2. Importance of Fruits and Vegetables
Plays vital roles in keeping balanced diet.
Main sources of essential dietary nutrients such as vitamins,
minerals and fibre.
India is next to China, with respect to production of
horticultural products. Fruits: 93.7 MTVegetables: 176 MT
Largest producer of Fruits like papaya, mangoes & guavas and
the second largest producer of Vegetables like potatoes, green
peas, cabbage and cauliflower.
3. Nearly 4.6-15.9% wastage annually due to lack of modern
harvesting practices and inadequate cold chain infrastructure
(MoFPI).
In India, only 4 % fruits and 2 % vegetables are processed.
Owing to their short shelf life and increasing demand for novel
products, the dehydrated F & V gradually come into the sight
of consumers and producers.
About 20% of the world’s perishable crops are dried to increase
shelf-life and promote food security.
4. Drying: Removal of known quantity of moisture from the food to a pre-
determined level i.e. a particular moisture content
Dehydration: Removal of moisture to very low level i.e. bone dry weight
or
Drying and Dehydration
• Both are among the oldest methods of preservation
• Removes moisture, stops the growth of bacteria, yeasts & molds that normally
spoil food
• Slows down but doesn’t completely inactivate enzymes
Drying: Removal of moisture under natural condition such as sunlight and
wind such as open sun drying, shade drying etc.
Dehydration: Removal of moisture by application of artificial heat under
controlled conditions of temperatures, humidity and air flow
5. Drying is basically governed by heat and mass transfer
principles.
When a wet substance is heated to an appropriate temperature,
moisture vaporizes at or near the surface.
As the moisture is vaporated from the surface, more moisture is
transported from inside layer to the surface.
Moisture transmission within the product depends upon its
inner structure
Mechanical separation processes like filtration, settling and
centrifugation often needs drying as final step.
6. Factors affecting
Initial moisture content of the raw material
Composition of raw material
Initial load of the food kept in drier (Thin and Deep bed drying)
Size, shape and arrangement of stacking of the raw material
Temperature, relative humidity and velocity of heated air
Rate of heat transfer on the surface of the food
Pre-treatment of the raw material prior to drying (peeling,
blanching, sulphuring etc.)
7. Advantages
• Helps in food preservation by reduction in water activity.
• Reduction in weight and volume (4-10 times from its initial
fresh weight /volume).
• Reduction in space requirement, cost of packaging, storage and
transport.
• In comparison to other methods, drying is the cheap and
simplest method of preservation.
• Dried foods add variety to the diet and supply convenient ready
to eat foods to the consumers.
• Nutrient concentration is very high per unit weight of dried
product.
8. Increased availability of particular fruit/vegetable
variety
• Every cultivar is unique in its taste and nutritional
properties.
• Mango cv. Langra, Chausa (Northern India)
• Mango cv. Banganpalli, Neelum (Southern India)
• Pineapple cv. Queen Pineapple (Tripura)
• Vegetables like Fenugreek, Spinach, Curry leaves,
Bittergourd etc. are seasonal in North India
9. • Psychrometric charts (humidity charts) are helpful in
understanding the relationship between temperature and
humidity of the air-water vapor system at constant pressure.
• Absolute humidity: Weight of water vapor per unit weight of dry
air.
• Saturation humidity: The absolute humidity at which the partial
pressure of water vapor in the air is equal to the vapor pressure
of free water at the same temperature (100% relative humidity).
• Relative humidity: the ratio of the partial pressure of water
vapor in the air to the vapor pressure of free water at the same
temperature. Which is also the ratio of the absolute humidity to
the saturated humidity at that temperature.
• Measuring instrument: Hygrometer
Psychrometry (Humidity)
10. • Wet bulb temperature: The equilibrium temperature reached by
an evaporating surface when the rate of heat transferred to the
surface by convection is equal to the rate of heat lost by
evaporation. It can be measured by a thermometer whose bulb is
covered by a wick saturated with water.
• Dry bulb temperature: The actual temperature of the air
measured by an ordinary thermometer.
• Dew point: It is the temperature to which a water vapor-air
system must be cooled to become saturated. The air holds the
maximum amount of water without condensation taking place.
• Measuring instrument: Thermometer
Psychrometry (Temperature)
11. Type of moisture
Equilibrium moisture: Product neither gains/looses moisture at
particular temperature and RH.
Free moisture: Liquid in excess of the equilibrium moisture
content at a given temperature and humidity. It can be easily
removed owing to the vapor pressure difference between the
product and ambient conditions.
Unbound moisture: Unbound moisture exerts a vapor pressure
equivalent to that of the free liquid.
Bound moisture: Bound moisture in a food material is that liquid
which is retained in a manner that exerts a vapor pressure less
than that of the free liquid at the given temperature.
15. Experimental procedure
While taking observations:
• Initially the time interval has to be kept small i.e. 5 min
• Later can be gradually increased to 10, 15, 20, 30, 45, 60, 75, 90 min
16. Moisture calculations
Where,
M is moisture content (%)
W is total weight
D is dry weight
MC (d.b.) = MC (w.b.)
1-MC (w.b.)
MC (d.b.) = 0.25
1-0.25
= 0.33%
Conversion of wet into dry basis
MC (w.b.) = MC (d.b.)
1+MC (d.b.)
MC (w.b.) = 0.33
1+0.33
= 0.25%
MC (d.b.)˃ MC (w.b.)
17. Calculations
Time (min) Weight of tray (kg) Moisture content (decimal)
0 2.00 0.90
5 1.95 ?
10 1.90 ?
15 1.80 ?
20 1.70 ?
30 1.50 ?
Where,
Wf = Final weight of the tray
Wi= Initial weight of the tray
MCi= Initial moisture content
MCf= Final moisture content
Wf = Wi {(1-MCi)/(1-MCf)}
Intermediate moisture content
1.95 = 2 (1-0.90)/(1-MCf)
MCf = 0.897
19. • Properties of drying air (dry bulb temperature, RH, velocity of
air and the surface heat transfer coefficient)
• Properties of food (moisture content, surface to volume ratio,
surface temperature) and rate of moisture loss.
• Fat/fibre content (higher content generally results in slower
drying, as water is trapped with in the food).
• Method of preparation of food (cut pieces lose moisture more
quickly than losses through skin).
Factors affecting Drying rate
21. 1. Predehydration treatments
2. Dehydration
3. Post dehydration treatments
Dehydration process
Predehydration treatments
Raw Material
Colour preservation
Raw material preparation
(Selection, Sorting, Washing, Peeling, Cutting, Blanching)
22. Shrinkage: Slow drying results in development
of internal stress/ruptures/compress and
permanently distort the cells to give a shrivelled
appearance. Such food material on rehydration
absorbs water more slowly.
Case hardening: Formation of impervious layer
over the surface of a dried food product (rich in
solutes and when initial drying temperature is
very high).
Browning: Change in the colour of food material
to light to dark brown colour due to
enzymatic/non-enzymatic browning
Effects of Drying on Foods
23. Preparatory steps: Washing, Coring (removal of stone/seed), Peeling
Fruits can be halved or sliced and some left whole
Thin, uniform, peeled slices dry very fast as the surface area for
drying is increased
Raw material preparation
24. Fruits, like apple, pear, banana, and pineapple are usually peeled
before dehydration.
Vegetables normally peeled include beets, carrots, potatoes, onions,
and garlic.
Prunes and grapes are dipped in an alkali solution to remove the
natural waxy surface coating which enhances the drying process
Sodium carbonate (0.5% or less) at 93.3-100oC facilitates drying
by forming fine cracks in the skin
Okra, green peas, beans, cabbage: Sodium bicarbonate is added to
the blanching water which raises the pH
25. Preservatives:
Ascorbic acid
Citric acid
Salt
Sugar
Sodium metabisulphite
Sulphuring
• Sulphur dioxide fumes acts as a effective additive to prevent
non- enzymatic browning
• Acts as a antioxidant in reducing loss of ascorbic acid and
carotene
Colour preservation
26. Types of Dehydration process
Generations Types of dryers Principle
First
generation
Solar, Tunnel,
Cabinet/Tray,
Rotary flow dryers
Use of hot air (convection), hot
surfaces (conduction) and
radiation
Ideal for solid foods, grains
Second
generation
Drum dryers
Spray dryers
Suitable for pastes,
suspensions, liquids, slurries
and concentrated solutions of
solids produce food powders
and flakes
27. Generations Types of dryers Principle
Third
generation
Osmotic dehydration
Freeze drying
Longer duration required
Better in maintaining
structural and quality
issues
Fourth
generation
Fluidized bed drying
Microwave drying
Electrohydrodynamic
drying
Heat pump drying
Reducing atmosphere
drying
Refractance window
drying (RW™)
These are latest
developments in
dehydration
28. Sun drying
Fruits are safe to dry due to high acid and sugar content
Vegetables should not be dried outside as they need constant
temperature & airflow
Vegetables are low in sugar and acid which increases the risks
for food spoilage
Minimum temperature of 30°C and RH below 60% is best for
sun drying
Cover to protect against insects/pests
Prunes, grapes, dates, figs, apricots, and pears
Simple process and requires small capital investment
29. Limitations
Uncontrolled and non-uniform drying
Dependent on the availability of sun energy
Large number of unskilled laborers
Very large area for drying
30. Solar drying
Also uses the sun as the heat source
Need to stir and turn food several times a day
Ventilation speeds up the drying time
Shorter drying times reduce the risks of food spoilage or mold
growth
31. Tray/Cabinet drying
Efficiently designed and contain electric element for heating, a fan
and vents for air circulation.
Heated air is ventilated through a mass of wet materials by
convection and it evaporates the moisture present in the material.
Initial temperature is generally kept at 43°C which is then gradually
increased to 60-66°C for vegetables and 66-71°C for fruits.
Drying at higher temperature causes ‘Case hardening’ preventing
moisture migration from the center to the outer part of the slice.
To save energy, sometimes the exit air from dryer is recirculated
(Recirculatory dryer).
33. Advantages Disadvantages
Low operating cost
Good heat transfer
Easy to clean
Enhanced economics
(optimum tray size, less
accidental spillage)
Time consuming
Required more labour to
load and unload
35. Drum drying
Used for drying high viscous liquid, such as baby foods, vegetable
puree, mashed potatoes, cooked starch
Heat is supplied to wet product by conduction
Video
36. Advantages Disadvantages
Rapid drying
Compact
Short heating time
Drying temperature can be
reduced by enclosing a
vacuum jacket
Product in flake form
Critical operating conditions
• Feed rate
• Film thickness
• Drum rotational speed
• Drum temperature
Not applicable for materials
that do not adhere to metal
37. Spray drying
A fine spray of liquid is injected into a blast of hot air in a chamber
Water evaporated within a seconds, leaving the solid part of the product
behind in powdered form
Major fruit juices dried: Mango, banana, orange, guava, bayberry,
watermelon, pineapple and vegetable juices: Tomato, sweet potato,
black carrot.
Commercial dryers can be very large of the order of 10 m diameter and
20 m high.
It requires addition of drying agents like matlodextrin, liquid glucose
etc. to increase the glass transition temperature.
38. Feed
Drying gas (cold)
Drying gas (warm)
Product
Pump (defining feedrate)
Aspirator
Heater
Filter
Drying gas in
Nozzle
Drying chamber
Collecting electrode
Grounded electrode
Process affecting parameters are :
Inlet air temperature
Relative humidity of air
Outlet air temperature
Atomizer speed
Video
39. Advantages Disadvantages
Dry particle size can be easily
controlled
Short residence time is required
Heat sensitive products can be
spray dried easily at relatively
high inlet temperatures
Minimum flavour loss
High product quality
The equipment is very
costly and bulky
Cleaning is time consuming
A lot of heat is wasted as
thermal efficiency is low
40. Concentrated solution Product
Water
Solute
Product’s own solutes
(minerals, sugars,
organic acids, etc.)
Osmotic dehydration
Removal of water by immersing the food in a solution of salt or
sugars of high osmotic pressure. Water is transferred from the food
to the solution by virtue of the difference in osmotic pressure.
41. Sugar syrup for fruit slices and salt (sodium chloride) or brine
with vegetables
60% aqueous solution of Saccharose or 25% aqueous solution
of sodium chloride normally used
About 50% water is removed by this treatment. Further drying
carried in tray dryer.
Suitable for fruits like Apple, Banana, Citrus, Mango, Papaya,
Peach, Pineapple etc.
Vegetables like Carrot, Onion,
Potato, Pepper, Tomato, Sugar
beet etc.
42. Advantages Disadvantages
Minimized heat damage while
drying
Least discolouration of fruits by
enzyamtic dehydration
Increase retention of volatile,
flavour and aroma
Improved textural quality and
lower energy consumption than
air drying
Time consuming
Sugar coating is not
desirable in some
products and quick
rinsing in water may be
necessary after the
treatment
43. Vacuum drying
Important process for heat sensitive materials.
Rapid drying at low temperature
They consists of a heavy shell to withstand a high vacuum
and a series of shelves heated by hot water, steam, hot oil.
Low temperature can be used under vacuum for certain
methods that might discolour or decompose at high
temperature.
Vacuum drying is considered too costly for large scale
adoption.
44. Freeze drying
Dehydration takes place in three stages.
First, the moisture of product is removed by
formation of ice crystals.
These ice crystals later on removed by
sublimation.
After removal of all the ice, the food may contain
small amount of water which may be removed by
evaporation.
Size/shape of original food product can be
retained and shrinkage is negligible.
Nutritional losses due to heat damage are
minimized.
Good retention of trace volatile flavour and aroma
compounds.
High capital investment, High processing costs
Video
45.
46. Fluidized bed drying
Product are dried under fluidized
condition
Warm air is blown upwards directly
underneath the food, causing it to
flow and remain separated
Used for the materials which have
high initial moisture content and
are lighter
Aonla, Pea, Coconut, Soyabean etc.
Video
47. Advantages Disadvantages
High rates of moisture removal
due to excellent gas-particle
contact which results in high
heat and mass transfer rates
It facilitates the drying of
thermo labile substances, since
contact time is short
It can be used either as batch
type or continuous type
The free movement of
individual particles eliminates
the risk of soluble material
migrating as may occur in
static bed
High energy consumption
Poor fluidization and low
flexibility especially if the feed
is too wet
The turbulence of the
fluidized state of granules may
cause attrition of some
materials resulting in
production of fines which can
be avoided by using suitable
binding agent.
48. Microwave drying
Uses electrical energy in the frequency range of 300 MHz to 300
GHz, with 2450 MHz being the most commonly used frequency.
In microwave drying, when the material couples with microwave
energy, heat is generated within the product through molecular
excitation.
The critical next step is to immediately remove the water vapour
achieved by passing air over the surface of the material hence
combining processes to form what is called “microwave convective
drying”.
Air temperature can be varied to shorten the drying time and is
dependent on the product’s characteristics.
50. Advantages Disadvantages
Fast volumetric heating
Higher drying rate
Short drying time
Enhanced quality of the product
Reduced energy consumption
Lower operating costs
High initial costs for
industrial scale dryers
Partial loss of aroma and
negative sensory changes
Product texture may be
affected
Specific sample size and
shape may be required for
effective drying
Microwave drying
51. Refractance window drying
When water is placed over a heating source,
infrared energy is transferred throughout the
water by convection. The heat energy then
radiates from the water, primarily through
evaporation
When water is covered by a transparent
membrane, such as plastic, and placed over a
heating source, evaporation and its associated
heat loss are blocked or "refracted," and only
conduction occurs. The plastic membrane acts
like a mirror reflecting the infrared energy back
into the water
52. But when a moist raw material is placed on the
plastic membrane's surface, the water in the
material creates a "window" that allows for the
passage of infrared energy through the material.
Heat behaves as if there were no membrane
present, and is directly transferred to the water
remaining in the material
In moments, the water in the material on
the plastic membrane's surface evaporates,
and the "window" of infrared energy closes
and "refracts" back into the heated water
source, no longer exposing the material to
heat.
53. • Thermostatic water bath (60 litre)
• Tray (Stainless steel)
Perforated Steel tray was fitted at the bottom for stability to
the film
Mylar™ film (250 µm) was placed for drying of pulp
Batch refractance window drying system
54. Pulp spread on tray for drying Batch assembly of RW drying system
Video
55. Vapour + Air
Inspection window
Hot water
Water heater
tank
Exhaust
Polyester
membrane
Dried product
Cold water
tank
Cold water
Continuous refractance window drying system
56. Comparison of different dryers
RW dried Mango flakes
Type of dryer Product temperature
Rotary dryer About 175˚C
Spray dryer 80-120˚C
Drum dryer 120-130˚C
RW dryer 70-72˚C
57. Advantages Future work to be done
Water is used as heating
medium
Higher drying rate
Better retention of heat sensitive
vitamins, color, phytochemicals
content and antioxidant activity
as compared to freeze-dried
products.
Reduced energy consumption
Lower operating costs
Optimization of RW drying
in terms of energy
consumption and process
design.
Development of a low cost
and compact RW system is
required to conduct small
scale studies.
It has been successfully evaluated for drying of green asparagus,
paparika, mango, tomato, strawberry puree, aloe vera, sapota
pulp, pomegranate puree, carrot puree, kiwifruit slices etc.
58. Heat pump drying
Hot air (1) is circulated over a
product belt inside the dryer
During this cycle the temperature of
the air decreases as its humidity
increases
About 1/3 of the cool humid air (2)
is circulated over the evaporator
The evaporator cools the air further
down below the condensation
temperature (3)
Below this temperature the air will be
dehumidified.
The cool, dry air (4) is than mixed
with circulation air from the dryer
The mixture (5) is heated to the
desired process temperature inside
the condenser and can be reused in
the dryer cycle (1)
Hot air
Cool humid air
Cool dry air
It has been successfully evaluated for drying of mango slices, dried apple, guava
potato pieces and honeydew melon.
59. Foam mat drying
Suitable technique for those F & V extracts which are heat
sensitive, high sugar content and viscous foods, which are
difficult to dry and sticky under relatively mild conditions.
Liquid food should be capable of foaming stable foam.
The foam is usually 2-3 mm thick and dried rapidly in two
stages by parallel and then counter current air flows.
Vegetable (solubilized soya protein) protein, gum and various
emulsifier (glycerol, monosterate and propylene glycol) are
used as foaming agents.
Generally, 3 types of foam mat dryers are available: Continuous
Foam Mat Dryer, Microwave Foam mat Drying and Foam Mat
Freeze-Drying.
61. Advantages Disadvantages
Rapid drying and low temperature
of products results in high quality
product
Foam drying is three times faster
than drying a similar type of liquid
Cheaper than vacuum, freeze and
spray drying method
Uniform heating, storage reliability
and simplicity
Rehydration of the dried product is
faster
High capital cost
Large surface area is
required for high
production rate
It has been successfully evaluated for drying of yacon (tuber crop) juice powder,
mango pulp, papaya powder, cherry powder, tomato powder.
62. Reducing atmosphere drying
The undesirable oxidation reactions occurred during the drying
process have to be minimized to preserve the nutritional and
sensorial values of foods; this goal can be achieved with a
process performed under as low as possible oxygen condition.
The air used in conventional methods as a drying atmosphere
could be replaced with some gases which are composed of a
reducing gas (hydrogen sulfide, hydrogen or ammonia),
microbicide gas (carbon dioxide) and inert gas (nitrogen,
helium or argon) to exclude oxygen implicated in the
deteriorative reactions
64. Advantages Disadvantages
Avoids oxidation reactions
Better product quality
Costs are high
Not easy to control
Reducing atmosphere drying
65. It is the study of the motions of ionized particles or molecules and
their interactions with electric fields and the surrounding fluid
It is an energy efficient non-thermal technology, suitable for the
treatment of heat-sensitive foods and food ingredients
EHD phenomenon is based on the high-voltage ionic discharge
between electrodes
Removal of water from a material placed in the strong electric field
Loss of water vapour takes place because of the molecular
reorientation in the direction of an electric field
Drying takes place by lowering the entropy which in turns lower the
product drying temperature.
Successfully evaluated for tomato, spinach, shrimp, radish,
cucumber, grape pomace, carrot, banana, apple.
Electrohydrodynamic drying
66. Principle of EHD Drying
Rapid rate of evaporation and exothermic interaction of electric field
with dielectric materials
68. Hybrid/combination drying
Hybrid drying techniques are becoming common since the
combined technology receives the benefits of individual process.
The number of combinations possible is vast and as technology
continues to improve more will be developed.
Adding a micro wave system to a fluidized bed system decreases
drying time while the fluidization improves drying uniformity,
thus reducing the burning.
Combination of hot air drying and freeze drying increased the
quality of dehydrated fruits and vegetables.
Combination drying with an initial conventional drying process
followed by a finish microwave process has proven to reduce
drying time while improving product quality and minimizing
energy requirements.
69. Post dehydration steps
Cool product 15-30 minutes before packaging
Don’t pack too soon or moisture buildup could occur
Don’t wait too long as it could pick up moisture from the air
Packaging in air tight plastic/glass jar
Shake jars to separate pieces and check for moisture
(condensation on sides of bag/jar)
If there is condensation, return fruit to dehydrator for more
drying
There is a chance mold will have already started growing in too-
moist fruit; discard if you find mold
71. EDP training on drying of Onion, Ginger and Garlic
• Training duration: 2-3 days
• Payment basis
• Hands on Training
• Technical guidance
• Certificate Issued
Contact email:
arisciphetabohar1998@gmail.com
72. Conclusion
Given the improvement in the quality of dehydrated foods, along
with the increased focus on instant and convenience foods, the
potential of dehydrated fruits and vegetables is greater than ever.
Almost all drying technologies possess some advantages, but also
have certain limitations that may lead to longer drying times,
quality degradation or non-uniformity in dried products as well
as increased energy consumption.
Due to high cost, using single unit operation to dry the produce is
not always cost effective.
In recent decades, several multi-stage dryer systems utilizing
various heat transfer mechanisms applied concurrently or
sequentially have become the dryers of choice in various
industries.
73. Conclusion
Care should be taken when assessing the energy costs as
electrical energy costs much more.
More research and development are needed for reliability.
Scale-up and cost estimates need to be examined carefully
when selecting hybrid dryers over conventional ones.
There are no drying cost estimates available since it is not yet
possible to confidently scale up recent drying techniques.
Validated mathematical models of hybrid drying and specific
dryer types are needed to encourage pilot scale testing of
innovative hybrid dryers.
