What is Freeze Dried ?
Freeze drying technology means the dehydration of foods with a freezing technique that changes water into ice crystals, and then lowers the air pressure in order to make the ice crystals vaporize at low temperature (i.e. 0 degree Celsius or lower) since the ice crystals can vaporize at air pressure of 4.7 mmHg or lower.
Freeze Drying Processes
The initial processes for freeze drying foods are similar to other drying methods that include the preparation of raw materials (e.g. washing, peeling and slicing fruits). The three following processes are explained below.
1. Freezing process – the temperate of foods are reduced and then below the freezing point in order to cause the water in the foods to form ice crystals. The freezing rate should be high in order to form small-sized ice crystals. There are various popular freezing techniques such as air blast freezing, cryogenic freezing and immersion freezing techniques.
2. Primary drying process – the foods are dehydrated by lowering the air pressure in order to cause the ice crystals to vaporize from the surface of the foods. The vacuum pressure should be lowered than 132 Pa and 132 mPa, respectively, in order to complete the vaporization process. Initially, the ice layer on the surface of the foods will vaporize, and then become the dry layer. Next, the ice crystals inside the foods will vaporize through the dry layer. The vaporization period is dependent on the size, shape and structure of the foods.
3. Secondary drying process – after the primary drying process is completed. Although the ice crystals are melted, the foods still contain moisture. Then, the foods must be dried by raising the temperature in order to achieve the appropriate moisture content.
The original properties of the freeze dried vegetables, fruits and foods can be preserved. The National Aeronautics and Space Administration (NASA) has utilized this “freeze drying technology” for preserving foods for the astronauts who stationed at space stations.