The purpose of freeze drying or lyophilization is to completely remove water from materials such as food, while leaving the basic structure and composition of the substance intact. It is an excellent method for preserving food for long periods of time.

Lyophilization is carried out using a simple principle of physics called sublimation. Sublimation is the transition of a substance from the solid to the vapour state, without first passing through an intermediate liquid phase. To extract water from foods, the process of lyophilization consists of:

1. Freezing the food so that the water in the food becomes ice;
   
   
2. Under a partial vacuum, sublimating the ice directly into water vapour;
   
   
3. Drawing off the water vapour;
   
   
4. The foods now being freeze-dried can be removed from the machine.
   
   

Lyophilization has many advantages compared to other drying and preserving techniques.

1. Lyophilization maintains food quality because the food remains below freezingpoint during the process of sublimation;
   The use of lyophilization is particularly important when processing lactic bacteria, because these products are easily affected by heat.
   
   
2. Lyophilized foods can usually be stored without refrigeration, which results in a significant reduction of storage and transportation costs.
   
   
3. Lyophilization greatly reduces weight, this makes products easier to transport. Many foods contain as much as 90% water. These foods are therefore, 10 times lighter after lyophilization.
   
   
4. Because, they are porous, most lyophilized foods can be easily rehydrated. Lyophilization does not significantly reduce volume; therefore water quickly regains its place in the molecular structure of the food.

The analysis of 100g of concentrated soup, performed by the Center for Agricultural Sciences at the Londrina state University (Brazil), at its Food Technology and Medicine department, gave the following results:

The most well known brand names in the food industry, using this process, are 'Nescafe' - coffee -, 'Tang' - soft drink - and 'Knorr' - soup -.

Currently, lyophilized meals are predominantly used by backpackers, bikers, hikers and other outdoor enthusiasts, in these instances meals are packaged in individual portions.