodate large and small volumes.
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