Progress in food dehydration Joseph S. Cohen and Tom C.S. Yang -Trends Food Sci. Technology.6, 20-25

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Progress in food dehydration Joseph S.
Cohen and Tom C.S. Yang
-Trends Food Sci. Technology.6, 20-25

• Lab. of Food Biomaterial
  Chemistry
• 
  Lee Si Yeon

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Introduction

• Dehydration is the oldest method of food
  preservation.
• Advantage
• decrease weight.
• Disadvantage
• decrease the quality.
• Two stage.
• 1.Constant rate stage.
• 2.Falling rate stage.

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Current dehydration techniques

• Solar (open-air) drying.
• Smoking.
• Convection drying.
• Drum drying.
• Spray drying.
• Fluidized-bed drying.
• Freeze drying.
• Other drying methods.

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1. Solar (open-air) drying

• Oldest drying process.
• Usage
• fruit, meat, fish, plants.
• Advantages
• simple, low cost
• Disadvantages
• large space required, high labor inputs,
  slow,
• difficult to control, insect and microbial.

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2. Smoking

• Almost as old as open-air drying.
• Usage
• meat, fish.
• Advantages
• added flavor, formed bactericidal properties.
  
• Disadvantages
• difficult to control, slow.

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3. Convection drying

• Hot air is to pass over the product, which has
  been placed in open trays.
• Usage
• low-value products.
• Advantages
• continuous process.
• Disadvantages
• difficult to control.

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4. Drum drying

• Products have been placed on the surface of a
  heated drum.
• Usage
• slurry, paste, fluid, gelatin.
• Advantage
• continuous.
• Disadvantages
• may require modification of liquid.

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5. Spray drying

• Method
• atomized (form small droplet)? droplets
  contact with hot gas?liquid is rapidly evaporated
  ?final product.
• Usage
• liquids, instant tea, coffee.
• Advantage
• spherical product.
• Disadvantages
• quality loss,
• not all materials can be dried in this way

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6. Fluidized-bed drying

• Usage
• small uniform particle, small vegetables.
• Advantages
• usually batch operation, uniform drying,
• maintain constant temperature, rapid.
• Disadvantages
• restriction on particle size.

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7. Freeze drying

• Principle.
• under high vacuum, frozen water removed from
  
• a food.
• Four rate-limiting step.
• 1. the external transfer of heat to the outer
  surface of the material from the heat source.
• 2. the internal transfer of heat within from the
  heat source.
• 3. the external mass transfer of water vapor from
  the surface.
• 4. internal mass transfer within the material.
• ?During the drying cycle, the thickness of dried
• layer increased, sublimation rate
  decrease.

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• Usage
• value-added products, fruit pieces, instant
  coffee.
• Advantages
• continuous operation, no restriction on
  particle size, no heat demage.
• Disadvantages
• slow, expensive.
• 
  
  
• 
  
  Freeze
  Drying

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8. Other drying methods.

• 1. Explosive puffing
• -Combination of high temperature and high
  pressure.
• -Usage small particle, honeycombo structure.
• -Advantage rapid.
• -Disadvantage High heat.
• 2. Osmotic drying
• -For sugar infused products.

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Novel dehydration techniques

• Microwave drying and dielectric drying.
• Microwave-augmented freeze drying.
• Centrifugal fluidized-bed drying.
• Ball drying.
• Ultrasonic drying of liquids.

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1. Microwave drying and dielectric drying

• Use the electromagnetic wavelength spectrum.
• Frequency -Microwave drying300-300000MHz.
• -Dielectric
  drying1-100MHz.
• Usagehigh value-added products.
• -Microwave drying (dry
  paste products)
• -Dielectric drying (cereal
  products)
• Advantages
• -low temperature, batch or continuous
  operation,
• good quality.
• Disadvantage-slow, expensive.

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2. Microwave-augmented freeze drying.

• Using microwave energy
• ?Freeze drying rate increase.
• Usagehigh value-added products.
• Fig. 1 Schematic diagram of a
  microwave-augmented freeze dryer.

Conventional freeze drying Using microwaves
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• Advantage Disadvantage
• ? Costs of the this equipment are expensive.
• ?But more efficient.
• ? Non-uniformity of energy within the chamber.
• ?Use of waveguides and a rotating tray.
• ? Variety of food products can be dries using
  this
• method.
• ? Rapid 1/31/2 time required.
• ? Low temperature.
• 
  
• 
  Microwave-augmented
  freeze drying

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3. Centrifugal fluidized-bed drying.

• Same principle as the conventional fluidized-bed
  dryer except that a rotating chamber is used.
• Usage
• Small particle, vegetable piece, powders.
• Advantages
• rapid, easy to control.
• Disadvantages
• loss of product integrity, noisy.
• Fig. 2 Schematic diagram of a
  Centrifugal fluidized-bed dryer

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4. Ball drying

• Usage
• Small particle, vegetable piece.
• Advantage
• Relatively low temperature (70?),
  continuous,
• rapid.
• Disadvantage
• Loss of product integrity, difficult to
  control.
• Fig. 3 Schematic diagram of
  a ball drying

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5. Ultrasonic drying of liquids.

• Usage
• Liquids.
• Advantage
• Rapid.
• Disadvantage
• Require low-fat solution.
• Fig. 4 Schematic diagram of an experimental
  ultrasonic dryer.

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Conclusion

• In the future, it is probable that other novel
  drying techniques will be developed and become
  available for specialized purpose.
• Furthermore, current techniques will also
  probably be further refined to make them more
  economical and will also be explored for use with
  other food products.