Ultrasonication as pretreatment for drying of tomato slices in a hot air-microwave hybrid oven

This study aims to investigate the effect of ultrasonic pretreatment on drying time and quality properties of tomato slices dried by microwave combined with hot air at 60 degrees C. The influence of ultrasound pretreatment (0, 20, and 40min) and microwave power (120, 150, and 180W) on drying time, color, total phenolic content, lycopene, vitamin C, and rehydration capacity of dried slices of tomato was studied. Results showed that as the microwave power level increased, drying time decreased significantly (about 46.4%). Ultrasound pretreatment decreased the drying time by 7.38% only at 120W microwave power and 40min of pretreatment compared to those without ultrasound pretreatment at the same microwave power. Depending on drying conditions, vitamin C and lycopene contents reduced from 433.94 to 81.89mg AA/100g dry solids and 3920.57 to 415.40mg/100g dry solids, respectively. The change in total phenolic content was not severe as much as vitamin C contents. Rehydration capacity of pretreated samples was larger than nontreated samples. The color values of dried tomato slices were in the acceptable range. Both microwave power and ultrasound pretreatment affected the quality of the final product significantly.