B. K. Bala
Department of Farm Power and Machinery, Bangladesh Agricultural University,
Mymemnsingh, Bangladesh
M. Borhan Uddin
Department of Food Technology, Bangladesh Agricultural University, Mymemnsingh, Bangladesh
M. A. Hossain
Bangladeh Agricultural Research Institute, Gazipur, Bangladesh
M. A. Hoque
Bangladeh Agricultural Research Institute, Gazipur, Bangladesh
Asparagus roots, Blanching, Color degradation, Hybrid solar dryer, Thin-layer drying model, Tray dryer
Bangladesh Agricultural Research Institute, Gazipur
Postharvest and Agro-processing
The hybrid dryer installed at Bangladesh Agricultural Research Institute (BARI) was used for drying of asparagus roots at 50C[25] and the tray dryer at BARI laboratory was used for drying of asparagus roots at 60 and 70C and three drying experiments were conducted. Fresh roots of asparagus were collected from herbal and medicinal plants in a village of the Natore district in Bangladesh. The roots of asparagus were processed in three different sizes (whole, split, and sliced). Three samples of each of the different shapes and sizes of the product were water blanched and another three samples were nonblanched. Fresh roots of asparagus were dried in three forms— whole, longitudinally split, and sliced. The roots of asparagus are cylindrical in shape and the average diameter of the asparagus root was 11mm (9.5<11<12 mm). Fresh roots of 100mm long were taken as whole. Roots of 100mm long asparagus were longitudinally cut into two parts and these were termed split samples. Cylindrical-shaped roots of asparagus were sliced into 10-mm-long pieces with a mechanical slicer. Three samples of each of the three forms were blanched at 80C for 5 min. The initial moisture content of the asparagus roots was determined by an oven-drying method at 105C for 24 h.[26]The dryer basically consists of a solar collector and a drying unit. The dimensions of the flat plate concentrating solar collector were 2.3m long, 1.6m wide, and 0.5m high. The transparent cover of the collector was 4-mm-thick clear glass. A black painted corrugated iron sheet about 200mm below the glass cover was used as an absorber plate. To increase the efficiency of the solar collector, a flat-type reflector made of a glass mirror was added at the top of the solar collector. The dimensions of the reflector were the same as those of the solar collector so that it could be used as a reflector in the daytime and as a cover at night or in adverse weather. This reflector is adjustable and could be adjusted according to the change of the sun’s angle during the day to collect a higher amount of sun rays. In addition, the collector was placed on four legs with a 140-mm wheel to turn the solar collector horizontally and change its direction according to the change of the sun’s angle. The solar collector was insulated by 50-mm-thick polystyrene. A centrifugal fan operated by a 0.75-kW, 220-V electric motor was connected at one side of the collector to draw the atmospheric air inside the collector and push out the heated air into the dryer at a desired air velocity. Air flow was controlled by a variac connected to the electric motor. For auxiliary heating, two electric heaters (2kW 2¼4 kW) were installed at the entry of the collector. A temperature controller was set to maintain constant temperature inside the dryer. The length and width of the solar dryer were the same as those of the collector (2.30m1.60 m). It was located directly under the solar collector and 200 mm under the absorber plate. It was divided into four parts with equal dimensions. In each of the parts there were two trays for drying. This allows the use of eight drying trays in the drying unit. The drying air was passed across the asparagus placed in thin layers on eight horizontally stacked trays and arranged in two vertical columns. Each tray was made of a wooden frame and plastic net with dimensions of 1040 mm780 mm. The drying air was heated up in the solar collector and passed to the drying chamber. The drying air from the solar collector was passed through a curved passage downward and then turned into the drying unit to flow over and under all the drying trays and exhausted through an outlet. The temperature in the dryer was maintained constant at 501C using a temperature controller and adjusting air flow by the variac. The mechanical tray dryer consists of a drying chamber, heater, electric fan, etc. The overall dimensions of the drying chamber of the tray dryer are 1.42m0.64m0.86 m. Inner dimensions of the chamber are 0.80m0.50m 0.60 m. There were arrangements for fixing five trays. Heated air was passed over the trays through 14 holes and passed out with same number of holes. Air was circulated over the electric heater installed on the bottom of the dryer with a fan sucking fresh air from the right side of the dryer. There was an arrangement to control the velocity of the air by reducing or increasing the opening of the holes. Sensors were used to detect the temperature level. When the temperature was higher than the desired temperature, the heating system was off and the reverse was true when the temperature was below the desired temperature. The asparagus roots in the form of whole, split longitudinally (5 mm thick; i.e., half of the diameter), and sliced (10 mm) were dried under blanched and nonblanched conditions at 50C using a solar hybrid dryer and at 60 and 70C using a tray dryer. Before starting an experimental run, the whole apparatus was operated for at least 1 h to stabilize the air temperature and air velocity in the dryer. Drying was started usually at 9:00 am and continued until it reached the final moisture content (about 9–10%, wb). Ambient temperature and temperature inside the dryer were measured with a digital thermometer (K202, Voltcraft, Berlin, Germany) connected with k-type thermocouples. Solar radiation was measured with a Lux meter (LX-9626, Shanghai, China) during the daytime. Velocity of drying air was measured with a thermo-anemometer (AM-3848, Shanghai, China). Weight losses of the samples in the solar dryer were recorded during the drying period at 2-h intervals with an electronic balance (EK-200 g, max 2000.01 g and Miltipas, USA). After completion of drying, the dried samples were collected, cooled in a desiccator to ambient temperature, and then sealed in plastic bags. The color of fresh and dried roots of asparagus samples was measured by a chromameter (CR-400, Minolta Co. Ltd., Osaka, Japan) in CIE (Commission Internationale l’Eclairage) Lab chromaticity coordinates. L, a, and b represent black to white (0 to 100), green to red (ve to þve), and blue to yellow (ve to þve) colors, respectively. Out of five available color systems, the Lab[27–29] and LCh[30] systems were selected because these are the most used systems for evaluation of the color of dried food materials. The instrument was standardized each time with a white ceramic plate. Three readings were taken at each place on the surface of root samples and then the mean values of L, a, and b were averaged. The different color parameters were calculated.
Drying Technology, 28: 533–541, 2010 Copyright # 2010 Taylor & Francis Group, LLC ISSN: 0737-3937 print=1532-2300 online DOI: 10.1080/07373931003618899
Journal