Explain about the freeze drying Method?

The Techniques of freeze-drying have been developed over the past half-century for the purpose of preserving certain biological materials, which are costly to produce, and which are highly unstable. Perhaps its most notable successes have been in the preservation of human plasma for transfusion purposes and in the preservation of the early samples of penicillin during the war. Today it has numerous applications in the pharmaceutical industry, but it falls far short of requirements for the food industry.

The cost of the process is also still too high to make it a practical proportion for many of the cheaper foods. In this method of removal of water the product is frozen and the temperature maintained below the triple point of the constituent aqueous solutions so that the water vapour can be sublimed from the frozen state. There is, therefore, a direct transfer from solid to vapour without the ice melting and passing through the liquid phase. The process is carried out under high vacuum to provide a high vapour diffusion potential and is accelerated by supplying heat in some convenient form, either radiant, conductive or from microwaves. It is generally considered that as a means of dehydration. It produces a dried product of the highest quality and therefore  is potentially an extremely attractive method.