If you visit the super market, you will find a wide variety of food items that carry the label "freeze dried". Vegetable soups, fruit juice concentrate, why, even cooked vegetables. You must have wondered what the word means. As the term suggests, it is the process of removing water from the foods, at very low temperatures.
Most foods contain very high percentage of water. Microorganisms thrive when there is water, spoiling the food and altering its taste. Removing water keeps food from spoiling for long periods of time. Also removing water makes the food lighter, making it easy to package and transport. Yet, removing water, must not alter the composition of the food. Its basic structure and composition of its nutrients must remain intact.
Freeze drying is one such method. The scientific principle in freeze-drying is sublimation, the conversion of a solid(ice) directly into its gaseous form(water vapour). A typical freeze-drying machine consists of three major components - a freeze-drying chamber, a freezing coil connected to a refrigerator compressor, and a vacuum pump.
First the temperatures are lowered in the freeze-drying chamber and the food item is frozen solid. This way the water content of the food is converted to solid ice. Next, the vacuum pump forces the air out of the chamber. This lowers the air pressure. Now when heat is applied to the food, under such low air pressures the ice directly gets converted to water vapour and is removed from the system and driven out by the pumps, leaving the food dry.
This retains the original composition of the food, yet preserving it for long periods. When you now want to eat the food, you add water to it to restore its original composition. Milk powder is a very familiar example of a freeze-dried food item, just find out how you use it and you know how the dried food is restored to its original shape and taste.