Freezing and freeze-drying

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Abstract
Most biological substances are unstable during storage owing to their high water content. This is why numerous attempts have been made over the last 100 years to prevent, by low temperature freezing, metabolic and biochemical degradations. The transformation of water into ice brings to an end all chemical reactions; however, it might also induce deleterious alterations into the delicate structure of many products. A brief review of the basic phenomena involved in the freezing process of biological substances helps to understand how the cooling cycles, storage temperature and subsequent rewarming have to be monitored in each individual case. Despite the fact that preservation in the frozen state offers wide possibilities, it is still difficult to apply in some instances, since it implies a continuous low temperature storage and transportation. This is why freeze-drying, or lyophilization, has been introduced. It is a two-step process in which the products to be preserved are deep frozen first, then dried by direct sublimation of the ice under reduced pressure. When completely dehydrated the substances can be stored almost indefinitely if kept in a dry, neutral atmosphere and in the dark. Fundamental aspects of the freeze-drying process will be discussed. Recent research work has shown that drying by sublimation from the frozen state can be performed with other systems than aqueous media. Solutions of lipophilic compounds, fats, phospholipids, steroids, in organic solvents can be ‘freeze-dried’ at low temperatures and high velocities. This particular technology opens new fields for the preservation of delicate materials of living origins.

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