Dehydration is a method of food preservation that involves removing water from foods through the application of heat. This reduction in water content inhibits microbial growth and enzyme activity, extending the shelf life of foods. However, dehydration also causes deterioration in food quality attributes like texture, flavor, and nutrition. Various factors influence the dehydration process, and different equipment like cabinet dryers, tunnel dryers, and spray dryers are used depending on the type of food being dried.

1. 1
Y.Bavaneethan.
Lecturer
Department of Food Technology
SLGTI. SriLanka.
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2. Dehydration
Dehydration (or drying) is defined as ‘the application of
heat under controlled conditions to remove the majority of
the water normally present in a food by evaporation.
The main purpose of dehydration is to extend the shelf life
of foods by a reduction in water activity.
This is inhibits microbial growth and enzyme activity, but;
• The processing temperature is usually insufficient to cause
their inactivation.
• Therefore any increase in moisture content during storage,
result in rapid spoilage.
Drying causes deterioration of both the eating quality and
the nutritional value of the food.
• The design and operation of dehydration equipment aim to
minimize these changes.
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3. Preservation effect
• Dehyrated foods have lower water activity (aw=0.2-0.6)
• This aw not enough (free water) for :
• Microorganism Growth (needs aw > 0.93, Specially
bacteria)
• Staphylococcus aureus (aw > 0.85)
• Mold ( need aw >0.6)
• Enzymatic reactions
• Chemical reactions (eg) Mailllard browning) (need
aw>0.3)
• Microorganisms are not killed,
• Keep Mos in-active stage.
• MO will resume growth after food is rehydrated (suitable
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4. Controlling Factors for Dehydration
• Two separate phenomena are involved in
dehydration.
 First, moisture must move from the interior to surface of
the material. This is occur two ways:
capillary action or diffusion.
 Movement by capillary action occur during early
stages of drying.
• Second, the surface water must be evaporated into
air.
• Evaporation rate depend on;
• condition of drying air, and
• concentration of water at the surface.
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5. Factors affecting dehydration
• Surface area
• Smaller food piece, more rapid the rate of moisture loss
• Temperature
• Increase in temperature will increase the dehydration rate
• Air velocity
• Maximize velocity of heated air moving around the food particles
• Humidity of drying air
• The drier the air, the moisture it can absorb
• % RH (relative humidity) of the drying air determines the final
moisture content of food
• Atmospheric pressure and vacuum
• Water boils at 100oC ( at a pressure of 1 atm= 760Hg)
• At lower pressure the boiling temperature will decrease
• Eg. Under vacuum, water will boil at 32oC
• Important for heat sensitive food products
• MOSTLY USED FOR FREEZE DRYING
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7. Aw for some dried foods
Product aW
Skim milk powder
Dehydrated soups
0.03
noodles 0.5
Fruit: berries, apples, apricots, raisins ( grapes) 0.6
Fruit juice concentrates, condensed milk 0.8
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8. Changes during dehydration
1. Cell/ tissue shrinkage
2. Case hardening
3. Chemical changes
1. Shrinkage
• Shrinkage water migrates- interior of the food surface
• Evaporates by the drying medium
• Water carries with it water soluble substances dissolved in it
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9. 2. Case hardening
• Rapid drying- compounds (such as sugar) form a
hard, impermeable “case” around the food piece.
• Can slow down the dehydration
• Common in high sugar products
• Tropical and temperate fruit products
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10. 3. Chemical changes
• Browning and flavor changes due to reactions
 Maillard browning = from increased solution concentration
• Denaturation of proteins, aggregation of polysaccharides
 Loss of water soluble binding capacity
• Loss of water soluble components
• Concentration on the surface of the food ( case hardening)
• Loss of volatiles ( especially flavor compounds)
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11. Dehydration Equipments
1. Bin Drying
• Bin drying systems are common in on-farm grain drying
operations.
• Bin is filled with grain and drying air
• As drying progresses; a layer of drying grain separates the
dried grain from the undried grain.
• “drying front” process
• Other drying methods include stirring of grain in the bin
during drying and use of continuous flow dryers to dry grain
before storage.
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12. 2. Cabinet Drying
• Cabinet dryers are usually small, insulated units with;
• A heater, circulating fan, and shelves to hold the product to
be dried.
• General procedure is to force heated air over multiple trays.
• However, greater energy efficiencies can be obtained if some
of the heated air is recirculated.
• Basic operation of a cabinet dryer with recirculation.
• Energy savings of 50% or more can be achieved with
recirculation.
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13. 3. Tunnel Drying
• Tunnel dryers are a large-scale modification of the cabinet
dryer concept.
• The drying chamber is a tunnel with multiple carts
containing trays.
• New carts of undried product are loaded at one end of the
tunnel.
• product are removed from the other end.
• Air flow in these dryers may be Parallel or Counter to the
movement of carts in the tunnel.
Tunnel dryer sketches showing (a) parallel and (b) counter flow operation
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14. 4. DrumDrying
• Large rotating drums are used for drying slurries (liquids
with a high solids content).
• A thin film of the slurry is deposited on the bottom of a
rotating drum as it passes through the slurry.
• The slowly rotating drum is heated and sometimes held
under a vacuum.
• The dried product is scraped from the drum (doctor blade)
before the one full circle rotation.
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15. 5. Fluidised-beddriers
• The main features of a fluidised-bed drier;
 evenly distribute the air at a uniform velocity around the bed
of material.
• Produce an homogenous region of air and prevent localised high
velocities,
 disengagement or ‘freeboard’ region above the bed to allow
disentrainment of particles.
• Hot air is blown through the bed, causing;
 the food to become suspended ( Problem)
To overcome this;
vigorously agitated (fluidised), exposing the maximum
surface area of food for drying.
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17. 6. SprayDrying
• Spray drying is used to dry liquid products. The product to be
dried is sprayed into a stream of heated air.
• The two major operations of concern in spray drying are;
• Droplet atomization and
• Powder collection.
• To optimize drying, droplets should be small and uniform in
size.
• Special procedures must be used to insure that atomization
is satisfactory.
• Collection of the dried powder also requires special
techniques.
• The powder particles are small and move easily within an
air stream.
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18. SPRAY DRYER
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19. 7. Freeze Drying
• Freeze drying involves the removal of moisture from a frozen
product .
• The temperature must be below freezing for that product (insure
that product remains frozen)
• And the vapor pressure must be maintained at a very low
level to permit moisture removal by sublimation.
• Because of the low temperature, low pressure, and low
drying rate, freeze drying is quite expensive compared
other drying methods.
• However, freeze drying produce high quality dried
products.
• “Freezer burn,” sometimes seen in frozen foods,
• undesirable of freeze drying. (very slow type freeze).
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21. Effect on Foods
Products undergo changes during drying that reduce their
quality compared to the fresh material.
1. Texture
Rupture, crack, compress and permanently distort the relatively
rigid cells,
• Change texture apperance
• To give the food a shrunken shriveled appearance.
Re-hydrated product absorbs water more slowly and does not
regain the firm texture of the fresh material.
Drying pieces of meat - severe changes in texture.
• Caused by aggregation and denaturation of proteins and loss
of water-holding capacity.
Case hardening effect.
• reduces the rate of drying to produce a food with a dry
surface and a moist interior.12/8/2017 Y.BAVANEETHAN 21

22. 2. Flavor and Aroma
Heat not only vaporises water during drying but also causes
loss of volatile components.
Volatile loss depends on;
• Temperature and moisture content of the food
• And the vapour pressure of the volatiles and their solubility in
water.
The open porous structure of dried food allows access of
oxygen,
• which is a second important cause of aroma loss due to
oxidation of volatile components and lipids during storage.
These changes are can reduced by:
• vacuum or gas packing
• low storage temperatures
• maintenance of low moisture contents
• addition of synthetic antioxidants
• preservation of natural anti-oxidants.12/8/2017 Y.BAVANEETHAN 22

23. 3. Colour
Causes of colour loss or change in dried foods;
• drying changes the surface characteristics of a food and
alter the reflectivity of surface.
Fruits and vegetables,
• Chemical changes to carotenoid and chlorophyl
pigments.
• Caused by heat and oxidation during drying
• Residual polyphenoloxidase enzyme activity causes
browning.
Prevented by treatment of fruits with ascorbic acid or
sulphur dioxide.
• However, sulphur dioxide bleaches anthocyanins, and
residual sulphur dioxide is also health problem.
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24. Calculation
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