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PRINCIPLES OF DEHIDRATION AND SUN DRYING

PRINCIPLES OF DEHIDRATION AND SUN DRYING. The main advantages of dehydration or sun drying over other methods of preservation include :. Light weight-between 60% and more than 90% of most foods is water, and most of this is removed by dehydration.

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PRINCIPLES OF DEHIDRATION AND SUN DRYING

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  1. PRINCIPLES OF DEHIDRATION AND SUN DRYING

  2. The main advantages of dehydration or sun drying over other methods of preservation include : • Light weight-between 60% and more than 90% of most foods is water, and most of this is removed by dehydration. • Compactness-most dehydration product occupy less space than original, frozen or canned material, especially if compressed into blocks. • Ambient storage-no refrigeration is required, but there are limitations on the maximum storage temperature for adequate storage life. • convenience

  3. The main disadvantages of dehydration or sun drying, some of which can be overcome with newer techniques and pre-dehydration treatments, include : • Heat sensitivity, as all food products are heat sensitive to some degree and can develop burnt flavours under badly controlled drying conditions. • Loss of volatile flavours. • Structural changes, as the result of shrinkage during water removal.

  4. Case hardening, which reduces rates and gives poor quality product. • Browning as the result of non-enzymics reactions involving concentrated reactants, and oxidation of lipid components. • Microbial deterioration if drying rates are initially slow, or if final moisture contents are too high.

  5. The main factors which affect the rate of drying of a foodstuff are : • The physical and chemical properties of the product (shape, size, composition, moisture content). • The geometrical arrangement of the product in relation to the heat transfer surface or medium. • The physical properties of the drying environment (air temperature, humidity, velocity) • The characteristics of the drying equipment (heat transfer efficiency).

  6. aw and Microbial Stability • aw, and not water content, determines the lower limit of available water for microbial growth. • Most bacteria do not grow below aw 0,91. • Most moulds cease to grow below aw 0,70. • Fungi have been shown to grow in the range 0,70-0,75.

  7. Environmental factors affect the level of water activity required for microbial growth. • The general principle which often applies is that the less favourable the other environmental factors (nutritional adequacy, pH, oxygen pressure, temperature) the higher becomes the minimum water activity at which microorganisms can grow.

  8. Methods of drying and dehydration • Sun drying and solar drying • Methods of dehydration Some of methods of dehydration suitable for solid foods are : • cabinet or tray drying • continuous belt drying • bin drying • fluidized bed drying • freeze drying

  9. Methods of dehydration suitable for liquid, pulpy or pureed foods includes : • drum drying, • spray drying, • foam mat drying, • tray drying, • freeze drying.

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