Introduction to Drying

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Introduction Drying is the removal of relatively small amounts of water or other liquid from a solid material to reduce the content of residual liquid to an acceptably low value. Usually, it is the final step in a series of operations before the final packaging. Water or other liquids may be removed from solids mechanically by presses or centrifuges or thermally by vaporization. In drying, the liquid is removed from solid by thermal vaporization. However, it is cheaper to remove liquid mechanically than thermally, and thus, it is advisable to reduce the liquid content as much as practicable before feeding the material to a dryer (McCabe et al., 2001). Dried foods can be stored for extended periods of time. Also, when materials are dried, they can be handled easier. In biological materials, especially food, drying is used as a preservation technique. Microorganisms that cause food spoilage and decay cannot grow and multiply in the absence of water. Also, many enzymes that cause chemical changes in fpood and other biological materials cannot function without water. When water is reduced below about 10%, the microorganisms are not active. There are some biological materials that cannot be heated by ordinary drying, but they can be freeze-dried (Geankoplis, 1995). Drying involves two processes occurring simultaneously. First, heat is transferred to evaporate the liquid. Second, mass is transferred as liquid or vapor from within solid to surface and as vapor from exposed surface (Bautista, 2012). Drying and evaporation are two different processes. In general, drying means removal of small amounts of water from the material. Evaporation, on the other hand, refers to the removal of large amounts of water from the material. In drying, the water is usually removed as a vapor by air. In evaporation, water is removed as vapor at its boiling point (Geankoplis, 1995).

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