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. The main purpose of dehydration is to extend the shelf life of foods by a reduction in water activity. This is inhibits microbial growth and enzyme activity, but; • The processing temperature is usually insufficient to cause their inactivation. • Therefore any increase in moisture content during storage, result in rapid spoilage. Drying causes deterioration of both the eating quality and the nutritional value of the food. • The design and operation of dehydration equipment aim to minimize these changes.
Preservation effect • Dehyrated foods have lower water activity (aw=0.2-0.6) • This aw not enough (free water) for : • Microorganism Growth (needs aw > 0.93, Specially bacteria) • Staphylococcus aureus (aw > 0.85) • Mold ( need aw >0.6) • Enzymatic reactions • Chemical reactions (eg) Mailllard browning) (need aw>0.3) • Microorganisms are not killed, • Keep Mos in-active stage. • MO will resume growth after food is rehydrated (suitable environment)
Movement by capillary action occur during early stages of drying. • Second, the surface water must be evaporated into air. • Evaporation rate depend on; • condition of drying air, and • concentration of water at the surface.vcapillary action or diffusion. v First, moisture must move from the interior to surface of the material. This is occur two ways: §Controlling Factors for Dehydration • Two separate phenomena are involved in dehydration.
Dehydration of fruit and vegetables is one of the oldest forms of food preservation techniques known to man and consists primarily of establishments engaged in sun drying or artificially dehydrating fruits and vegetables. Although food preservation is the primary reason for dehydration, dehydration of fruits and vegetables also lowers the cost of packaging, storing, and transportation by reducing both the weight and volume of the final product. Given the improvement in the quality of dehydrated foods, along with the increased focus on instant and convenience foods, the potential of dehydrated fruits and vegetables is greater than ever.
Dried or dehydrated fruits and vegetables can be produced by a variety of processes. These processes differ primarily by the type of drying method used, which depends on the type of food and the type of characteristics of the final product. In general, dried or dehydrated fruits and vegetables undergo the following process steps: predrying treatments, such as size selection, peeling, and color preservation; drying or dehydration, using natural or artificial methods; and postdehydration treatments, such as sweating, inspection, and packaging.