Freeze Drying

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WORKING KNOWLEDGE
FREEZE-DRYING

by Herbert Aschkenasy

F

reeze-drying is possible because under the right conditions, a solid material such as ice can change directly into a gas without first passing through a liquid phase. This process, called sublimation, gradually removes all ice from food and other biological matter or even from inorganic substances such as ceramics. As a method of preserving many organic materials, freeze-drying is ideal. The freezing immobilizes the object, allowing it to retain its original shape. The absence of water discourages the growth of microorganisms and pre-

HUNDREDS OF FOODS can be freeze-dried; after the water is removed, some fruits, such as oranges, can then be ground into a powder for use in candy.

vents other chemical changes associated with spoilage. Also, because water sublimates so readily, the conditions needed to freeze-dry a food will not eliminate most other constituents, such as the acetaldehyde molecules that give citrus fruits some of their flavor. The rudiments of freeze-drying were known to the Peruvian Incas of the Andes, who stored their potatoes and other foodstuffs on the heights above Machu Picchu. There the cold tempera-

HERBERT ASCHKENASY is the president of Oregon Freeze Dry in Albany, Ore.

WATER in materials subjected to this process vaporizes onto cold condenser plates (shown at sides of chamber in bottom photograph ). The dried products are taken from the chamber and stored in containers that seal off oxygen and water.

FRUIT DUST that can be used as fillings for chocolate candies is made from freeze-dried strawberries that have been ground into a powder (right ).

184

Scientific American September 1996

Working Knowledge

Copyright 1996 Scientific American, Inc.

PHOTOGRAPHS COURTESY OF OREGON FREEZE DRY

INDUSTRIAL FREEZE-DRYING involves putting food or other materials in a cold room (top ) at temperatures as low as 50 degrees below zero Fahrenheit (about –46 Celsius). The items are then moved to vacuum chambers (middle ).

tures froze the tubers, and the water inside slowly vaporized under the low air pressure. Wide use of the process commercially only began during World War II, to preserve blood plasma needed at the front lines. Since the 1960s, it has been applied to upward of 400 foods, from meat to fruits and vegetables. A few foods, such as lettuce and watermelon, are not good candidates for freeze-drying; consisting almost entirely of water, they disintegrate when frozen and dried. The process does preserve desirable microorganisms such as cheese cultures. It can even be used as a form of taxidermy and for the preservation of flowers. Freeze-drying is more costly than simply chilling food to preserve it. But freeze-dried food in an airtight container may last for decades without spoilage; it only needs to be exposed to water to reconstitute it. We once rehydrated a 23-year-old beef stew military ration for a group of military officers, all of whom found the meat to be palatable.

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