Dehydration and it’s equipments basic details

1. DEHYDRATION AND ITS EQUIPMENTS
By Yadnesh Chaudhari and Bhupesh Borse

2. INTRODUCTION
➤ Most fruits and vegetables contain
enough moisture to permit the activity of
enzymes and micro-organisms for
spoilage and drying is necessary to reduce
the water activity.
➤ Dehydration means the process of
removal of moisture by application of
arti
fi
cial heat under controlled conditions
of temperature, humidity and air
fl
ow.
➤ Dehydration removes biologically active
water thus stopping the growth of
microorganisms.

3. ADVANTAGES OF DEHYDRATION
➤ Helps in food preservation by reduction of water activity
➤ Reduction in weight and volume of the food by 4 to 10 times from its initial fresh
weight volume.
➤ Reduction in space requirement due to reduction in bulk and hence involves lower
cost of packaging, storage and transport.
➤ Dried foods add variety to the diet and supply convenient ready to eat foods to the
consumers.
➤ Nutrient concentration is very high per unit weight of dried product.

4. MECHANISM OF DEHYDRATION
➤ Dehydration involves the simultaneous application of heat and removal of water from the food.
➤ When hot air is blown over a wet food, the water vapour di
ff
uses through a boundary
fi
lm of
air surrounding the food and is carried away by the moving air.
➤ A water vapour pressure gradient is established from the most interior of the food to the dry
air provides the driving force for water removal from the food.
➤ The boundary
fi
lm acts as a barrier to both heat transfer and water vapour removal during
drying.
➤ Water vapour leaves the surface of the food and increases the humidity of the surrounding air,
to cause reduction in the water vapour pressure gradient which reduces the rate of drying.
➤ Therefore, the mopping air should be faster to reduce the thickness of boundary
fi
lm and
hence to achieve faster rate of drying.

5. This water vapour passes through the interstitial spaces between the cell walls of food cells during
dehydration. Therefore velocity of the air flow must be higher and low mositure to create this pressure.

6. CASE HARDENING
➤ During drying process the control of air
temperature and its circulation in the
system is important.
➤ If the temperature is too high in the
beginning, a hard shell will develop on
the surface of the food.
➤ The shell is impermeable and prevents
the removal of mositure from the interior
portion of the food and moisture will trap
inside the food material.
➤ This known as case hardening.

7. FACTORS AFFECTING DEHYDRATION
➤ Initial moisture content of the material
➤ Composition of raw material
➤ Initial load of the food kept in drier.
➤ Size, shape and arrangement of stacking of the raw material
➤ Temperature, relative humidity and velocity of air used for drying.
➤ Rate of heat transfer on the surface of the food
➤ Pre-treatment of raw material prior to drying (peeling, blanching,
sulphuring,etc.)

8. DEHYDRATION EQUIPMENT
➤ Hot air driers
➤ Heated surface driers
➤ Freeze driers
➤ Osmotic dehydrators

9. HOT AIR DRYER
➤ In hot air dryers the food is in contact
with a moving stream of hot and dry air.
➤ Heat is supplied to the product mainly by
convection.
➤ Hot air dryers operate within an open
cycle.
➤ These do not use any kind of desiccant to
reduce the moisture.
➤ Types:- kiln, cabinet tray, tunnel,
conveyor, bin,
fl
uidised bed, pneumatic,
rotary, spray, foam mat and trough dryer

10. Kiln Dryer

11. Cabinet tray dryer

12. Tunnel dryer

13. Conveyor dryer

14. Bin Dryer

15. Fluidised bed dryer

16. Rotary dryer

17. Spray dryer

18. Belt trough dryer

19. HEATED SURFACE DRYER
➤ Unlike hot air dryers, the heat in heat
surface dryers is supplied to the food by
conduction, thus resulting in higher
thermal e
ffi
ciency.
➤ Drying can be carried out in absence of
oxygen to protect foods that are sensitive
to oxidation.
➤ It is not necessary to heat large volumes of
air before drying commences.
➤ Heated surface dryers require far less usage
of energy in comparison to hot air dryers.
➤ Types:- vacuum drum, roller, vacuum shelf
and explosion pu
ff
dryer

20. Vacuum drum dryer

21. Roller drum dryer

22. Vacuum shelf dryer

23. Vacuum shelf dryer

24. FREEZE DRYER
➤ It is the sublimation or removal of water content
from frozen food
➤ The dehydration occurs under a vacuum, with the
product solidly frozen during the process.
➤ In the process, food is frozen to -18 degree Celsius
in lower chamber and then the food is dried at 30
degree Celsius for 24 hours, then at 20 degree
Celsius under high vacuum in the upper chamber.
➤ Final product is highly hygroscopic.
➤ Usually, fruit pulps are freeze dried.
➤ Because the food is frozen, heat damage does not
occur.
➤ The material is dried by sublimation of ice without
passing through liquid phase.

25. Freeze dryer

26. OSMOTIC DEHYDRATION
➤ Highly acidic fruits like grapes and apples
that have sensitive aroma can be dried by
using osmotic dehydration
➤ Fruits here are kept in a hypertonic
solution like sugar syrup and kept for 4
hours to overnight.
➤ In the process, the water oozes out in
syrup due to osmosis.
➤ The fruits are then drained out of syrup,
rinsed and further dried in hot air drier to
desired moisture content.