PRINCIPLES OF DEHIDRATION AND SUN DRYING

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PRINCIPLES OF DEHIDRATION AND SUN DRYING
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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

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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.

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• 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.

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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).

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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.

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• 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.

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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

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