The origins of freeze drying can be traced back to the 15th century, where a rudimentary form of freeze drying was used by the Incas. Their crops were stored on the mountain heights above Machu Picchu where the cold mountain temperatures froze their food stores, and the water inside slowly vaporized under the low air pressure of the high mountain altitudes.
Modern freeze-drying techniques were initially developed during World War 2. Blood being sent to Europe from the US for medical treatment of the wounded required refrigeration, but due to the lack of simultaneous refrigeration and transport, many blood supplies were spoiling before reaching their destination. The freeze-drying process was developed as a commercial technique, enabling blood to be rendered chemically stable and viable without requiring refrigeration. Shortly thereafter, the freeze-dry process was applied to penicillin and bone, and freeze-drying became recognized as an important technique for the preservation of numerous biologicals.
Since that time, freeze-drying has been used as a preservation or processing technique for a wide variety of applications to include food, pharmaceuticals, restoration of water damaged materials, ceramics, and many others. Freeze-dried foods are closest to their natural composition with respect to structure and chemistry, ensuring essential vitamins and nutrients are maintained. The freeze-drying process became widely popular when it was used by NASA in the creation of freeze-dried ice cream, a popular astronaut food. It is also widely used to produce essences or flavorings to add to food.
How freeze-drying 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: prefreezing, primary drying, and secondary drying.
Prefreezing:
Food items that are freeze-dried must first be prefrozen 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.
Primary Drying:
After prefreezing, 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.
Secondary Drying:
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.