Dehydration (or drying) is defined as the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation (or in the case of freeze-drying by sublimation). This definition excludes other unit operations that remove water from foods (eg, mechanical separations, membrane concentration, evaporation, and baking) as these normally remove much less water than dehydration. The main purpose of dehydration is to extend the shelf life of foods by a reduction in water activity. This inhibits microbial growth and enzyme activity, but the product temperature is usually insufficient to cause inactivation. The reduction in weight and bulk of food reduces transport and storage costs and, for some types of food, provides greater variety and convenience for the consumer. Drying causes deterioration of both the eating quality and the nutritive value of the food. The design and operation of dehydration equipment aim to minimize these changes by selection of appropriate drying conditions for individual foods. Examples of commercially important dried foods are sugar, coffee, milk, potato, flour (including bakery mixes), beans, pulses, nuts, breakfast cereals, tea, and spices.

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