Dielectric properties of carrots affected by ultrasound treatment in water and oil medium simulated systems

The ultrasound treatment of vegetables can damage their tissue structure and release water and soluble solids altering the dielectric properties of the material. Changes in the dielectric properties will influence the microwave processing of ultrasound pretreated material. In order to investigate the effect of ultrasound pretreatment on dielectric properties, carrots were ultrasonicated in simulated water and oil systems. Microwave heating and microwave-assisted vacuum frying of carrot slices were also completed to explore the relation between dielectric properties of microwave pretreated samples and microwave absorption. Ultrasound of carrots in water simulated system showed that ε' and ε″ of carrots tended to decrease with the increase of ultrasonic power (from 480 W to 640 W) and time (from 10 min to 30 min). Ultrasound treatment of carrots in salt solutions increased ε' and ε″ of carrots. The combined pretreatment of microwave blanching with ultrasound in salt solutions increased ε″ (from 19.72 to 29.90). Conversely, in oil simulated system, the ε' and ε″ of carrots also decreased as the ultrasonication power and time increased. Besides, the dielectric properties decreased as the temperature of oil medium increased from 35 °C to 75 °C. With improved dielectric properties, the dehydration rate during frying of carrot increased due to the enhanced microwave absorption. Therefore, the combined pretreatment of microwave blanching with ultrasound in salt solutions largely improved the dielectric properties of carrots, and reducing the required time of microwave vacuum frying.