Introduction

Freeze-drying is a process, in which a product is first frozen and then dried by sublimation of the ice. The total process involves four steps: freezing; sublimation of the ice, called main drying (MD); desorption of the water bound to the solid, called secondary drying (SD); and packing in containers to exclude absorption of water and/or oxygen from the atmosphere. By freeze-drying a product unstable in water is transformed into a dry, stable product. The process has to be developed to satisfy four demands on the finished product: its volume remains that of the frozen substance; the structure and the biological activity of the dried solid correspond as far as possible to those of the original substance; the dried product remains stable during storage, if possible at temperatures up to + 40°C and for up to 2 years; and with the addition of water the original product is quickly reconstituted. This article summarizes the problems and solutions to achieve these aims.

Theoretically, sublimation of ice can be done at atmospheric pressure; however, the vapour pressure of ice between — 10°C and —40°C is approximately 2.6-0.13 mbar and 1 kg of ice has a volume of approximately 470 m3 at — 10°C and 8400 m3 at — 40°C. To transport these volumes at atmospheric pressure the gas volume has to be approximately 400-8000 times larger than that of the vapour. Therefore all freeze-drying plants today are vacuum plants, in which the air is reduced to some 10% of the vapour pressure, to allow the free flow of vapour at velocities up to 100 m s-1.

The first organ tissues were freeze-dried in 1890 and in 1932 a vacuum freeze-drying plant was built, but only after 1940 did it become an industrial process with the freeze-drying of blood plasma and penicillin.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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