Effectiveness of Design of Solar Dryers on Dehydration of Vegetables (Tomato, Green Onion Leaves)

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1 Universities Research Journal 2011, Vol. 4, No. 3 Effectiveness of Design of Solar Dryers on Dehydration of Vegetables (Tomato, Green Onion Leaves) Tin Lin Maung 1 and Yee Yee Win 2 Abstract Based on preliminary investigations under controlled conditions of drying experiments, solar tunnel dryer and solar cabinet dryer with turboventilator were designed and constructed to dry tomato and green onion leaves. Solar tunnel dryer (STD) design uses direct sunlight and has low resistance to airflow. STD is weather protected food dehydration tunnel that can dry food economically and hygienically. Solar cabinet dryer with turbo-ventilator (SCD) is an innovative design with natural draught induced with turbo-ventilator. Turbo-ventilator runs on external wind and creates necessary draught and maintains good airflow through the solar dryer giving excellent performance. As the turbo-ventilator works on outside wind only, no power is required and unit is truly a renewable energy gadget. This paper describes the design considerations followed and presents the results of calculations of design parameters. A minimum of 6 ft 2 (0.557m 2 ) solar collector area is required to dry a batch of 2-5 kg sliced vegetables in 6-10 hours (~two days drying period). The average initial and final moisture content considered were ~95% and ~10% wet basis, respectively. The average ambient conditions are 35ºC air temperature and 56% relative humidity. The weather conditions considered are of Southern Yangon Region, Myanmar. Tomato and green onion leaves were chosen systematically prepared and dehydrated. Their characteristics such as ph, acidity, moisture content, color, ash content, fibre content, sugar content and rehydration ratio were determined. Introduction Sun drying is still the most common method used to preserve agricultural products in most tropical and subtropical countries. However, being unprotected from rain, wind-borne dirt and dust, infestation by insects, rodents and other animal, products may be seriously degraded to the extent that sometimes become inedible and the resulted loss of food quality in the dried products may have adverse economic effects on domestics and international markets. Some of the problems associated with open-air sun drying can be solved through the use of a solar dryer which comprises of collector, a drying chamber and sometimes a chimeny. The conditions in tropical countries make the use of solar energy for drying food practically 1. Assistant Lecturer, Dr., Department of Industrial Chemistry, East Yangon University. 2. Professor (Head), Dr., Department of Industrial Chemistry, East Yangon University.

2 402 Universities Research Journal 2011, Vol. 4, No. 3 attractive and environmentally sound. (Mulhlbauer, W., J. Mullere, and A. Esper, 1996) Solar dryers have the principal advantage of using solar energy--a free, available, and limitless energy source that is also non- polluting. Solar dryers use the energy of the sun to heat the air that flows over the food in the dryer. As air is heated, its relative humidity decreases and it is able to hold more moisture. Warm, dry air flowing through the dryer carries away the moisture that evaporates from the surfaces of the food. Solar dryers can be classified based upon the exposure of the crop to direct or indirect solar radiation, or the method of air flow through the dryer which may be by natural or forced convection. Natural convection solar dryer has low buoyancy induced air flow. Forced convection solar dryer have increased air flow induced with fan. Solar cabinet dryer with turboventilator is the indirect forced convection solar dryer, whereas solar tunnel dryer is the direct natural convection solar dryer. (Bala B.K., 2009) In this research work, the chosen fruits and vegetables for dehydration are tomato and green onion leaves. Dehydrated leaves in the form of flour, powder, flakes, and granulated are used in cookery as seasonings, condiments and also ingredient of instant- noodles. Materials and Methods Design of Solar Dryers The two types of solar dryers constructed are solar tunnel dryer and solar cabinet dryer with turbo-ventilator. Solar Tunnel Dryer The dryer dimension is 3ft wide x 6ft (0.91m x 1.82m) long and is constructed with materials readily available in Southern Yangon Area. The frame consists of 3 ft x 6ft (0.91m x 1.82m) plywood sheet and two 1 x 4in (0.0254m x m) side boards (Figures 1, and 2). Hoops made of 1in (0.0254m) bamboo sticks support a clear polyethylene film cover. The peak height of polyethylene cover is 1ft (0.3048m) above the drying tunnel. The bottom of the dryer is insulated with ⅓in (0.85cm) foam insulation board. The entire solar dryer frame is supported on a bunch. Drying trays, 3ft wide x 4ft (0.91m x 1.22m) long, have knitted polyethylene shade cloth attached to the bottom of the ¾ x 1½in (1.905cm x 3.81cm) wood frames.

3 Universities Research Journal 2011, Vol. 4, No Air ducts convey heated air from the 6 ft 2 (0.557m 2 ) solar collector to below the drying trays, allowing heated air to rise through the bottom of the polyethylene shade cloth. Figure (1) Views of Solar Tunnel Dryer Fig (2) Solar Tunnel Dryer Solar Cabinet Dryer with Turbo-ventilator The solar cabinet dryer with turbo-ventilator consists of drying cabinet, solar collector and turbo-ventilator. 26in wide x 41.5in long (0.6604m x m) corrugated galvanized sheet, painted black is fitted to the bottom of the drying chamber and tilted at 30 º. Galvanized sheet was covered with 26in wide x 47.5in long (0.6604m x m) transparent glass plate. There are three trays in the drying chamber. 13.5in wide x 26in length x 2in height (0.3429m x m x m) trays are made of mosquito wire net. The height below the trays is 45in (1.143m). The top of the trays are covered with turbo-ventilator made of iron sheet. The black colour of the corrugated galvanized sheet absorbs the sun rays and heats the air above. The warm air rises, flows through the drying

4 404 Universities Research Journal 2011, Vol. 4, No. 3 cabinet and leaves the drying chamber through the top turbo-ventilator. Cool environment air is sucked off through the bottom opening. Turboventilator works on outside wind and exhausts air from drying cabinet inducing draught. The solar dryer only works with direct solar radiation and works best during dry periods when there is little humidity in the air Fig (3) Views of the Solar Cabinet Dryer with Turbo-ventilator

5 Universities Research Journal 2011, Vol. 4, No Turboventilator Drying Tray Solar Collector Fig (4) Solar Cabinet Dryer with Turbo-ventilator Table (1) Design Conditions and Assumptions of Solar Tunnel Dryer for Tomato and Green Onion Leaves Conditions Tomato Green Onion Leaves Location EYU Campus EYU Campus Drying Period November November Loading Rate(m p ) (kg) 5 2 Initial Moisture Content (M i ) (%w/w) Final Moisture Content (M f ) (%w/w) Ambient Air Temperature (T am ) (ºC) Ambient Relative Humidity (RH am ) (%) Maximum Allowable Temperature (ºC) Drying Time (t d ) (hr) Wind Speed (m/s) Thickness (mm) 5 5

6 406 Universities Research Journal 2011, Vol. 4, No. 3 Table (2) Design Conditions and Assumptions of Solar Cabinet Dryer with Turbo-ventilator for Tomato and Green Onion Leaves Conditions Tomato Green Onion Leaves Location EYU Campus EYU Campus Drying Period November November Loading Rate(m p ) (kg) 3 1 Initial Moisture Content (M i ) (%w/w) Final Moisture Content (M f ) (%w/w) Ambient Air Temperature (T am ) (ºC) Ambient Relative Humidity (RH am ) (%) Maximum Allowable Temperature (ºC) Drying Time (t d ) (hr) 10 6 Wind Speed (m/s) Thickness (mm) 5 5 Table (3) Values of Design Parameters for Solar Tunnel Dryer for Tomato and Green Onion Leaves Parameters Tomato Green Onion Leaves Initial Humidity Ratio,w i (kg H 2 O/kg dry air) Initial Enthalpy, h i (kj /kg dry air) Equilibrium Relative Humidity,RH f (%) Final Humidity Ratio,w f (kg H 2 O/kg dry air) Final Enthalpy, h f (kj /kg dry air) Mass of Water to be Evaporated, m w (kg) Average Drying Rate, m dr (kg H 2 O/hr) Air Flow Rate, m a (kg dry air/hr) Volumetric Air Flow Rate, V a (m 3 /hr) Total Useful Energy, E (MJ) Solar Collector Area, A c (m 2 )

7 Universities Research Journal 2011, Vol. 4, No Table (4) Values of Design Parameters for Solar Cabinet Dryer with Turbo-ventilator for Tomato and Green Onion Leaves Parameters Initial Humidity Ratio,w i (kg H 2 O/kg dry air) Tomato Green Onion Leaves Initial Enthalpy, h i (kj /kg dry air) Equilibrium Relative Humidity,RH f (%) Final Humidity Ratio,w f (kg H 2 O/kg dry air) Final Enthalpy, h f (kj /kg dry air) Mass of Water to be Evaporated, m w (kg) Average Drying Rate, m dr (kg H 2 O/hr) Air Flow Rate, m a (kg dry air/hr) Volumetric Air Flow Rate, V a (m 3 /hr) Total Useful Energy, E (MJ) Solar Collector Area, A c (m 2 ) Dehydration of Tomato and Green Onion Leaves Materials Sound ripe tomato and good, fresh and green onion leaves, from Thanlyin Township, Yangon Division were used. Sodium bicarbonate was purchased from local markets. Method of Preparation of Dehydrated Tomato Sound ripe tomatoes were thoroughly washed with water and removed stems and blemishes. These tomatoes were put in a large pot of boiling water for no more than 1 min (30 45 seconds is usually enough) and then plunged them into a waiting bowl of ice water. With a gentle tug, the skins were practically slide off the tomatoes. Tomatoes were then sliced into 5mm thickness. Olive oil was used to lightly grease the trays of solar dryer (this will prevent the tomatoes from sticking to the tray). Sliced tomatoes were arranged on the trays and then dried in solar tunnel dryer at 48ºC for (9.5) hr. As another

8 408 Universities Research Journal 2011, Vol. 4, No. 3 way, they were dried in solar cabinet dryer with turbo-ventilator at 45ºC for about 10 hours to obtain complete drying. Method of Preparation of Dehydrated Green Onion Leaves Mature green onion leaves were trimmed to remove the undesired portions (bulb and roots), followed by washing and cutting into small pieces (5mm). Consequently the weighed pieces were blanched for 15 minutes in steam vapour and then dipped in 1% sodium bicarbonate solution for about 5 minutes at room temperature (adapted from Yi Yi Myint et. al., (2007)). Finally they were then dried in solar tunnel dryer (46ºC) for 5.5 hours and another way, they were dried in solar cabinet dryer with turbo-ventilator at 46ºC for about 6 hours to obtain complete drying. Methods of Analysis The essential determinants, showing the quality of each type of dehydrated leaves including, ph, moisture content, ash content, fibre content, colour, acidity, sugar content, total solid content, rehydration ratio and organoleptic properties were investigated. Results and Discussion The mean average day temperature in November at Southern Yangon Region, Myanmar is 35ºC and RH is 56 %. From the psychometric chart, the humidity ratio is 0.02kg H 2 O/kg dry air. Since Southern Yangon Region is rainy and tropical district and also near the sea, the humidity is high. From the result of preliminary experiments on the crops, the drying temperature range for the two dryers is 40-50ºC and final moisture content of products for storage is ~10% w.b. The corresponding relative humidity is 50-60% (sorption isotherms equation). Based on the results obtained during the test of dryer, temperature above 40ºC was recorded against the ambient temperature in the drying chamber. For solar tunnel dryer, design parameters and design conditions of tomato and green onion leaves are shown in Tables (1) and (3) whereas design parameters and design conditions of these two vegetables for solar cabinet dryer with turbo-ventilator are shown in Tables (2) and (4). In solar tunnel dryer, ideal height of polyethylene cover is important and ranges between 8 to 16 inches (20-41cm) above the drying trays. A

9 Universities Research Journal 2011, Vol. 4, No solar tunnel dryer with taller peak height has a larger air volume to absorb water vapour but it takes longer to heat the volume of air. The taller peak height also has less wind resistance allowing faster air flow, but the path of airflow is higher above the drying trays with less mixing of moisture-laden air at the drying tray. A shallow peak height has a smaller volume to absorb water vapour but the smaller volume heats up faster and the airflow is directed closer to the drying tray to remove the moisture from the products. So, the peak height of 1ft (30.48cm) is chosen for this solar tunnel dryer. The amount of produce place in the dryer also affects the humidity in the dryer and the drying rate. The solar cabinet dryer is an indirect dryer by inserting blackened galvanized plate directly beneath the clear glass cover and which removes the exposure of the crop to direct sunlight. If the drying trays are in any way over-packed, air flow is obstructed and crop spoilage occurs. Dryers of this type must be used with care. An exhaust fan such as turbo-ventilator, fitted at the top of the cabinet can facilitate air movement through such dryers. This unit does not require external power because of use of turbo-ventilator. The characteristics of fresh and dehydrated (tomato and green onion leaves) such as colour, ash content, fibre content, ph, total solid content, sugar content and rehydration ratio can be seen in Tables (5) and (6). In Table (5), colour of the dehydrated tomato using SCD was nearly equal to fresh tomato. With STD, red and yellow color were reduced. Table (6) show that colour of dehydrated green onion leaves using SCD was nearly equal to those using STD. The increase of color intensity after dehydration was reduced in solar tunnel dryer than solar cabinet dryer. This was due to the direct heat and low resistance to air flow in solar tunnel dryer. Other characteristics of dehydrated products were increased because of dehydration. The effect of the type of dryer on the characteristics of dehydrated fruits and vegetables are recorded in Table (7). The results point out that solar cabinet dryer with turbo-ventilator was more favourable for dehydration of fruits and vegetables because indirect and forced convection can retain better characteristics than solar tunnel dryer. As solar tunnel dryer is direct and natural convection dryer, flavour of the products were slightly reduced than with solar cabinet dryer with turbo-ventilator.

10 410 Universities Research Journal 2011, Vol. 4, No. 3 Table (5) Characteristics of Fresh and Dehydrated Tomato Dehydrated Tomato Characteristics Fresh Tomato Solar Cabinet Dryer with Turbo-ventilator Solar Tunnel Dryer Colour R 15.5, Y 3.9 R 15.4, Y 3 R 4.3, Y 1.7 Moisture (%) Fiber (%) Ash (%) ph Sugar Content (%) Rehydration Ratio Table (6) Characteristics of Fresh Green Onion Leaves and Dehydrated Green Onion Leaves Characteristics Colour Fresh Onion Leaves Y 60, B9 Dehydrated Green Onion Leaves Solar Cabinet Dryer with Turboventilator Solar Tunnel Dryer Y 20.7, B 10.9 Y 20.6, B 12 Shift-made Hot air Dryer* Y 20.6,B 60,R 2 Moisture (%) Ash (%) Total Solid Content (%) Rehydration Ratio * adapted from Yi Yi Myint et. al., (2007)

11 Universities Research Journal 2011, Vol. 4, No Table (7) Effect of Dryers on the Characteristics of Tomato and Green Onion Leaves Vegetables Dryers Color Smell Shelf life Tomato Green Onion Leaves STD Pale red Pleasant smell SCD with Turbo-ventilator red Pleasant and pungent smell STD green Pleasant smell SCD with Turbo-ventilator Green Pleasant and pungent smell 6 months (browning occur) 6 months (browning occur) 1 year (do not change) 1 year (do not change) (i) (ii) Figure (7) Dehydrated Tomato Using (i) Solar Tunnel Dryer (ii) Solar Cabinet Dryer with Turbo-ventilator (i) (ii) Figure (8) Dehydrated Green Onion Leaves Using (i) Solar Tunnel Dryer (ii) Solar Cabinet Dryer with Turbo-ventilator

12 412 Universities Research Journal 2011, Vol. 4, No. 3 Conclusion Solar dryers are very usable and it will help the community to have nutritious processed food. Solar driers are simple in construction and can be constructed using locally available materials such as glass, wood, galvanized plate, bamboo and plastic, by the local craftsman. Solar cabinet dryer with turbo-ventilator is an indirect, forced-convection solar dryer. Solar tunnel dryer is a direct, natural convection solar dryer. The quality of the dehydrated products using solar cabinet dryer with turbo-ventilator is superior in color than those using solar tunnel dryer since solar cabinet dryer is indirect and good ventilation. Solar cabinet dryer with turbo-ventilator can be used for many types of fruits and vegetables with good quality in color and flavor. Solar tunnel dryer can also be used for fruits and vegetables whose quality is independent of color and flavor. Based on this research work, the farmers in rural areas can also use simple solar drying technique in their farm to preserve their home-grown vegetables after harvesting and the products can be used in off-season. Acknowledgements We would like to express our sincere gratitude to U Kyaw Ye Tun, Rector, East Yangon University, Dr. Mar Lar Aung, Pro-Rector and Dr. Kyaw Kyaw Khaung, Pro- Rector, East Yangon University, for their permission and the financial support in this research work. We are deeply indebted to Dr. Yee Yee Win, Professor and Head of the Industrial Chemistry Department, East Yangon University, for giving permission to use research facilities in the department. We are grateful to Dr. Than Htaik, Director General (Retired), Department of Cottage Industry and Dr. Yi Yi Myint, Professor, Department of Industrial Chemistry, Yadanarbon Univeristy (Former Associate Professor, Department of Industrial Chemistry, East Yangon University) for their kind help in construction of solar cabinet dryer with turbo-ventilator. Thanks are also extended to the teaching staff, especially, Daw Naw Zar Htwe, Daw Aye Thida, Daw Soe Wai Phyo and Daw Myat Yu Maw, Demonstrators of Industrial Chemistry Department, East Yangon University for their help in carrying out this research.

13 Universities Research Journal 2011, Vol. 4, No References Bala B. K. (2009), Solar Drying of fruits, vegetables, spices, medicinal plants and fish: Developments and Potentials, International Solar Food Processing Conference, Ehiem J.C. and etal, (2009) Design and Development of An Industrial Fruit and Vegetable Dryer, Research Journal of Applied Sciences, Engineering and Technology 1, Maxwell Scientific Organization. Mulhlbauer, W., Mullere J., and Esper A., (1996), Sun and Solar Crop Drying, Research and Development. Yi Yi Myint et. al., (2007), Study on the Dehydration of Green Onions, Chive (Chinese Leek) Leaves and Green Mustard Leaves in Southern Yangon Area, Department of Industrial Chemistry, East Yangon University.