The document discusses various aspects of dehydrating fruits and vegetables including: 1. Dehydration involves applying heat under controlled conditions to evaporate water from foods and can be used to preserve foods. 2. Common dehydration methods described are solar, spray, convective (cabinet, tray, etc.), freeze, fluidized bed, infrared, vacuum, flash and osmotic drying. 3. Dehydration is widely used in the food industry to reduce weight and space needs for storage, avoid refrigeration, and extend shelf life of perishable foods. However, dehydrated foods can lose nutrients over long storage and require rehydration before eating.

1. Dehydration of Fruits
and vegetables
Aligarh Muslim University

2. Contents
Introduction
Objectives of
dehydration
Methods of
dehydration
Applications of
dehydration
methods in Food
industry.
References
Conclusion
Disadvantages

3.  Dehydration (or drying) is defined as ‘the application of heat under controlled
conditions to remove the majority of the water normally present in a food by
evaporation.
 Dehydration has been used widely for this purpose since ancient times, the
earliest known practice is 1,2000 B.C. by inhabitants of the Middle east and
Asia regions.
 When hot air is blow over a wet food, heat is transferred to the surface and
latent heat of vaporization causes water to evaporate.
 Water vapors diffuse through a boundary film of air and is carried away by the
moving air. This create a region of lower water vapor pressure gradient which is
established from the moist interior of the food to the dry air. The gradient
provides the driving force for water removal from the food.
1. What is
Dehydration ?
Fig 1.1 Mechanism of dehydration

4. Improving properties
To improve or keep the good properties
of a material, e.g. flow ability,
compressibility.
Moisture Removal
To avoid or eliminate moisture without
damaging the quality.
Diversifying the food
Diversify the supply of foods with
different flavors and textures, thus
offering the consumers a great choice
when buying foods.
Food preservation
Destruction of micro-organisms to
increase the shelf life of the food.
2. Objectives of Dehydration

5. Strawberry
Kiwi
Apple
Pineapple
Mango
Potato
Garlic Turmeric
Spring Onion Carrot
Dehydrated Fruits and Vegetables
Dehydrated foods should have < 0.6 water activity.

6. Dehydration
Convective drying
Solar Drying Spray Drying
Freeze Drying
Osmotic
dehydration
Flash drying
Infrared Drying Vacuum Drying
3. Methods of Dehydration

7. 3.1 Solar
Drying
• Solar drying is a non-polluting process and
uses renewable energy.
• Abundant energy source that cannot be
monopolized.
• The simplest of the sun dryers consists of
an open surface on which wet food
product is spread on a thin layer (usually
1–5 cm thick) and is dried by the energy
from the sun.
Picture Source:
https://rbdc.com.pk/portfolio-items/solar-dryer-training/
Fig 3.1.1 Schematic
diagram of Solar dryer
Fig 3.1.2 Solar dryer

8. 3.2 Spray Drying
 Spray drying is used to dry liquid
products. The product to be dried
is sprayed into a stream of heated
air in an atomized form.
 The atomization may be done by a
rotary or a nozzle atomizer.
 Dried product is separated from
the airstream using cyclone
separators, settling chambers, or
bag filters.
Picture Source:
https://www.freund-vector.com/products/vsd-800-pilot-and-production-spray-dryer/vsd-200/
/
Fig 3.2.2 Schematic diagram of Spray dryer
Fig 3.2.1 Spray dryer

9. 3.3
Convective
drying
Cabinet
dryer
Rotary
Flow
conveyer
dryer
Tunnel
dryer
Tray
dryer
Truck
tray
dryer

10. • Cabinet dryers are usually small, insulated units with; A heater, circulating fan, and shelves to
hold the product to be dried.
• General procedure is to force heated air over multiple trays. Material flows onto each shelf
from the one above, forming a pile.
• The rotation of the tray (shelf) assembly carries the freshly formed pile past a stationary blade
set to level the pile to fill the tray at a uniform height.
• At the end of revolution, another stationary blade wipes the tray and material flows onto the
shelf below. Dried product is discharged through a chute in the bottom of the housing.
• Used for fruits ( grapes, dates, apples), Vegetables (onion, cabbage).
Picture Source:
https://worldindustrialreporter.com/tray-dryer-turbo-dryer/
Fig 3.3.1.2 Turbo dryer
Fig 3.3.1.1 Schematic diagram of Turbo dryer

11.  In freeze drying, water is removed at low temperatures (about 10°C) by sublimation rather than
evaporation. Therefore, freeze drying is done under vacuum (at absolute pressures of about 2 mm Hg) on
frozen products.
 Used for coffee, fruits, juice, vegetables, herbs, dairy, etc..
3.4 Freeze Drying
Picture Source:
https://cryovit.com/en/freeze-dryers-lyophilizers-from-the-manufacturer/industrial-freeze-dryer-model-cc-300/
https://in.pinterest.com/pin/268879040230530282/
Fig 3.4.1 Process of Freeze drying Fig 3.4.2 Freeze dryer

12. 3.5
Fluidized
bed drying
 Evenly distribute the air at a uniform
velocity around the bed of material.
 The food material is supported on a
perforated metal at the bottom of a drying
chamber.
 The hot air is forced through the material
at high velocities such that the particulate
material is fully suspended in the hot
airstream.
 All particles are completely exposed to
drying air, resulting in high rates of heat
transfer.
Picture Source:
http://unitedpharmatek.com/Processing/Fluidized-Bed-Drying/Lab-Fluid-Bed-Dryer.html
Fig 3.5.1 Schematic diagram of
Fluidized bed dryer
Fig 3.5.2 Fluidized bed dryer

13. 3.6 Infrared Drying
 In infrared drying, the solid food is exposed to a source of
infrared heating increasing the temperature of its surface.
 Infrared radiative heating produces heat fluxes of about 4.5
to 12.0 K watt/m2.
 This brings dehydration of fruits and vegetables in shorter
periods at low temperature of 28°C.
 Combined convective and radiative drying is done for heat
sensitive foods.
Picture Source
https://www.sciencedirect.com/science/article/abs/pii/S0023643805000216
https://foodtechprocess.com/en/dryers/358-infrared-dryer-idu.html
Fig. 3.6.1 Schematic diagram of Infrared dryer
Fig 3.6.2 Infrared dryer

14. 3.7 Vacuum Drying
 Vacuum drying provides opportunity to remove water from
food products at significantly lower temperatures than those
required at atmospheric pressure conditions.
 The object to be dried is placed in an enclosed container to
vent air and reduce pressure with the vacuum pump in order
to artificially increase the water vapour partial pressure.
 The dryer door is tightly shut and steam is passed through the
space between trays and jackets so that the heat transfer
occurs by conduction.
 Water vapour from the feed is sent into the condenser and
after drying vacuum pump is disconnected and dried product
is collected from the trays.
Picture Source:
https://pharmacygyan.com/vacuum-dryer/
https://gstshykj.en.made-in-china.com/product/SjtmsbwCwiVW/China-Fruits-and-Vegetables-Vacuum-
Drying-Machines-Industrial-Vacuum-Tray-Dryer-Vacuum.html
Fig 3.7.1 Schematic diagram of Vacuum dryer
Fig 3.7.2 Vacuum dryer

15. 3.8 Flash Drying
 Flash dryers consisting of inert media have
been employed at pilot scales to dry
slurries and suspensions, which are sprayed
onto them.
 The small particles are fluidized by the
high-speed steam of the drying gas and are
circulated to the top of the dryer into the
classification chamber.
 Classification is initiated by centrifugal
force, with large particles returning to the
drying chamber for further size reduction
and drying.
Picture Source:
https://cassavaindia.com/products/starch/flash-dryer/ Fig 3.8.2 Schematic diagram of Flash dryer
Fig 3.8.1 Flash dryer

16.  Osmotic dehydration is based on the principle that when cellular materials are
immersed in a hypertonic aqueous solution, a driving force for water removal sets
up because of the higher osmotic pressure of the hypertonic solution. .
 Cut fruits and vegetables are immersed in concentrated solutions of sugar and
salt.
 Due to the difference in conc. of these foodstuff and the solution, it looses some
quantity of water to the external solution.
 About 50% of water is removed by this treatment and further drying is done by
other dehydration methods.
Picture Source:
https://www.mdpi.com/2076-3417/11/3/1269/htm
Fig 3.9.2 Typical Osmotic dehydration system

17. Modern production technology has greatly
increase yields of fruits and vegetables.
One-fourth of highly perishable fruits and
vegetables are not consumed due to
spoilage during storage transportation and
processing.
Proper processing minimizes post harvest
losses.
Dehydrated foods has less weight and
required less space for storage.
Dried and dehydrated foods are less costly
to produce.
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5
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4. Applications of
dehydration
methods in food
industry
Avoid the need for use of refrigeration
systems.

18. This process takes some time for the food
to be completely dry.
The taste and appearance of food may
change.
.
Dehydrated food is prepared well and
stored for a very long time, it can lose its
nutrients, thus decreasing the quality of
food.
Dehydrated foods has excessive calories,
which might lead to weight gain.
Typically requires preparation to
reconstitute food.
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5. Disadvantages
of dehydration
It may increase the cost of production.

19.  Drying is an industrial preservation method in which water content and water activity of the fruits and vegetables are decreased by heated
air to minimized biological, chemical and microbial deterioration.
 Dehydration is a process of simultaneous heat and mass transfer. To obtain the dehydrated product of high quality, the drying process
should be such that it allows effective retention of color appearance, flavor, taste and nutritive value, comparable to fresh vegetables.
 The technique of drying is probably the oldest method of food preservation practiced by mankind for the extension of food shelf life.
 The use of artificial drying to preserve agricultural commodities is expanding, creating a need for more rapid drying techniques and
methods that reduce the large amount of energy required in drying processes.
5. Conclusion

20. 6. References
1. Nowacka M, Dadan M, Tylewicz U. Current Applications of Ultrasound in Fruit and Vegetables Osmotic Dehydration
Processes. Applied Sciences. 2021; 11(3):1269. https://doi.org/10.3390/app11031269
2. Guiné, R.P., & Dets, C. (2018). The Drying of Foods and Its Effect on the Physical-Chemical, Sensorial and Nutritional
Properties.
3. M. Shafiur Rahman. (2007). Dehydration in food processing and preservation.
4. Jayas, Digvir. (2016). Food Dehydration. 10.1016/B978-0-08-100596-5.02913-9.
5. Ahmed.N, Singh.J,Chauhan.H, Gupta.P, Anjum.A, Kour.H(2013) Different Drying Methods: Their Applications and Recent
Advances. International Journal of Food Nutrition and Safety, 4(1): 34-42.
6. Renee Boyer , Using Dehydration to Preserve Fruits, Vegetables, and Meats. publication 348-597
7. Sagar V. R. and Kumar P(2010) Recent advances in drying and dehydration of fruits and vegetables: a review. J Food Sci
Technol 47(1):15–26
8. Barbosa-Cánovas, G.V., Vega-Mercado, H. (1996). Osmotic Dehydration. In: Dehydration of Foods. Dehydration of Foods.
Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-2456-1_8

21. Thanks