Study on Combined Hot-air and Microwave Vacuum Drying for Scallion

Combined drying of hot-air and microwave vacuum was applied for scallion. The effects of different drying conditions and drying methods on its drying time and sensory quality including color, superficial aspect and shape of dried scallion was investigated. The effects of combined hot-air and microwave vacuum drying scallion under different cutting length and different microwave power was discussed. Also the performance comparison of different drying methods including hot-air drying, microwave vacuum drying, combined hot-air and microwave drying, combined hot-air and microwave vacuum drying were demonstrated. The results proved that scallion cutting length had significant effect on drying time and sensory quality; the higher microwave power resulted in little shorter drying time and obvious lower scallion quality. The total drying time of combined hot-air and microwave vacuum drying was 2.8h under the drying condition of 0.65kW microwave power, 0.085MPa vacuum level, 5mm scallion cutting length while hot-air dried at 60°C for 2.5h. It decreased about 37.8% than hot-air drying, and increased 3.67 times than microwave vacuum drying. But the sensory state was less poor than hot-air drying, and better than microwave vacuum drying, as well as combined hot-air and microwave drying.