Introduction: Thin layer drying of agricultural products using an infrared dryer (IR) is one of the interesting and low cost methods of dehydration. Quality of dried products could be increased if the engineering aspects and proper selection of performance parameters be well considered in the designing of a dryer. The dryer that uses two or more drying methods in combination, produces in the most cases the higher quality product with lower energy consumption.
Materials and methods: In this study, an available IR dryer was modified and a combined IR and hot- air dryer was constructed. According to the reported quality factors in the literatures and market desire, the developed dryer performance was evaluated during the drying of banana slices. The studied factors were the effects of the thickness of slices (4 and 6 mm) and the surface temperature of the product (55, 65 and 75 °C) on the drying kinetic and quality of the dried product. Some various mathematical models were fitted to the experimental data and results and among them the best fitted model was selected.
Results & discussion: Based on the results, the different surface temperature and thickness of the slices had a significant effect on the drying time and color changes of the samples. However, they did not affect the density of samples. Drying time of the banana slices (to reach the moisture ratio of 0.04) with the thickness of 4 mm was 155, 105 and 80 min at the surface temperatures of 55, 65 and 75 °C, respectively. At the thickness of 6 mm, the drying time was 230, 130 and 100 min, respectively at the mentioned temperature. The minimum color change were observed at the thickness of 4 mm (ΔE=32.41) and surface temperature of 55°C (ΔE= 28.99). The samples dried at the temperature of 55 °C had the best quality. Evaluation of the various mathematical models indicated that the Page model is the most suitable to predict the drying kinetic of banana slices under the studied condition. Comparing the obtained results with the reported ones for just hot-air or IR drying of banana slices; it is obvious that in point of product quality and drying time, the combined IR and hot- air dryer has better conditions