What Is Freeze-Drying?

• Freeze-drying is when a material is frozen and drained of its water. This preserves it for a long time and makes it lighter to transport. Some materials, such as fruit, contain 90 percent water, so the weight is substantially decreased. Later, water is added to rehydrate it almost back to its original condition.
  
  

Types of Freeze-Dry Machines

• Commercial freeze dryers comprise several shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum. Rotary freeze dryers are used with liquids, such as vaccines. Manifold freeze dryers are used for small containers and remove about 95 percent of the water. Tray freeze dryers are used for a variety of materials and create the driest product for long-term storage; they do freezing and primary and secondary drying.
  
  

Testing and Preparation

• The material is tested for contamination and spoilage. Foods such as meat must be cooked beforehand. Fruits and vegetables generally require just washing, but some, such as peas and corn, are boiled before drying. Coffee bean oil may be added to coffee to maintain its aroma.
  
  

Freezing

• The material is spread out on cold, metal trays that are stacked onto wheeled carts, which are moved into a room to be frozen to temperatures as low as -58°F to -112°F, which is below the triple point (the lowest temperature where the solid, liquid and gas phases of water can coexist). The frozen material is stored until the drying process begins.
  
  

Primary Drying

• The frozen material is moved into a vacuum drying chamber. Liquids may be ground into smaller pieces. The chamber is sealed, and a vacuum pump reduces the pressure to the triple point. Heat is added through conduction, radiation or microwaves to a temperature still below zero. Heat causes sublimation, whereby the ice inside the material is changed from solid to gas for removal. It can take up to a few days to remove about 95 percent of the water.
  
  

Secondary Drying

• Next residual water molecules need to be removed. The temperature is increased slightly higher than the primary drying but still below zero or slightly above zero. This breaks the bond between water molecules and the frozen material, so that the rest of the water can be removed. The pressure is lowered even more, resulting in the removal of about 96 percent to 99 percent of the water. The vacuum pressure is cut off with a gas, such as nitrogen, to keep the pressure stable until the material is sealed.
  
  

Packaging

• The material is removed from the drying chamber and quickly tested for moisture content and purity. It may be ground into smaller pieces or blended together with other products. Almost as soon as the material comes out of the drying chamber, it is packaged in airtight containers to prevent absorption of water in the air. Various kinds of packaging are used, including pouches, envelopes, bags and jars.