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The Role of Vacuum Technology in Freeze Drying

While the best-known freeze-dried product is instant coffee, fruit, vegetables, and even cooked meats can reap the benefits of extended shelf life through this technology.

Fig. 1: Schematic representation of a freeze dryer: 1. Drying chamber, 2. Condenser (cold trap), 3. Vacuum system, 4. Opening gate, 5. Heatable intermediate plates, 6. Cooling coil. Image courtesy of Busch Vacuum Solutions.
Fig. 1: Schematic representation of a freeze dryer: 1. Drying chamber, 2. Condenser (cold trap), 3. Vacuum system, 4. Opening gate, 5. Heatable intermediate plates, 6. Cooling coil. Image courtesy of Busch Vacuum Solutions.

According to Busch Vacuum Solutions’ Uli Merkle of global marketing department, “Freeze drying is a modern extension of traditional drying methods to protect food from spoilage for longer periods of time. This process is only made possible by modern vacuum technology because freeze drying food takes place under vacuum.”  

While soluble (instant) coffee is probably the most well-known freeze-dried product, freeze drying is also suitable for fruit and berries as well as for cooked meat or vegetables. Herbs and spices are also preserved through freeze drying by removing all but a few percent of the water from these products.  Says Merkle, “During freeze drying, the structure of the food remains unchanged and the dried cells can easily absorb water again during preparation. By drying under vacuum, the aromas are largely preserved.”

The process

Freeze drying (Fig. 1) is a physical process that uses the principle of sublimation. Due to the vacuum in the drying chamber, the frozen water sublimates into water vapor. The ice skips the "liquid" state and becomes directly "gaseous" from "solid."

Fig. 1: Schematic representation of a freeze dryer: 1. Drying chamber, 2. Condenser (cold trap), 3. Vacuum system, 4. Opening gate, 5. Heatable intermediate plates, 6. Cooling coil. Image courtesy of Busch Vacuum Solutions.Fig. 1: Schematic representation of a freeze dryer: 1. Drying chamber, 2. Condenser (cold trap), 3. Vacuum system, 4. Opening gate, 5. Heatable intermediate plates, 6. Cooling coil. Image courtesy of Busch Vacuum Solutions.

Before the actual drying process, the product must be spread over trays inside a cooling chamber to deep-freeze. The frozen product is then placed in the actual drying chamber (Fig. 2). Though, in some freeze dryers, cooling takes place directly in the drying chamber. Freeze dryers for continuous processes are also used in food processing.

“After cooling, the drying process takes place in the air-tight chamber by reducing the air pressure via a vacuum pump until a vacuum of 1 to 0.5 mbar is reached,” says Merkle. “The evaporation process of the frozen water then starts at -58 to -40° F. The water vapor is suctioned out of the drying chamber by the vacuum pump into a downstream condenser. In this "ice trap" cooled to at least -94° F, the water vapor sublimates and condenses as ice on the cooling coil. Most of the moisture is removed from the product in this process step, called primary drying.”

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