Progress in food dehydration Joseph S. Cohen and Tom C.S. Yang - Trends Food Sci. Technology.6, 20-2

1. Progress in food dehydrationJoseph S. Cohen and Tom C.S. Yang -Trends Food Sci. Technology.6, 20-25 Lab. of Food & Biomaterial Chemistry Lee Si Yeon

2. 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.

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

4. 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.

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

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

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

12. 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.

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

14. 1. Microwave drying and dielectric drying • Use the electromagnetic wavelength spectrum. • Frequency -Microwave drying:300-300000MHz. -Dielectric drying:1-100MHz. • Usage:high value-added products. -Microwave drying (dry paste products) -Dielectric drying (cereal products) • Advantages -low temperature, batch or continuous operation, good quality. • Disadvantage-slow, expensive.

15. 2. Microwave-augmented freeze drying. • Using microwave energy →Freeze drying rate increase. • Usage:high value-added products. Fig. 1 Schematic diagram of a microwave-augmented freeze dryer. Conventional freeze drying + Using microwaves

16. 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 : 1/3~1/2 time required. ◀ Low temperature. Microwave-augmented freeze drying

17. 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

18. 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

19. 5. Ultrasonic drying of liquids. • Usage :Liquids. • Advantage :Rapid. • Disadvantage :Require low-fat solution. Fig. 4 Schematic diagram of an experimental ultrasonic dryer.

20. 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.