Drying is one of the easiest and commonly used food processing unit operations that can provide entrepreneurship development to a large extent. Drying of agricultural produce and selling of dried products can generate a huge amount of income.

1. Drying of fruit & vegetables: An
approach for Entrepreneurship
Development
TH. BIDYALAKSHMI DEVI, RENU BALAKRISHNAN, THONGAM
SUNITA, SHAGHAF KAUKAB
ICAR-Central Institute of Post Harvest Engineering and Technology, Ludhiana,
Punjab-141004

2. Overview of Lecture
Introduction
Moisture content determination
Behaviour of solid during drying
Factors affecting drying rate
Aims to be fulfilled during drying
General flow-chart for drying/dehydration
Examples of dry fruit and vegetables
Classification of dryer
Types of dryer for drying fruit and vegetables

3. Losses after
harvesting
Huge amount of
Harvest and Post
harvest losses in
the country
Post harvest
losses are high at
farm level due to
lack of primary
processing
Introduction
Contd…

4. How to solve the problem of post harvest losses
• On farm primary processing
• Development of novel processing techniques
• Appropriate packaging and storage methods
• Cold chain management
Common
Processing/pres
ervation
techniques
Drying
Storage
Salting
Canning
Packaging
Freezing
Contd…

5.  Presence of high moisture content of the produce is the main
reason for deterioration.
 Drying may be defined as removal of liquid, generally water,
present in a wet solid to get a relatively liquid free solid product
 Drying is one of the most important and commonly followed
post-harvest unit operations for preservation of agricultural
products and also an initial operation for further processing.
 Drying is the most easy and commonly used method among all
the preservation techniques.
 This unit operation can lead in entrepreneurship development
just by drying and selling of agricultural produce.
Drying of Agricultural Produce
Contd…

6. Reduce microbial, fungal and insect attack
Increase the shelf life of the product
Decrease weight and bulk size and convenient for handling and
transport
Preliminary processing for storage and further processing
cheaper than the other methods of preservation with less
requirement of equipment.
Storage of dried food products does not require special facilities
like refrigeration etc. Dried food products are simple to store and
pack because of their low volume.
Why Drying??

7. • The moisture content of a substance is expressed in percentage by weight on wet basis.
But the moisture content on dry basis is more simple to use in calculation as the quantity
of moisture present at any time is directly proportional to the moisture content on dry
basis. The moisture content, m, percent, wet basis is:
𝑚 =
𝑊𝑚
𝑊𝑚 + 𝑊𝑑
𝑥 100
Where Wm= weight of moisture and W d= weight of bone dry material
The moisture content , M, dry basis, per cent is:
𝑚 =
𝑊 𝑚
𝑊 𝑑
𝑥 100 = 𝑚 =
𝑚
100−𝑚
𝑥 100
• Two additional useful equations for determination of moisture content
•
𝑊′ 𝑚
𝑊1
=
𝑚1−𝑚2
100− 𝑚2
=
𝑀1−𝑀2
100+ 𝑀1
;
𝑊′ 𝑚
𝑊2
=
𝑚1−𝑚2
100− 𝑚1
=
𝑀1−𝑀2
100+ 𝑀2
Where
W1= Initial weight of wet material=(Wm+ Wd)kg
W2= Final weight of dry product, kg
W’
m= weight of moisture evaporated, kg
m1, m2 = Initial and final moisture content respectively, per cent wet basis
M1, M2= Initial and final moisture contents respectively, per cent, dry basis.
Moisture content determination

8.  Direct Method
The air-oven drying method can be accomplished in a single stage or double stage in
accordance with the grain sample.
• Single stage method
 Grind 2-3 gm sample
 Keep the sample in the oven for about 1h at 130o ± 2oC
 Place the sample in a dessicator and then weigh
 Indirect method
• Moisture meters
All commonly used methods are based on electrical property of product. An electrical
current unit, resistance or capacitance, is measured and then converted into moisture
content.
Resistance: Measures the electrical resistance of product when a current is applied between
two electrodes in a constant and known volume.
Capacitance: Measures an electrical current between two plates of a condenser which
constitute the walls of a recipient. A precise weight of sample is required.
In both techniques, temperature corrections are required for accurate measurements. Most
of moisture meters are equipped with temperature correction software.
Moisture content determination

9.  Every food exerts a characteristic vapor pressure at a certain temperature and
moisture content. All porous food materials, when in contact with moist air, adsorb
or desorb water molecules to attain equilibrium moisture content. If the moisture
content of the food material does not vary with time for a given combination of
water vapor pressure and air temperature, it is then said that it has reached the
equilibrium moisture content (EMC) of the material at that water vapor pressure
and temperature.
 This equilibrium moisture content depends very strongly on the partial pressure of
the water vapor in the surrounding air and rather weakly on the air temperature
that are commonly experienced in drying and storage of foods.
Equilibrium Moisture Content (EMC)
Importance of (EMC)
• The EMC helps to decide the stability of food at particular moisture content in the given
environment.
• If exposed to air, high moisture foods, loose moisture whereas low moisture foods gain
moisture in humid air.
• EMC determines the minimum moisture content to which food can be dried under a
given set of conditions. Also, it determines the maximum amount of moisture the
dehydrated food can absorb during storage.

10. Behavior of solids during drying
Steps of drying:
• In conventional drying process, wet solid
absorbs heat and increase its temperature
• Thus, the moisture begins evaporating and
cools the drying solid.
( Absorbing heat and increase in temperature is
indicated by the segment of AB).
Between point B and C, the moisture
evaporating from the surface is replaced by
water from the interior of the solid at a rate
equal to the rate of evaporation. The rate of
drying is constant and known as the constant-
rate period.
At point C, the surface water is no longer
replaced at a rate fast enough to maintain
continuous film. Drying spots begin to appear,
the rate of drying begins to fall off. The moisture
occur referred to as the critical moisture
content.

11. Between point C and D, and the rate of drying falls
steadily and the dried spot increases. The drying time
during C to D is known as first falling-rate period (or
unsaturated surface drying).
At Point D, the surface is completely evaporated, and
the rate of drying depends on rate of diffusion of
moisture from inside to the surface of solid. This point
is known as second critical point.
Between point D and E, the rate of drying falls rapidly
than the first falling rate, and drying time during D to E
is called second falling rate period.
At point E, when dying state is equal to zero, the
equilibrium moisture period begins, and the solid is in
equilibrium with its surroundings (Temp. and moisture
content remain constant). Continuation of drying after
point E is waste of time and energy.
Contd…

12. • Temperature
• Velocity of air
• Surface area
• Size of product
• Tray load
• Relative humidity of air
FACTOR AFFECTING DRYING RATE

13. • Minimal chemical and biochemical degradation reactions
• Selective removal of water over other salts and volatile flavor and aroma
substances
• Maintenance of product structure (for a structured food)
• Rapid and simple rehydration or redispersion
• Storage stability: less refrigeration and packaging requirements
• Desired color
• Lack of contamination or adulteration
• Minimal loss in product quality
• Rapid rate of water removal (high capacity per unit amount of drying
equipment)
• Inexpensive energy source (if phase change is involved)
• Minimal solids handling problems
• Facility of continuous operation
• Noncomplex apparatus (reliable and minimal labor requirement)
• Minimal environmental impact
Aims to be fulfilled during drying

14. GENERAL FLOW-CHART FOR DRYING/DEHYDRATION
Fruits and vegetables
Washing
Preparation
Pre-treatment
Drying/Dehydration
Packaging
Storage

15. Example of dry fruits
DRIED BANANA
DRIED GRAPES
DRIED APPLE
DRIED PEACH
DRIED FIG

16. Example of dry vegetables
DRIED MUSHROOMS DRIED TOMATO
DRIED ONION
DRIED POTATO
DRIED SPINACH

17.  Dryer can be classified based on the drying medium (air, gas or
steam), based on the physical nature of the product (solid, slurry,
liquid) and operating conditions (temperature, high pressure,
atmospheric, vacuum), based on the method of material handling
involved (tray, tunnel, rotary, vibratory, gravity, dispersion), etc.
Classification of dryers based on the method of operation:
• (a) Batch dryer
• (b) Continuous dryer
Classification based on mode of heat supply to the drying process:
• (a) Convection dryer
• (b) Conduction dryer
• (c) Radiation dryer
• (d) Combined dryer

18.  Tray dryer
 Tunnel dryer
Belt Dryer
Freeze dryer
Vacuum dryer
Solar Dryer
Osmotic drying
Refractance Window dryer

19. References
• https://slideplayer.com/slide/12755888/
• ecoursesonline.iasri.res.in/mod/page/view.php?id=
3208 1/
• https://www.slideshare.net/RAGHAVENDRAHIREH
AL/principles-and-methods-of-preservation-by-
drying#:~:text=3.,humidity%20and%20air%20flow
%20etc.

20. Thank You