Performance of a portable indirect solar dryer for drying various fruits: Drying characteristics and quality assessment

In this paper, the performance of an indirect solar dryer (ISD) for multiple fruits and vegetables, including apples, tomatoes, oranges, and bananas, was investigated experimentally using two consecutive 8-h tests (16 h) and continuous overnight 32-h tests. The air blowing inside the drying chamber on four trays to continuously eliminate moisture from fresh fruits was supplied by 2 DC fans powered by 12V batteries charged by a 50W PV panel. The designed solar dryer is lightweight, made with low-cost materials, portable, and adaptable for different fruits and vegetables, and it is suitable for farmers in remote areas. The quality assessment of dried fruits was evaluated using pH, colour, and mineral composition measurements. The average solar radiation was comparable, and it ranged from 708 to 762Wm^-2 throughout the 8-h experiment tests, affecting the drying chamber and thermal profiles. Maximum temperatures of 63.5, 62.7. 61.5 and 66.5 °C were recorded in the first tray during day 2 of the 8-h test for apples, tomatoes, oranges, and bananas, respectively. During the 32-h continuous tests, the maximum temperatures were 66.5, 53.5, 60.2, and 62.9 °C on the top tray. Final moisture content on the first tray was achieved at 11 h for apples (15.42 %), 10 h for tomatoes (5.14 %), 13 h for bananas (29.14 %), and 16 h for oranges (33.44 %) in the 16-h tests. On day 1, the average drying efficiencies were 12.03, 12.84, 8.50, and 6.70 %, whereas the values of 0.44, 0.53, 1.75, and 2.23 % were obtained in day 2 for apples, tomatoes, bananas, and oranges, respectively. The experimental data were fitted to five drying kinetics models. Continuous drying in 32-h tests for the four fruits showed a decrease in moisture content of 16, 1,37, and 18 %, for apples, tomatoes, oranges, and bananas, respectively. However, the oranges did not completely dry up during the 32-h tests. The drying efficiencies were 6.36, 4.50, 2.82, and 3.25 %. A noticeable change in colour between the fresh and dried fruit slices was observed, whereas a pH < 6.0 suggested the preservation of nutrients and inhibition of the proliferation of microorganisms. Mineral composition showed that K, Ca, P, and Mg were the most abundant elements in dried samples. Overall, this study demonstrates the effectiveness of indirect solar drying to preserve the quality of fruits and vegetables while promoting sustainable development goals by reducing post-harvest losses and promoting eco-friendly practices.