Dryers advantaged and disadvantages for foods

Drying of foods

1. DRYING of Foods

2. What is drying? Drying is one of the most preferred methods of food preservation that use the application of heat to remove the majority of water present in food through evaporation.

3. Does drying and dehydration imply same meaning ? DRYING Drying means get rid of the free water in a given compound It requires simple method of moisture removal like sun drying DEHYDRATION Dehydration means the removal of all water from the given compound It is a controlled method of moisture removal commonly using sophisticated equipment's

4. Factors affecting drying rates - Type, variety, & size of the materials : The moisture content , internal composition & the size of the cut product affects the rate of drying. Like the more moisture content , the more slow the rate of drying. The larger the size the more the drying time.

5. 2. Air temperature : The higher the air temperature the faster the drying rate as an increased air temperature is capable of carrying more moisture diffused from the product .

6. 3. Air velocity If the air moves fast through the surface of the product, the highly moisture air flows away while low humid air fills that place & carry the diffused moisture from the surface.

7. 4. Air humidity Humidity is the measure of saturation of the air. In simple term the amount of moisture in air. More humidity means more moisture. So an increased humidity slower the rate of drying.

8. 5. Initial moisture content The more the moisture content in the product, the more the drying time.

9. 6. Depth of drying layer The moisture moves from the interstitial space of the product through diffusion. So the larger the surface area and the thinner the layer , the faster the drying rate

10. General guidelines for drying of food SPEED Foods should be dried rapidly, but not so fast that the outside gets hard before the moisture inside gets the chance to evaporate.

11. Temperature Each fruits and vegetables has a critical temperature above which a scorched taste develops. The temperature should be high enough to evaporate moisture from food but not that high to cook the food.

12. Humidity & ventilation The higher the temperature and the lower the humidity, the more rapid the rate of dehydration will be. Too fast drying cause “ case hardening”.

13. Uniform drying Spreading thin layers of uniformly sized pieces of foods will help in uniform drying

14. Nutritive value Dried fruits are good source of concentrated sugar, vitamins and minerals.

15. Importance of drying Increase the shelf life of food Inhibits microbial growth & enzyme activity Reduced weight and volume reduces transportation and storage cost Lowers processing cost

16. Food driers These devices use a heat source and air flow to reduce the water of foods. The commonly used driers are- 1. Drum drier 2. Spray drier 3. Freeze drier 4. Vacuum drier 5. Tunnel drier 6. Fluidized bed drier 7. Tray drier

17. Drum dryer Type of drier used to dry high viscous liquid foods, purees, pastes etc . Fig : Single drum dryer Fig : Double drum dryer

18. Advantages & Disadvantages Relatively low cost capital & operation cost Plant is moveable & requires low floor space Easy to handle Easier access for maintenance Less risk of damage caused by metal objects. Produce cooked flavor & brown colored product Providing zoned temperature control needed to vary drying temperature profile Some products like dried fruits and juice tends to be sticky and semi-molten Some products tend to crimp, roll up & stick to the doctor blade.

19. Spray dryer In a spray dryer, liquid or solid material in slurry is sprayed in the form of fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counter flow. There are two modes of contact- 1. Co-current 2. Counter current

20. Mode of action ( co-current & counter current)

21. Application Limited to foods that can be atomized Foam spray drying : One type of spray drying foams liquid food, such as milk or coffee before spraying it into the dryer. The result is a faster drying rate from the expanded foamed droplet of higher surface area and lighter density dried product. BIRS spray dryer : This has been developed to handle thermoplastic material and highly heat sensitive products.

22. Advantages and Disadvantages Very short drying time A large proportion of the flavor, color and nutritive value is retained High capital cost Requirement for a relatively high feed moisture content to ensure that the food can be pumped to the atomizer

23. Freeze drying Freeze drying is a process where food is first converted into ice, where is then dried by sublimation of water from food into vapor. Frozen water will sublime if the temperature is 0 degree C or below and the frozen water is placed in a vaccum chamber at a pressure of 4-7mm or less.

24. Application 1. Used to dehydrate sensitive high value liquid foods like coffee and juice 2. Especially suited to dry solid foods of high value such as straw berries, whole shrimps, mushroom slices etc.

25. Tunnel dryer Consists of tunnel where the trays of food are placed Hot air is blown across the trays A truck of finished products is removed from the end of the tunnel Air movement can be parallel flow or counter current flow

26. Tunnel drier

27. Advantages & Disadvantages Most common in drying fruits and vegetables Reduces labor cost More uniform drying condition Relatively large quantity of foods in short time Large installation is required Investment is required High operating cost