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• Food spoilage is caused due to molds, yeast , bacteria and enzymes.
• Preservation can Reduce wastage and facilitate export to high-value
• Methods of Preservation are : canning, freezing, pickling, curing and
• Drying was done by open Sun under the open sky.
• Slow process.
• Reduction in product quality due to insects and micro-organisms growth.
• Spoilage of product due to rain, wind, moist, dust ,birds & animals, fungal growth.
• Time consuming.
• Requires large area.
• Useful from energy conservation point of view.
• Occupies less area.
• Improves quality of product.
• Protects environment.
SOLAR DRYERS USEFUL FOR …
• Agricultural crop drying
• Food processing industries for dehydration of fruits and vegetables
• Fish and meat drying
• Dairy industries for the production of milk powder
• Seasoning of wood and timber
• Textile industries for drying of textile materials
SOLAR DRYING ESSENTIALS.
• A drying Chamber in which Food is
• A Solar collector that heats the air .
• Air flow system.
CLASSIFICATION OF FOOD DRYERS
Food is exposed to the sun and wind by placing in
trays, on racks or on the ground. Food is rarely
protected from Predators.
Food is enclosed in a container with a clear lid
allowing sun to shine directly in food. Vent holes allow
for air circulation.
Fresh air is heated in a solar heat collector & then
passes through food in the drier chamber. In this way
the food is not exposed to direct sunlight.
Combines the direct and indirect types; a separate
collector preheats air and direct sunlight adds heat to
the food and air.
Combines solar heat with other source such as fossil
fuel or biomass.
Uses electricity or fossil fuels as a source of heat and
• Consist of wood poles covered
with plastic sheet.
• The food to be dried is placed on a
rack above the ground.
• Main purpose is to protect from
dust, rain, wind and predators.
• Disadvantage of being easily
damaged by rain.
• Used for small scale food drying.
• Wooden box with hinged
• Inside is painted black & food
supported on mesh tray above
• Air flows into the chamber
through holes in front & exits
from vents at top of back wall.
• Rigid Rectangular frame, the length of
which being 3 times the width resting
on a support with an axis.
• This support is oriented north-south &
is high to allow the frame to be tilted
at 30 towards east in the morning and
in west in afternoon.
• The material for drying is kept on
number of wooden frame 100 x 50.
• Is a large wooden or metal box.
• The air passes through the air duct
into the drying chamber and over
drying trays containing food.
• The moist air is discharged through
the air vents at the top of column.
• As the air enters below the bottom
try, this tray will dry first.
• The idea of a greenhouse dryer is to replace the function of the solar collector by a green house.
• The roof and wall of this solar dryer can be made of transparent materials such as glass, fiber glass.
• The transparent materials are fixed on a steel frame support or pillars with bolts and nuts and rubber
packing to prevent humid air or rain leaking into the chamber.
• To enhance solar radiation absorption, black surfaces should be provided within the structure.
• Inlet and exhaust fans are placed at proper position within the structure to ensure even distribution of
• More appropriate for large scale drying.
NATURAL GREENHOUSE DRYER
• Earliest form was practically
realized at Brace research
institute glass-roof solar dryer.
• Consist of two parallel rows
of drying platforms.
• A fixed slanted glass roof over
the platform allowed solar
radiation over the product.
NATURAL-CIRCULATION SOLAR DRYER
CONSISTS OF :
• Transparent semi-cylindrical drying chamber with an attached cylindrical chimney, rising
vertically out of one end.
• While the other end is equipped with a door for air inlet and acess to the drying chamber.
• Drying operates by the action of solar-energy impinging directly on the crop within the
• Black absorbing curtain within the chimney absorb the solar radiation and are warmed.
• Heated air flows up the chimney to the outside of the dryer, fresh replenishing air is drawn
in other end of the dryer.
ADVANTAGES AND DISADVANTAGES TYPES OF SOLAR FOOD
+ Least expensive
- UV radiation can damage
+ Products protected from UV
- More complex and
expensive than direct sun
+ Less damage from temperature
+ Less damage from temperature
- UV radiation can
+ Ability to operate without sun
reduces chances of food loss
+Allows better control of drying
+fuel mode may be up to 40x
faster than solar
- May cause fuel
Behavior of agricultural crops during drying depends on :
• Size and shape
• Initial moisture content
• Final moisture content
• Bulk density
• Thickness of the layer
• Mechanical or chemical pre-treatment
• Temperature of grain
• Temperature, humidity of air in contact with the grain
• Velocity of air in contact with the grain
• The drying time is short under sunny conditions.
• Use of the solar energy as the only energy is recommended for smallscale dryers, where the risk of spoilage of big quantities of crops due
to bad weather is low.
• If large scale solar dryers are used for commercial purposes it is
strongly recommended to equip the dryer with a backup heater for bad
• Depends on shape of the crop
• Shape of crop should not be big to ensure that the preparation (
washing, slicing and pre-drying processing) of the product to be
dried can be completed within time duration.
• On other hand, it should be big enough to enable the user to generate
income and thus to create new jobs .
Types of food
Moderate time to dry
Fruits and vegetables
Short time to dry
In field or near
Vitamin loss from sun
Factors to consider in selecting a solar food dryer
USAGE OF SOLARASSISTED SPOUTED BED
DRIER IN DRYING OF
• Drying of peas provides effective and practical preservation in order to reduce the losses
• Direct sunlight is well-known & easy method of reducing the moisture content.
• But it is slow process.
• Polluted from dust, dirt, insects, animals or microbial combination, environmental
• Solar drying can be used as a safer and efficient method.
WHY SPOUTED BED DRYERS ??????
• Introduces high drying rate and shorter drying times due to continuous particleair contact.
• Even at lower drying temperatures, system can provide effective drying which
is important for heat-sensitive products.
• Also used for food products like wheat, corn, oats and cereal seeds.
To evaluate the effect of solar-assisted sprouted bed and open sun drying:
• Drying rates
• Quality of parameters of peas
• Bulk density
• Apparent density
• Bulk and internal porosity
• Solar-assisted spouted bed dryer :
Consisting Solar collector to get hot air
Air blower to provide spouting
Spouting column for drying of sample.
• Hot air was provided in the solar collector.
Dimensions : 1.80,1.66 and 2.60 m
Positioned to south with 40 angle.
Covered by glass sheet and well-isolated.
Base was covered with gravels to store energy and to keep air temperature
higher for longer time.
They were blackened to absorb solar radiation more.
To transport air from the solar collector into a sprouting column.
Ball valves were used to fix the air velocity.
• Spouted-bed column :
Diameter of 15cm and maximum spoutable bed depth has 100 cm.
Cone angle was 45 .
The nozzle was located at the center of the bottom of the cone with a diameter
of 2.54 cm.
OPEN SUN DRYING :
• Sample were placed in a cage to minimize the environmental
influences without interrupting the solar radiation.
• Sample were put on the mesh and cage was positioned parallel to
DRYER velocity as 0.60m/s.
• The flow rate of air was adjusted to minimum spouting air
• Air blower was operated half an hour before the experiment to adjust steady
temperature in spouting column.
• 250gm of sample was loaded into spouting column.
• Weight of the sample was measured every 30min during drying.
• Temperature of air in a solar collector, at the inlet and exit of the spouting column
and in surrounding were determined by thermocouples.
• During drying solar radiation was also measured by Pyranometer in
units of W/m2.
Open Sun drying:
• Samples were simply laid on a mesh were laid on a mesh under direct
• For each run,250gm of sample were weighted with an electronic
• In order to avoid environmental influences, it was put in a cage placed
parallel to solar collector.
• Weight change and outside temperature were recorded for every half an
• Dried samples were stored in closed plastic bags to avoid change in
moisture content for further analysis.
• Drying experiments were repeated 3 times.
DETERMINATION OF MOISTURE
• Moisture content of samples were determined by using a moisture
analyzer ( OHAUS, MB45, Switzerland).
• About 10 g of samples were put into the sample holder part and
dried until the constant weight was obtained.
• Moisture content data was given in kg water/kg of dry matter.
• The Color of samples was determined by a color reader
• The color values were expressed as CIE L* (Light and Dark), a*
(red and green) & b* (yellow and blue).
• A Container with a known volume and weight, which were 20ml and 24.9gm, respectively.
• Filled with the sample.
• The container was tapped without compressing the sample and the excess was removed by
sweeping the surface of container with a ruler.
• The weight of the sample with the container was determined by electronic balance.
• Tapping and weighing procedure continued until constant weight was reached.
• Then the bulk density was calculated by taking ratio of weight of sample and volume of container
• A 100ml burette with a graduation of 1 ml was filled with a certain amount of
• About 3g of sample was immersed in the water and the volume displaced by the
samples was recorded.
• Then, the apparent density was calculated as the ratio between sample weight
and displaced volume in kg/m3 .
SOLAR IRRADIATION AND
• Ambient temperature changed from 20 to 27.4 C.
• While temperature of collector part of solar dryer changed between 40 to 68 C during
• Outside temperature did not change so much, temperature in the solar collector part of the
solar dryer varied in accordance with solar radiation.
• Solar irradiation values ranged from 585 to 950 W / m2.
• Air temperature in Solar collector was almost the same during solar-assisted spouted bed
• Drying rate was much higher and therefore drying time was lower for drying in solarassisted spouted bed as compared to open-sun drying.
• Significant difference was found between drying methods in affecting bulk and apparent
densities, shrinkage, rehydration capacity of peas dried using solar-assisted spouted bed
dryer were higher than those dried in open air.
• There was no quality difference between solar-assisted spouted bed dried peas and open
sun dried peas in terms of color.
• Since the quality of peas dried by solar assisted spouted bed method were acceptable and
drying takes place in a shorter time, this method can be recommended to be used for
TABLE-LIKE SOLAR DRYING, TANZANIA
• Two designs proposed to women by the International Centre for Research om Women and the Tanzania Food and
• One dryer was made of wood, which is light weight, portable, expensive.
• Another was mud brick and less expensive than wooden dryer.
• Each dryer could produce 1.5 kg of dried vegetables.
• If vegetables were thinly spread to facilitate faster drying.
• On an average, the dryers were used three times a week, with drying times ranging from four to six hours per use
depending on type of vegetables and intensity of sun.
BANANA DRYER, BRAZIL
• Introduced by costa is operating in a mixed mode.
• Uses indirect heating through forced convection
• High yield & good quality
The system is composed of :
• Electrical Ventilator allowing an independent air flow rate
• A solar collector in which its characteristics and dimensions were determined according to
hot air flow rate and operating temperature
• Made of flexible PVC, in a cylinder shape with two cones at both ends.
• At the bottom, 50% of the collector is made of dark plastic in order to absorb heat.
• At the top transparent plastic, in which the solar rays penetrate.
Solar dryers are :
• It is more efficient
• It is safer
• They cant be used in cloudy weather.
• During fair weather, they can become so hot inside that they may
damage the crop.
• Serpil Sahin, Gulum Sumnu, Ferihan Tunaboyu, “Usage of Solar-assisted spouted bed
drier in drying of peas”
• S. VijayaVenkata Raman, S. Iniyan, Ranko Goic, “Renewable and sustainable Energy
reviews: A review of solar drying technologies”
• Roger G. Gregoire, “Understanding Solar dryers”
• Matthew G. Green, Dishna Schwarz, “ Solar drying technology for food preservation”
• Werner Weiss, Josef Buchinger, “ Solar Drying”