Fst 151 food freezing food science and technology 151 Special topics: 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).