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7. freeze-drying is not a new process

Harris (19) notes that sublimation of ice has been known as early as the middle of the eighteenth century, but the fires attempt to freeze-dry tissue was made by Richard Altman in Leipzig in 1890. His technique was to use a form of dehydration in which the tissue, previously frozen at -15 C (5 F), was placed in a dessicator, then evacuated with a vacuum pump.

During the early decades of the 1900s sublimation was little more than a series of laboratory experiments. However, the potential for industrial application of the results obtained by drying from the frozen state were apparent: the temperatures used in sublimation were low enough so that many unstable substances like those found in blood, viruses, microorganisms, other biologicals and pharmaceuticals would not undergo changes; with low temperatures there was a minimum loss of volatile substances in some foods; there was no bubbling or foaming to cause surface changes in drying; in most cases the dissolved substances remained evenly dispersed and distributed, the remaining residue highly porous with a solid framework, the solubility rapid; bacteriological growth and enzymatic changes could not take place under frozen conditions.

During the 1930s scientists and engineers saw that by establishing proper conditions of vacuum for removal of water vapor and by rapid application to the frozen product, the process could be made industrially workable (20). However, it was not until shortly before World War II that the laboratory models were made commercially available. The requirement for a vast supply of blood plasma was instrumental in the development of freeze-dry equipment and techniques. And after the war there was a virtual explosion in the use of vacuum freeze-drying for biologicals, pharmaceuticals and food. The use of this process has spread to include not only the salvage of water-damaged or wetted archival and library materials, but also of wetted textiles, saturated archaeological artefacts such as leather, cloth, wood, wrecks of old vessels sunk at sea, taxidermy, natural history exhibits for museums.

7.1 Examples of its use

7.1.1 Pharmaceuticals

The first time freeze-drying was used on a large scale industrial basis was during World War II for the preservation of blood plasma, the fluid that surrounds the red blood cells. The dehydrated plasma was sealed in packets and when needed was rehydrated with sterilewhater. Since that time, the list has grown substantially to include medicinals that cannot be stored in liquid or frozen state: vaccines against rabies, measles, mumps, rubella; penicillin and other antibiotics; antivenin to treat snakebite; human skin cells to spread over burns; many cancer drugs such as cisplatin and doxorubin (21).

7.1.2 Foodstuffs

Probably the product most suitable for mass production by freeze-drying is also the most familiar: instant coffee. In freeze-drying the coffee extract is frozen then introduced in the vacuum chamber where the moisture is sublimed; a solid mass is left which is then reduced to granules, These are rehydrated with hot water to make a cup of coffee. Other products include meats, vegetables, and fruits. Some fruits like strawberries lend themselves superbly to freeze-drying: one kilogram of freeze-dried pieces gives ten kilograms of strawberries after rehydrations (22). Other products that lend themselves to the process include spices, dairy products, and food packs.

7.1.3 Biological specimens

Examples of biological specimens which are freeze-dried in laboratories on a routine vases are bacterial or virus suspensions, enzymes, fungi, protozoa, tissue, bone and skin for surgical grafting (12). In the microbiological area museums and taxidermists freeze-dry exhibit specimens such as rodents, reptiles, birds, crustaceans, insects and fishes. In freeze-drying of these specimens there is faithful retention of appearance and tissue structure - prevention of shrinkage which is vitally important. Animals as large as a six foot alligator have been successfully freeze-dried; apparently the only limitation to the size of the animals is the capacity of the vacuum chamber in which they are dehydrated (23).

7.1.4 Archaeological artifacts

When archaeologists find ancient artifacts of wood, cloth or leather which have been in wet or damp sites, they face the immediate problem of drying the object without irreversible shrinkage, surface checking or other deformations. Freeze-drying is one of the methods employed successfully in drying pre-treated (polyethylene glucol impregnation) wood in order to prevent the normal movement of water which would cause the collapse of damaged cells (24). Leather items have bees successfully preserved by using a similar technique (25). And Neolithic textiles along with braided work have been salvaged by freeze-drying after thorough cleaning and desalinization


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