Effects of ultrasound blanching followed by hot air-coupled microwave drying on the quality of perilla leaves

To improve the drying efficiency and quality of dried perilla leaves, this work investigated the impact of ultrasound and blanching (UB) pretreatment followed by hot air coupled microwave drying (MHD) on the drying kinetics, activation energy, color difference and microstructure of perilla leaves. The effects of 2 UB duration (1 min, 3 min) and drying temperature (50, 60, 70 °C) on the chlorophyll, flavonoid and volatile content of perilla leaves were also evaluated. The results revealed that UB pretreatment increased the drying rate, improved the effective moisture diffusivity and reduced the activation energy. It was found that drying at 70 °C without UB pretreatment had the lowest ΔE (14.68) and highest chlorophyll (2.38 mg/g), whereas UB pretreatment could retain a greater content of flavonoids (0.116 mg/g) at 50 °C for 1 min and preserve better odor characteristics. Taken together, these findings suggest that UB pretreatment is an alternative method for maintaining good quality for the MHD of perilla leaves.