Solar food drying

SOLAR FOOD DRYING

Rakhi Vishwakarma

B.E Chemical

Roll no 78

FOOD PRESERVATION

• Food spoilage is caused due to molds, yeast , bacteria and enzymes.

• Preservation can Reduce wastage and facilitate export to high-value

markets.

• Methods of Preservation are : canning, freezing, pickling, curing and

drying.

NATURAL DRYING

• 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.

SOLAR DRYERS

• 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

dried.

• A Solar collector that heats the air .

• Air flow system.

DryingChamber

Airflow

SolarCollector

CLASSIFICATION OF SOLAR DRYERS

CLASSIFICATION OF FOOD DRYERS Classification Description

Open air Food is exposed to the sun and wind by placing in trays, on racks or on the ground. Food is rarely protected from Predators.

Direct Sun Food is enclosed in a container with a clear lid allowing sun to shine directly in food. Vent holes allow for air circulation.

Indirect Sun 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.

Mixed Mode Combines the direct and indirect types; a separate collector preheats air and direct sunlight adds heat to the food and air.

Hybrid Combines solar heat with other source such as fossil fuel or biomass.

Fueled Uses electricity or fossil fuels as a source of heat and ventilation.

TENT DRYER • 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.

Tent dryer

BOX DRYER • Used for small scale food drying.

• Wooden box with hinged transparent lid.

• Inside is painted black & food supported on mesh tray above dryer.

• Air flows into the chamber through holes in front & exits from vents at top of back wall.

SEESAW DRYER • 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.

Seesaw dryer

CABINET DRYER • 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.

Cabinet dryer

GREENHOUSE DRYER• 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

the dryer.

• 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 dryer.

• 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 DRYERS

Classification Advantages Disadvantages

Direct Sun + Least expensive

+ Simple

- UV radiation can damage food

Indirect Sun + Products protected from UV

+ Less damage from temperature

- More complex and expensive than direct sun

Mixed mode + Less damage from temperature extremes

- UV radiation can damage food

- Complex

Hybrid + 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

- Expensive

- May cause fuel dependence

NON-TECHNICAL ASPECTS

DRYING BEHAVIORBehavior 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

WEATHER CONDITIONS

• The drying time is short under sunny conditions.

• Use of the solar energy as the only energy is recommended for small-

scale 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

weather.

Sunshine

High

Hot

Dry

Direct Small

Humid

Moderate

Dry

Direct Small

Humid

Cold

Dry

Humid

Add Collector

area

Indirect

Large

Low

Add auxiliary heat source

CAPACITY

• 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 .

SELECTION, CLEANING & PRE-TREATMENT

• Selection ( Fresh, undamaged )

• Cleaning ( washing & disinfection )

• Preparation ( peeling, slicing )

• Pre-treatment ( Sulfurization, Salting )

• Drying

• Packaging

• Storage or sale

Types of food

Grains

Short time to dry

In field or near

preparation area

Portable dryers

Moderate time to dry

Permanent dryers

Factors to consider in selecting a solar food dryer

USAGE OF SOLAR-ASSISTED SPOUTED

BED DRIER IN DRYING OF PEAS….

• Drying of peas provides effective and practical preservation in order to reduce the losses

after harvest.

• 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

conditions.

• Solar drying can be used as a safer and efficient method.

• Among different types of dryers, spouted bed dryers may be more efficient for drying

peas.

WHY SPOUTED BED DRYERS ??????

• Introduces high drying rate and shorter drying times due to continuous particle-

air 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.

OBJECTIVES….To evaluate the effect of solar-assisted sprouted bed and open sun drying:

• Drying rates

• Quality of parameters of peas

• Color

• Bulk density

• Apparent density

• Bulk and internal porosity

• Microstructure

• Shrinkage

• Rehydration

MATERIALS AND METHOD

MATERIALS :

• Peas

• Solar-assisted spouted bed dryer :

Consisting Solar collector to get hot air

Air blower to provide spouting

Spouting column for drying of sample.

EXPERIMENTAL SET-UP • 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.

• Blower:

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

solar collector.

DRYING OF PEAS SAMPLE IN

DRYER & OPEN SUN.

SOLAR-ASSISTED SPOUTED DRYER • The flow rate of air was adjusted to minimum spouting air velocity as 0.60m/s.

• 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

sunlight.

• For each run,250gm of sample were weighted with an electronic balance

.

• 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

hour.

• 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 CONTENT

• 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.

ANALYSIS OF DRIED SAMPLES

COLOR

• The Color of samples was determined by a color reader

(Minolta,CR10,Osaka,Japan).

• The color values were expressed as CIE L* (Light and Dark), a*

(red and green) & b* (yellow and blue).

BULK DENSITY • 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 in kg/m3.

APPARENT DENSITY

• A 100ml burette with a graduation of 1 ml was filled with a certain amount of

water.

• 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 TEMPERATURE

• Ambient temperature changed from 20 to 27.4 °C.

• While temperature of collector part of solar dryer changed between 40 to 68°C during

experiments.

• 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.

DRYING

QUALITY OF DRIED PRODUCT

Pictures of pea ( A : Open sun dried ; B : Solar-assisted spouted bed drier

PHYSICAL PROPERTIES OF FRESH PEAS AND PEAS DRIED USING

DIFFERENT METHODS Physical

Properties Fresh Open sun dried Solar-assisted

spouted bed dried

Bulk Density(kg/m3)

385 ± 0.00 610 ±13.23 407 ± 17.08

Apparent Density(kg/m3)

750.0 ± 0.00 1023.8 ± 41.20 754.8 ± 9.60

Diameter(mm) 7.45 ± 0.47 5.82 ± 0.71 6.53 ± 0.43

Shrinkage (%) - 50.24 ± 16.16 31.90 ± 11.29

Bulk porosity 0.49 ± 0.000 0.40 ± 0.018 0.46 ± 0.023

Internal Porosity 0.46 ± 0.021 0.24 ± 0.014 0.32 ± 0.036

Rehydration Capacity (%)

- 290.00 ± 4.00 306.50 ± 22.83

• Drying rate was much higher and therefore drying time was lower for drying in solar-

assisted 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

drying of peas.

APPLICATIONS & DEVELOPMENTS

FRUITS & VEGETABLES DRYING

TABLE-LIKE SOLAR DRYING, TANZANIA • Two designs proposed to women by the International Centre for Research om Women and the Tanzania Food and

Nutrition center.

• 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.

Scheme of the solar dryer for bananas

Pictures of the solar banana dryer

CONCLUSION Solar dryers are :

• Faster

• It is more efficient

• It is safer

• Healthier

• Cheaper

• They cant be used in cloudy weather.

• During fair weather, they can become so hot inside that they may

damage the crop.

REFERENCES

• 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”

August 2001

• Werner Weiss, Josef Buchinger, “ Solar Drying”