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Vacuum freeze-drying, also known as lyophilization or cryodesiccation, is a drying process that extracts moisture and oxygen under a pressurized and low temperature environment and leaves evenly distributed air spaces between the product fibers. With the extraction of moisture and oxygen, which is the primary cause of food deterioration in fruits, vegetables and other freeze-dried products will retain their original cell structure, flavor, aroma and nutritional value for an extended period of 10 to 15 years. These products can be re- hydrated quickly to their original shape and density with the addition of water.
HOW FREEZE-DRY WORKS
Simply put, freeze-drying is the removal of water from a frozen product using a process called sublimation. Sublimation occurs when a frozen liquid transforms directly to a gaseous state without passing back through the liquid phase. The process of freeze-drying consists of three phases: pre-freezing, primary drying, and secondary drying.
Freeze-dry food must first be pre-frozen below its eutectic temperature, or simply put, freezing the materials (solute) that make up the food. Although a product may appear to be frozen because of all the ice that is present, in actuality it is not completely frozen until all of the solute is frozen as well.
After pre-freezing, ice must be removed from the product through sublimation. This requires careful control of two parameters; temperature and pressure. The rate of sublimation depends on the difference in vapor pressure of the product compared to the vapor pressure of the ice collector. Molecules move from the higher pressure sample to the lower pressure sample. Since vapor pressure is related to temperature, it is also necessary for the product temperature to be warmer than the ice collector temperature.
After primary drying, all ice has sublimated but some liquid is still present in the product. Continued drying is necessary to remove the remaining water. The process for removing this excess water is called isothermal desorption. The excess water is desorbed from the product by making the product temperature higher than the ambient temperature.
During the entire freeze-drying process, the exact freezing methodology and proper storage is very important.
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