THIN- LAYER DRYING OF DICED CASSAVA ROOTS

Fresh cassava (Manihot spp) roots were obtained from a farm and used in this study. They were peeled and diced using a special dicing machine into cubes of side 0.5 cm. The cubes were dried in thin layers (one to three layers) in a drier that was specifically designed and fabricated in the Department of Agricultural Engineering and Land Planning, Morogoro, Tanzania for the purpose. The drier had a motor driven fan, a heater and a tray chamber, with thermometers for determining entry and exit temperature (dry and wet bulb) conditions of the air. The input variables were: depth of thin layer (0.5, 1.0 and 1.5 cm), drying temperature of the air (55 and 65 oC) and drying time (from 0 min until the sample attained equilibrium, at intervals of 25 min). The response variable was the moisture content of the cassava cubes. Weather conditions during the experiments were also monitored. A parallel sun drying experiment was carried out to compare thin layer drying on the sun and thin layer drying in the fabricated dryer. For the fresh cassava that was used in the experiments, a duplicate sample was placed in an oven at 75 oC for 7 hours to determine the initial moisture content. The results indicated that the average moisture content of fresh cassava roots was about 75.4 %(w.b.). Both temperature and depth of layers were found to affect the drying characteristics of cassava cubes, with single layer and higher drying temperature giving faster approaches to equilibrium moisture content. Comparing sun drying of one layer at an average temperature of 25 oC with artificial drying at the above named temperatures, it was found that sun drying took 2 to 3 days to reduce the moisture content to Equilibrium Moisture Content (EMC), while this was achieved within 150 min and 125 min with artificial drying at 55 oC and 65 oC respectively. The generally accepted thin layer drying equations were fitted to the drying data of cassava cubes, and the Page model was found to agree with the drying data of one, two and three layers with high accuracy for artifial drying, but not for sundrying. The exponential model only agreed accurately with drying of one layer.