This document discusses principles and methods of preservation by drying. It begins by defining drying and dehydration as processes for removing water from food using non-conventional energy sources like sunlight and wind or under controlled conditions. The benefits of dried food include unlimited shelf life when properly stored, reduced transportation and storage costs, and portability. Various factors that affect drying rates and common types of drying methods are described. Pre-treatments like blanching and post-treatments like sweating and screening are outlined. Changes that occur during drying and determining dryness in fruits and vegetables are also summarized.

1. PRINCIPLES AND METHODS OF PRESERVATION BY
DRYING

2. COLLEGE OF HORTICULTURE, BENGALURU
DEPARTMENT OF POST HARVEST TECHNOLOGY
PRESENTATION ON:
PRINCIPLES AND METHODS OF PRESERVATION BY
DRYING
UNIVERSITY OF HORTICULTURAL
SCIENCES, BAGALKOT
SUBMITTED BY:
RAGHAVENDRA HIREHAL

3. DRYING:
Removal of water from the food by non-conventional
energy sources like sunlight and wind.
DEHYDRATION:
The process of removal of water from the food under the
controlled conditions like temperature, relative humidity and air flow
etc.

4. DRYING AND DEHYDRATION
•One of the oldest methods of preserving food.
•Both drying and dehydration mean the removal of water.
•Removes moisture stops the growth of bacteria, yeasts & moulds that
normally spoil food.
•Slows down but does not completely inactivate enzymes.

5. BENEFITS OF DREID FOOD
 The main benefit of dry food is that it is convenient for the owner –
it’s easy to measure, easy to serve and easy to store.
 They have unlimited shelf life under proper storage conditions.
 Transportation, handling and storage costs are reduced.
 Provide an important modern marketing requirements.
 Easy and economical way to save surplus food for a later time.
 Portability – Can take it anywhere you go.
 Helps in minimizing post harvest loses.

6. GENERAL FLOW-SHEET FOR DRYING/DEHYDRATION
STORAGE
PACKING
POST DRYING/DEHYDRATION TREATMENTS
DRYING/DEHYDRATION
SPREADING ON TRAYS/CONTAINERS
PRE-TREATMENTS
PREPARATION
WASHING
FRUIT/ VEGETABLES

7. MECHANISM INVOLVED
1. HEAT AND MASS TRANSFER
 Heat into product
 Moisture out of product
2. DEVELOPMENT OF DRIED THICK LAYER
At initial stages of drying moisture from surface of food is
removed later it becomes slow because of development of dried
thick layer, which is due to loss of more moisture from outer surface
of food and remaining of more moisture in the centre of food.

8. 3. ESTABLISHMENT OF MOISTURE GRADIENT
 When dried thick layer is formed and act as insulation against
rapid heat transfer.
 Water retain in the centre of food which have moisture gradient to
get out on the surface which will not loose faster and establishes
moisture gradient.
4. ESTABLISHMENT OF NORMAL EQUILIBRIUM
RELATIVE HUMIDITY (ERH)
 This is the humidity at which the product neither looses nor gains
moisture from atmosphere.
 Below this atmospheric humidity level, the food can be further
dried while above this humidity, it cannot.
 Rather it picks up moisture from atmosphere.

9. FACTORS AFFECTING DRYING RATE
Temperature
 Velocity of air
 Surface area
 Size of product
 Tray load
 Relative humidity of air
 Atmospheric pressure and vacuum

10. TYPES OF DRYING
1. Sun drying, solar drying:
2. Atmospheric drying including batch (kiln, tower & cabinet
driers) and continuous (tunnel, belt, belt trough, fluidized
bed, foam mat, drum and microwave)
3. Sub-atmospheric dehydration (vacuum shelf/belt and freeze
driers)
The scope has been expanded to include use of low
temperature, low energy process like osmotic dehydration.

11. PRE AND POST DEHYDRATION TREATMENTS FOR
DRYING

12. I. PRE-TREATMENTS
1. Blanching- Partial pre-cooking treatment in which vegetables are
generally heated in boiling water.
 To inactivate natural enzymes.
 To remove hardness.
 To reduce bacterial load.
2. Sulphuring – Whole/pieces are exposed to fumes of burning sulphur in
closed chamber known as sulphur box.
 To check growth of mould.
 To act as preservative/antimicrobial agent.
 To prevent oxidation and darkening.
3. Fruit juice dip.
4. Honey dip.
5. Syrup blanching.

13. ll. POST-DEHYDRATION TREATMENTS
1. Sweating – It is a process to hold the dehydrated foods for
equalization of moisture before packing.
2. Screening – The unwanted size of pieces of products are removed
by passing the dried products through various screens.
3. Inspection – The dried product is inspected to remove discolored
pieces. These are removed manually.
4. Fumigation – It fix the insect including their eggs. Great care is
necessary in using fumigants as CO2 and (HCN) gas are highly
poisonous. The mixture of Ethylene Dichloride, Ethylene oxide are
also used for fumigation.

14. CHANGES DURING DRYING:
 Food materials do not have perfect elasticity and water is no
removed evenly throughout food as it is dried and causes shrinkage.
 Due to high surface temperature and unbalanced drying, dry skin
will form and causes case hardening.
 Enzymatic browning of products due to poor blanching.
 Caramelization of sugars due to excess heat.
 Loss of volatile flavor constituents.
 Partial loss of some essential nutrients like Vit C.

15. Determining Dryness of Fruit
 Drying fruit can take 6 hours for thin/small pieces or 10-12 hours
for larger juicy fruits such as peach or apricot halves.
 Dried fruit will feel leathery, won’t stick to itself.
 Cut fruit should have no visible moisture.
Determining Dryness of Vegetables
 Dry vegetables until brittle or “crisp”
 Low moisture (10%)

16. DRIED PRODUCTS
 DRIED FIGS

17.  PRUNES

18.  RESINS

19.  CHENPI (SUN DRIED MANDRIN PEELS)

20.  DRIED PEACHES

21.  DRIED APRICOT

22.  DRIED STRAWBERRIES

23.  DRIED MANGO SLICES

24.  SUN DRIED POTATO CHIPS

25.  DRIED TOMATO

26.  DRIED ONIONS

27.  DRIED MUSHROOMS

28. DEFECTS CAUSES PREVENTION
MOULDING High product moisture,
above ERH
Reduce moisture content.
Pack in hermetic airtight
packages
INFESTATION Presence of larvae or
insects in dried products
Storage room disinfection
with fumigants.
Disinfection by heat (60-
65°C) of products before
packing
BROWING Chemical reaction
Enzyme catalyzed
reactions
Reduce water content,
store at low temp.
Enzyme in activation by
blanching before drying
REDUCED REHYDRATION
RATIO
Too high temperature at
final stage of storage
Operate final
temperatures as
recommended
SPOILAGE OF DRIED PRODUCTS

29. THANK YOU