Development of an intermittent drying process of onion

This study was carried out to develop an intermittent energy-saving drying process that best preserves the quality attributes of the onion. Onion slices, 5 mm thick were dried at 45, 55 and 65°C under air flow rates of 20, 24 and 28 dm3/s. The first critical drying time determined was between 0.75 and 3.5 h while the second was between 3.25 and 14 h. The most energy-efficient couple (temperature; air flow) was (65°C; 24 dm3/s). An intermittent drying process (step-down) was developed and these critical drying times and the most energy-efficient couple were put into consideration. This step-down process was compared to the most energy saving continuous drying at 65°C under the air flow of 24 dm3/s. The results showed an improvement on the energy efficiency of 12.70% of the intermittent process compared with continuous drying and the nutritional quality, particularly the fat content (166.04%), the vitamin C (53.85%) and proteins (2.50%) were significantly maintained. The modeling of drying kinetics (by ten empirical models) showed that the model Two term exponential and the model of Midilli described both the continuous drying and the developed intermittent process