Alternative Freeze Drying Methods

U.S. food freeze drying patents issued between 1961 and 1976, an important period of development, have been re

Heating plate

Tray

Frozen particles

To vacuuum pump

To vacuuum pump

Melted condensate

—Heater coil

Condenser being defrosted

Active condenser

Melted condensate

—Heater coil

Heating plate

Tray

Frozen particles

To vacuuum pump

Condenser being defrosted

To vacuuum pump

Active condenser

Figure 3. A batch freeze drier in which dual, switchable condensers are used and in which trays enter on a rack of shelves supported on an overhead track. Source: Reproduced with permission of Atlas-Stord Inc.

viewed by Gutcho (1). Some of these patents describe now-standard freeze drying methods and equipment, methods for freeze drying particular products, or methods for preparing such products for drying. Others describe freeze drying methods that for one reason or another have been abandoned or are rarely or infrequently used today. Some describe infrequently used heating methods: direct-contact heating; microwave heating; infrared heating; induction heating; use of layers of expanded metal mesh or of ribbed trays to facilitate heat transfer; or the use of heat pumping to couple heating and condensation. Others describe spray freeze drying, various types of atmospheric freeze drying, circulation of low-pressure gas through the drying material, or vapor removal by adsorption instead of condensation. Still other patents describe use of moving belts, vibrating conveyers, helical conveyors or vertical stacks of scraped trays as particle conveying devices in continuous freeze driers. Food freeze drying patents and literature for the years 1977 to 1998 were reviewed in preparing this article. Aside from development of the previously described continuous freeze drier, innovation in the field has slowed down. Atmospheric freeze drying in sorbent-containingflu-idized beds and use of microwave heating in freeze drying are still of interest, and articles dealing with these methods continue to appear from time to time.

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