Fst 151 food freezing food science and technology 151 Special topics: Freeze Drying



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Freeze drying

Freeze drying equipment

  • Freeze dryer consists of:
    • Vacuum chamber (with trays to hold the food during drying)
    • Heater to supply latent heat of sublimation.
    • Refrigerator coil for condense the vapors directly to ice
    • Vacuum pump removes non-condensable vapors.

Types of freeze dryers

  • Conduction type
  • Irradiation type
    • (Convection heating is not important in the partial vacuum of the freeze drier cabinet.)
    • (The surface temperature of the food does not exceed 60ºC.)

Contact ( or conduction) freeze dryers:

  • Contact ( or conduction) freeze dryers:
      • Foods is placed onto trays which rest on heater plates.
      • Slow drying process ( heat transferred by conduction to only one side of the food).
  • Accelerated freeze dryers:
      • Food is held between two layers of expanded metal mesh and subjected to a slight pressure on both sides.
      • Heating is by conduction, but heat is transferred more rapidly into food by the mesh than by solid plates, and vapor escapes more easily from the surface of the food.
  • Radiation freeze dryers:
      • Infrared radiation from radiant heater is used to heat shallow layer of food on flat trays.
      • Heating is more uniform than in conduction types.
  • Microwave and dielectric freeze dryers:
      • Radio frequency heater have potential use in freeze drying but not widely used (Difficult to control because water heated more than ice).

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