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What is freeze-drying?

Freeze-drying is a method of preserving food. During this process, water is removed from the food while it is still frozen, by a process known as sublimation. The frozen food is cooled to about -30° C. Then it is placed on trays in a refrigerated vacuum chamber, and heat is carefully applied. As a result, any water in the food is changed directly from ice to water vapour without first changing into water. Freeze-dried products include: soups, tea, and instant coffee.