This document summarizes various aspects of food drying, including: - Drying removes water from foods to extend shelf life without refrigeration by inhibiting microbial growth and enzymatic activity. - Key factors that influence drying rate are temperature, humidity, air velocity, and drying time. Common drying methods include hot air drying, freeze drying, and sun drying. - Important dryer types are kiln dryers, tunnel dryers, fluidized bed dryers, and spray dryers. Proper dryer selection depends on production needs and food properties.

1. Food Drying
By
Debomitra Dey

2. DRYING
o Oldest method of Preservation.
o Important Unit Operation, can occur is conjunction with other
processes.
o Removal of water/moisture from a product to pre-
determined level, the low water activity attained by food
products extends the shelf life of dried foods without the
need for refrigerated storage or transportation.
o For e.g. Liquid milk is highly perishable, whereas milk
powders are more stable and easy for preservation and
handling.
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3. WATER ACTIVITY
o Dehydration accomplishes preservation in two ways:
1. Removes the water necessary for growth of microbes and
for enzymatic activity.
2. Removal of water, as it increases the osmotic pressure by
concentrating salts, sugars, acids thus creating
environment which is unfavourable for growth of
microbes.
o Water is a key factor in food dehydration, it is the ratio of
Vapour pressure of food product to vapour pressure of
water at the same temperature. Also defined as the
Equilibrium relative humidity.
𝒂𝒘 =
𝑷
𝑷𝟎
=
𝑬𝑹𝑯
𝟏𝟎𝟎
The maximum water activity of dried food is below 0.70,
which is below the minimum value for food pathogens.
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4. MOISTURE CONTENT
o Expressed either in wet basis i.e. mass of water per unit mass
of wet material or dry basis i.e. mass of water per unit mass
of dry solids
o Equilibrium Moisture content- moisture content after
equilibration with specified conditions of temperature and
humidity
o Water within food will be either bound or unbound to solids,
this influences the drying process and stability
o Bound water exerts vapour pressure less than that of liquid
water at the same temperature.
o Unbound water is moisture in excess of bound moisture, also
called free moisture. Held primarily in voids of the solid.
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5. 12/9/2017 5
MOISTURE CONTENT OF FOODS BEFORE AND
AFTER DRYING

6. RATE OF DRYING
The drying rate depends upon the following factors-
o Vapour pressure of water at drying temperature
o Vapour pressure of water in external
environment
o Equilibrium vapour pressure of water in the food
o Moisture content of food
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7. MECHANISM OF DRYING
1. INITIAL SETTLING DOWN- No loss of moisture, reaches
to wet bulb temperature.
2. CONSTANT RATE PERIOD- CMC(Critical Moisture
content) is reached, moisture from food product to
atmosphere > moisture from atmosphere to outer surface
of grain.
3. FALLING RATE PERIOD- Rate of drying falls in this
period. Products can be packaged after this process, as
this marks the end of the process of drying.
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8. CONSTANT RATE PERIOD
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Free Moisture (kg H20/kg dry solid)
Drying
rate
(kg
H20/
m2h)
B
A (ISP)
C
D
E
AB-Initial Settling Phase, BC- Constant Rate Period, CE-
Falling Rate Period
DRYING RATE CURVE

9. FACTORS THAT INFLUENCE DRYING
OF FOOD
1. Drying Temperature- This varies with food and the method
of drying, greater the temperature difference between
heating medium and food, greater the rate of heat transfer
2. Relative Humidity of air- Sorption characteristic of food to
be dried should be known, as EMC is the lowest moisture
content that can be achieved under given set of temperature
and humidity conditions.
3. Velocity of air- Higher the velocity of air, more efficient the
process of drying
4. Drying Time- Drying time depends upon the type of food
and its moisture content and temperature
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10. o Preservation of foods
o Decreasing the weight and bulk of food to economize
shipping and canning costs
o Production of convenience products such as instant
coffee, milk powder, instant mash potatoes
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PURPOSE OF DRYING

11. DRYING PROCESSES
o Drying processes falls in 3 categories:
1. Air and contact drying- Heat is transferred
through food materials either from heated air
or heated surface.
2. Vacuum drying- Heat transfer is generally by
conduction sometimes by radiation.
3. Freeze drying- the water is sublimed off from
frozen food, structure of the food product is
maintained.
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12. 12/9/2017 12
DRYERS

13. METHODS OF DRYING
1. SUN DRYING
2. USE OF MECHANICAL DRIERS
a) BY HEAT
b) DIRECTCONTACTWITH HEATED SURFACE
c) APPLICATION OF ENRGY FROM RADIATING
MICROWAVE OR DIELECTRIC SOURCE
3. FREEZE DRYING OR LYOPHILIZATION
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14. APPLICATION OF DRYER
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15. SELECTION OF DRYER
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1. Production capacity
2. Initial moisture content of product
3. Particle size distribution or shape of the product
4. Drying characteristic of product
5. Maximum allowable product temperature
6. Explosion characteristics (spray or fluid bed dryer)
7. Moisture isotherm
8. Physical data of material

16. SUN DRYING
o Oldest method of preservation of agricultural products (Imre, 1995;
Bolin & Salunkhe, 1982)
o Cheap method. Sun drying of food refers to foods that are dried under
the direct sun
o Radiant energy from sun provides heat to evaporate the water, while
wind helps to move the moisture and accelerates the process.
o Disadvantages- difficultly in controlling drying conditions, protection
from insects, pests, etc., availability of sun. 12/9/2017 16

17. HOT AIR DRYERS
o The food is in direct contact with moving stream
of hot air.
o Heat is supplied to product by convection
o The driers that are used for drying solid foods are-
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1. Kiln dryers
2. Cabinet tray
dryer
3. Pneumatic
dryer
4. Fluidized bed
dryer
5. Rotatory
dryer
6. Spray dryer
7. Bin dryer
8. Tunnel dryer
9. Conveyor
belt dryer

18. KILN DRYER
o Consists of a two-floor installation, burner or
furnace on the ground floor.
o Product is placed on the slatted floor and hot air is
forced from the lower floor.
o Drying times are quite long due to extensive
number of product processed during drying cycle
almost 20 tons per day.
o Dryer allows processing of liquid slurries to solid
materials, products like hops, apple rings, malt,
grains can be dried using Kiln dryer.
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KILN DRYER

20. TUNNEL DRYER
o Consists of a tunnel that is 24m long with
rectangular cross section of 2*2m.
o Wet food is spread in even layers on tray of slatted
wood or metal mesh.
o Trays are assembled in stacks on trucks which
passes through the tunnel and hot air with velocity
of 2.5-6.0 m/s velocities either in concurrent or
counter current to direction of food.
o Vegetables and fruits cut into pieces can be dried
in semi continuous basis.
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21. 12/9/2017 21
TUNNEL DRYER

22. CABINET, TRAY OR COMAPARTMENT DRYER
o Insulated cabinet with air circulating fan which
moves the air through heater and then through
baffles which direct air between the trays of food.
o They vary in the number of trays, and have varied
application in the industry, which includes drying
fruits and vegetables at 1-20 tons/day.
o The installation is cheap and flexible.
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CABINET,TRAY OR COMPARTMENT DRIER

24. CONVEYOR, BELT DRIER
o Similar to tray dryer, but wet material is conveyed
through a tunnel on perforated (mesh) conveyors.
o The heated air is either directed up or down
through the conveyor, the layer of product either
maybe perpendicular or parallel to the surface
spread in thin layer.
o Dryers consists of two or more conveyors,
sometimes maybe up to five, they called Multiple
conveyors
o Used for drying vegetables and fruits mainly in
diced form.
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25. 12/9/2017 25
CONVEYOR, BELT DRYER

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MULTI PASS CONVEYOR DRYER

27. PNEUMATIC DRIER
o Generally used for the finishing of dried powder or
granulated materials.
o The incoming material is usually pre-dried (Spray,
fluidized, etc.) to moisture level below 40% (wb).
o Dried product are separated from exhaust air by
cyclone or filters.
o Commercially used pneumatic driers are Flash and
Full ring dryer, used for drying powders like
potatoes, corn and starch.
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28. ROTARY DRIER
o Rotatory dryers are used to dry particulate solid foods
(Kelly, 1995).
o The wet material is rotated in cylindrical shell and heated
air is passed through as the material is agitated.
o Drying medium is either hot air or combustion gases co-
currently or counter-currently.
o Size of dryers in usually 0.3m diameter-2m in length to
5m diameter-90m length.
o Application includes in drying of meat pellets, granulated
sugars, treatment of cocoa beans.
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ROTARY DRIER

30. SPRAY DRIER
o It involves a suspended particle processing
operation that consists of particle formation as
well as drying
o It commonly used for processing of milk
powders, coffee, tea, whey solids, ice cream
mix, milk based baby foods, yeast extract,
edible proteins.
o Three steps of drying-
1. Atomization of feed
2. Spray air mixing and moisture evaporation
3. Separation of dry product from outlet air
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31. FLUIDIZED BED DRIER
o Heated air is forced up through a bed of solids
under fluidized conditions so that solids are
suspended in air.
o Heated air acts both as fluidizing and drying
medium.
o It is used for drying peas, beans, carrots, coffee,
salt, sugar, meat cubes, flours, etc.
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FLUIDIZED BED DRYER

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CONTACT DRIERS
Consists of hot surface used as source of heat, necessary
for evaporation of moisture from thick liquids, pulps,
pastes or slurries.

34. DRUM/ROLLER DRIER
o Can be used for drying liquid and slurry forms of food,
and foods which can withstand relatively high
temperature for short time.
o Used for drying milk, soup mixes, mashed potatoes.
o Double drum can be used for drying tomatoes,
pumpkin purees.
o Consists of single, double, twin drum with feed roller
and vacuum casing
o Film of feed material is uniform in depth, economic for
use as high rates of drying.
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35. 12/9/2017 35
A
B
A- SINGLE DRUM DRYER,
B-TWIN DRUM DRYER

36. VACUUM SHELF DRIER
o Heat sensitive foods in solid or liquid state are
dried by heat conduction.
o Vacuum is used for very heat sensitive foods e.g.
fruit juices.
o Low temperature drying with no agitation or
compression.
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38. VACUUM BAND DRIER
o It can be used to drying of liquids and slurries such as fruit
juices concentrates, tomato concentrates and coffee
extracts.
o ‘Puff Drying’ is a method to obtain rapid drying by
adjusting the vacuum to make the material puff by the
evolution of gases.
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39. NOVEL DRYING TECHNOLOGY
1. MICROWAVE DRYING- Microwave heating in
combination with hot air circulation is used for
drying. It can reduce drying time. Microwave-
vacuum drying has advantages over conventional
drying, as there is little shrinkage, reduced loss
of nutrients and flavours.
2. RADIO FREQUENCY DRYING- Uses frequency
in the range of 3-3000MHz, the penetration of RF
is large as compared to MW. The largest
application is in the finishing of dried post baked
biscuits and cereal product.
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40. FREEZE DRYING
o Removal of moisture as a result of sublimation of water (ice
to vapour) without phase change from solid to liquid.
o Latent heat is conducted inward through the layer of dried
material, water vapour is also transferred through the layer
of dried material, thus heat and mass transfer occur
simultaneously during freeze drying.
o Freeze dried products can be stored for unlimited time,
most physical, chemical, biological, organoleptic properties
is retained.
o Essentially used for preserving exotic fruits, spices, herbs,
instant dry soups, instant coffee.
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41. OSMOTIC DEHYDRATION
o Osmotic dehydration is the phenomenon of removal of water from
lower concentration of solute to higher concentration through
semi permeable membrane results in the equilibrium condition in
both sides of membrane (Tiwari 2005).
o Osmotic dehydration is preferred over other methods due to the
retention of colour, aroma, nutritional constituents and flavour.
o The most important products of commercial importance available
in market made from fruits are murabbas of gooseberry (Aonla),
apple, candies of different fruits and vegetables like pethas, sweets
of parwal made by osmosis in sugar syrup..
o Common methods of applying these processes include drying,
spray drying, freeze drying, freezing, vacuum packing, canning,
preserving in syrup (osmotic dehydration), sugar crystallization,
food irradiation and adding preservatives or inert gases such as
carbon dioxide
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42. FOAM DRYING
o Two methods for foam drying- Foam mat and Foam spray
drying
o Stable gas-liquid foam is prerequisite for foam drying.
Natural foam like egg white, vegetable proteins like soya
protein, gums, emulsifiers are used.
o Liquid foods are formed into a stable foam by the
addition of a stabiliser.
o The foam is spread on a perforated belt to a depth of 2–
3mm and dried
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FOAM MAT DRYING

44. CHANGES THAT OCCUR DURING DRYING
1. Movement of solids- Soluble solids move towards the surface
because of Shrinkage, concentration gradient set up between
surface and the wet center.
2. Shrinkage- May influence the drying rates, due to changes in
surface areas, pressure gradients.
3. Cellular structure- Blanching makes cells more permeable hence
cooked meat, vegetable dry easily.
4. Case hardening- Common phenomenon in food with high
concentration of sugars and other solute.
5. Porosity- Escaping steam tends to puff product which desirable in
bakery products, makes structure more porous so as to facilitate
mass transfer and increase drying rate.
6. Chemical and other changes- Browning reactions, texture
changes, Flavour,Texture, Aroma, Denaturation of proteins.
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45. REFERENCES
1. Earle RL, Unit operations in Food Processing, Second Edition,
Pergamon Press,1983, Pg. 85-95
2. Unit operations in Agricultural Processes
3. Sivashankar B, Food Processing and Preservation, PHI Learning Pvt
Ltd, 2010, Pg. 219-229
4. Ramaswamy H, Marcotte M, Food Processing Principles and
Applications,Taylor and Francis,2009, Pg. 233-302
5. Yadav Ashok, Singh Satya, Osmotic dehydration of fruits and
vegetables: a review, Journal of Food Science and Technology,
September 2014,Volume 51, Issue 9, pp 1654-1673
Image source- Internet and above mentioned references
Video source- Internet
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