Solar food drying

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Information about Solar food drying

Published on March 7, 2014

Author: RakhiVishwakarma


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. Airflow Drying Chamber Solar Collector


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 Direct Sun Advantages + Least expensive Disadvantages - UV radiation can damage food + Simple Indirect Sun + Products protected from UV - More complex and expensive than direct sun + Less damage from temperature Mixed mode + Less damage from temperature extremes - UV radiation can damage food - 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 Complex - Expensive - May cause fuel dependence


DRYING BEHAVIOR 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

WEATHER CONDITIONS • 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 weather.

Sunshine Low High Add auxiliary heat source Humid Humid Dry Cold Moderate Hot Dry Direct Direct Dry Humid Add Collector area Indirect Small Small Large

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 & PRETREATMENT • Selection ( Fresh, undamaged ) • Cleaning ( washing & disinfection ) • Preparation ( peeling, slicing ) • Pre-treatment ( Sulfurization, Salting ) • Drying • Packaging • Storage or sale

Types of food Grains Moderate time to dry Fruits and vegetables Short time to dry In field or near preparation area Vitamin loss from sun exposure High Low Permanent dryers Portable dryers Indirect dryer Direct dryer Small quantity Factors to consider in selecting a solar food dryer Large dryer


• 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.

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.

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 : • 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.


SOLAR-ASSISTED SPOUTED 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 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.


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

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.



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 Bulk Density(kg/m3) 385 Apparent Density(kg/m3) 750.0 0.00 1023.8 Diameter(mm) 7.45 0.47 Shrinkage (%) 0.00 Open sun dried 610 13.23 Solar-assisted spouted bed dried 407 17.08 754.8 9.60 5.82 0.71 6.53 0.43 50.24 - 41.20 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 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 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”

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