The document discusses various methods and principles of food preservation through drying, emphasizing its significance in preventing spoilage and retaining nutritional value. It covers both traditional and modern drying techniques, including sun drying, solar drying, and mechanical methods, while highlighting the advantages of drying such as cost-effectiveness and reduced storage needs. Additionally, it details the factors affecting drying rates and the different industrial drying methods available.

1. Food preservation
by Drying
PREMRAJA N,
FTM 20084,
College of Nutrtion and Food Technology,Chennai.

2. Introduction:
1. The process of treating and handling food in such a way
to stop or greatly slow down spoilage and prevent
foodborne illness also maintaining nutritional value,
texture and flavour.
2. The primary objective is to prevent food spoilage, to
increase the shelf-life and to retain most of the nutrients
as before.
FOOD PRESERVATION:

3. METHODS OF FOOD PRESERVATION:
1. Sun Drying.
2. Fermentation
Modern methods:
1. Solar drying.
2. Canning.
3. Freezing.
4. Freeze drying.
5. Refrigeration
6. Irradiation, etc.
Old methods:

4. “DRYING”

5. DRYING or DEHYDRATION:
1. Removal of water by evaporation, from a solid or liquid
food, to obtain a solid product sufficiently low in water
content with most of the nutrients and other characteristics
such as flavour, taste etc, are retained.
2. Microorganisms need free water for their growth and
multiplication.
3. When food is dry preserved, the water content of the food is
reduced to critical level below which microorganisms can’t
grow.

6. FOOD PRESERVATION BY DRYING:
 One of the oldest method of food
preservation.
 Reduces water activity which
prevents bacterial growth.
 Reduces weight of food.

7. FOOD PRESERVATION BY DRYING:
 Sun and wind are used for drying.
 Meat and fruits like apples, apricots
and grapes are some examples of
drying with this method.
 Slows down the enzymatic activity
but doesn’t inactivate them.

8. FACTORS AFFECTING DRYING:
 Greater the surface area, greater the drying.
 Low humidity aids drying.
 Increase in air current in surroundings, speeds up drying.
 Lower the atmospheric pressure, lower the temperature required to
evaporate water.

9. ADVANTAGES OF DRYING:
 Cheaper and can be accomplished with fewer equipment.
 No refrigeration needed for storage.
 Takes little space for storage, due to reduction in size.
 If the moisture content is reduced to 1 to 5 percent then the product can be
stored for more than 1 year.

10. DRYING UNDER VARYING EXTERNAL
CONDITIONS
Batch drying on trays.
Through-flow batch drying in a fixed bed.
Continuous air drying on a belt or in a tunnel.

11. Industrial drying can be grouped as:
There are different types of drying methods are in practice, let’s
see some of those methods.
 Natural methods
 Mechanical methods

12. TYPES:
METHODS MECHANICAL:
• There is influence of only natural
sources like air, light, temperature
,etc.,
• No control over temperature.
• In this method of drying is aided by
artificial machines or applications.
• Example: Sun, drying, solar drying,
shade drying.
• Example: forced heated air, direct
contact of heated surface, microwave,
etc.,
NATURAL METHODS:

13. SUN DRYING:
 Drying food products under sunny conditions.
 Minimum energy is required.
 Minimum 35°C and low humidity needed.
 The maximum moisture content which can be reduced
by this method is upto 15%.
 Dried fruits which can be used as such after soaking,
while dried vegetables are usually soaked in water
overnight and then cooked.

14. SOLAR DRYING:
 It is designed to collect and enhance solar radiation for
drying.
 It generates high air temperature and low humidity
which fastens drying.
 This dryer is faster than sun drying and require less
drying area.
 Can’t be used in cloudy days.

15. Cont..,
 These are of three types:
1. Absorption type – product directly
heated by the sun.
2. Indirect type – product is exposed to
warm air which is heated by means of
solar absorber or heat exchanger.
3. Drier – combination of first and
second type.

16. SHADE DRYING:
 This type is used for the food product which lose
their colour while exposed to direct sunlight.
 Generally herbs, green and red chillies, okra,
beans etc are dried under shaded and good air
circulation.

17. 17
MECHANICAL DRYING
CONVECTIVE CONDUCTIVE DIELECTRIC
RADIATION COMBINED
 Fluidized bed
dryer
 Cabinet dryer
 Tunnel dryer
 Spray dryer
 Rotary dryer
 Flash dryer
 Drum dryer
 Agitated
pan dryer
 Infrared shelf
dryer
 Sun dryer
 Microwave
oven
 Microwave
tunnel
 Radiofrequ
ency dryer
 Microwave
convective
dryer
 Microwave
spouted
dryer
 Infrared
convective
dryer
 Pump
dryer

18. CONVECTIVE DRYING(AIR DRYING):
• Hot and dry gas (usually air) is used to supply the
heat.
• A typical example is tray drying.
• The moist food is placed on a tray.
• Hot and dry air is passed over the food (cross-flow
drying) or through it (through flow drying).

19. FLUIDIZED BED DRYER:
 Hot and dry air is used for fluidization and for
drying.
 Can be applied to particulate, nonsticky foods
with particle size of 0.05–10 mm.
 Due to the efficient heat and mass transfer, the
product is dried rapidly.
 Part of the hot air may be recirculated.
19
1. FLUIDIZED BED DRYER:

20. CABINET DRYER:
• Used for batch drying of solid foods.
• Foods are placed on trays in a closed compartment.
• The trays rest on shelves with adequate spacing.
• For uniform drying, direction of airflow may be
reversed or the trays may be rotated periodically.
• Air inlet temperatures are usually in the range of 60–
80°C.
• Frequently found in rural installations where they
are used for drying fruits (grapes, dates, apples),
vegetables (onion, cabbage), and herbs (Ex:mint).

21. ROTARY DRYERS:
• Mainly used in the chemical and mineral industry.
• In the area of food, the most common application is
dehydrating waste materials (citrus peels, vegetable
trimmings) and animal feedstuffs (alfalfa).
• Rotary dryers consist of a metal cylinder with internal
flights or louvers.
• The cylinder is slightly inclined.
• Hot air is blown in cocurrent or countercurrent direction.
• Most of the drying takes place while the material falls
• through the air blast.

22. SPRAY DRYERS:
 Used for drying liquid solutions and suspensions, with the
objective of producing light, porous powders.
 It is the standard method for the production of milk and
whey powders, coffee creamers, cheese powder, dehydrated
yeast extract, instant coffee and tea, isolated soybean protein,
enzymes, maltodextrin, egg powder, and many other
products in powder form.
 The liquid is dispersed (atomized) as a spray of fine droplets
into very hot air inside a large chamber.
 Because of their small size and the high temperature of the
air, the droplets are dried in a matter of seconds and
transformed into particles of solid powder.

23. PNEUMATIC DRYER:
• Also known as flash dryer.
• In this method, the particles are dried while being moved
in a stream of hot, dry air.
• This method is often used for removing “free” moisture
in the constant drying rate period
• This method is often used as a method of predrying,
followed by another type of drying.
• The use of pneumatic drying in the food industry is
limited to the drying of flours, starch, gluten powder,
casein powder, etc.

24. • Process where heat is transferred to the wet material from a heated surface
in contact with it.
• The moist food is brought to contact with a hot surface (in a particular
application, with superheated steam) and the water is “boiled-off”.
• The principal mechanism is conduction, with the exception of superheated
steam drying where heat is transported by convection.
• Can be applied to foods in liquid or slurry form.
CONDUCTIVE DRYING(BOILING):

25. DRUM DRYING:
• In this application, the heated surface is the envelope of a
rotating horizontal metal cylinder.
• The cylinder is heated by steam condensing inside, at a
pressure in the range of 200–500 kPa bringing the
temperature of the cylinder wall to 120–155°C.
• The wet material is applied on the drum surface as a relatively
thin layer by a variety of different methods.
• The dried product is removed from the drum with the help of a
blade.
• Drum drying is extensively used in the production of instant
mashed potatoes, precooked cereals, soup mixtures and low-
grade milk powder.

26. DRYING BY RADIATION:
• Infrared drying (IR) uses the energy from IR radiation
to directly heat the bulk material of the polymer
granule/flake.
• The delivered energy is applied directly to the granule
with no other transfer medium. The applied energy
causes internal heating and molecular oscillation and
therefore heats the bulk material of the granule and
any internal moisture.
• A stream of cooler ambient air surrounds the granule
and the internal heat drives the moisture out into the
cooler air stream that removes it from the process.
INFRARED DRYING:

27. Cont..,
• The driving force for IR drying is the difference in temperature between the
vaporised water and the ambient air stream.
• This creates a partial pressure gradient from the inside of the granule to the outside
and a strong driving force for moisture removal.

28. Ohmic heating:
• Ohmic heating is a technique whereby a material is heated by passing an electric current
through it.
• In Ohmic heating, like in microwave heating, heat as such is not delivered to the food
through its surface but is generated inside the food by conversion of another type of
energy to heat.
• A temperature gradient is not required for heating to occur.
• The rate of heating does not depend on thermal conductivity or heat transfer coefficients.
• Heating is rapid and uniform, at least theoretically.
• Since there is no heat transfer surface, there is practically no fouling.
• These characteristics give ohmic heating a distinct advantage over conventional heat
transfer.

29. DIELECTRIC DRYING:
• Dielectric heating is yet another process of uniform sterilization of food products.
• Radio frequency and microwave heating are used following the principle of dielectric
heating.
• The process allows heating a dielectric material with the help of high frequency
alternating electric field, microwave electromagnetic radiation, or radio wave.
• Radio frequency and microwave are noticed to act on polar compounds, namely,
water coordinating itself around the food by reverse electric field.
• Dielectric components are identified by the presence of permanent or induced
dipoles, which acts as capacitor when allocated in between two electrodes.
• The frequency of microwave heating is maintained between 300 MHz and 300 GHz.

30. Cont..,
• Water shows immense dielectric properties for which it is commercially used in
dielectric heating.
• Microwave heating is utilized in a wide range of applications including thawing,
cooking, and heat sterilization.
• Sterilization of food occurs with no change in quality of the food at a very short time.
Microwave heating, in recent days, has become a prevailing process of
inhibiting microbial contamination in food industries.
• Water shows immense dielectric properties for which it is commercially used in
dielectric heating.

31. References:
Food process engineering and technology, third edition,
zeki berk.
McDonald CJ, Lloyd SW, Vitale Ma, Petersson K, Innings F.
(2000).
NIZO Dairy Conference on Food Microbes.
Fundamental aspects and applications (pp. 149–166).
Technomic Publishing company.

32. Thanks!