Preparation of anthocyanin-rich mulberry juice by microwave-ultrasonic combined pretreatment.

The work aims to study the process of microwave-ultrasonic combined treatment to obtain anthocyanin-rich mulberry juice. A Box-Behnken design was employed to analyze the effects of microwave time and citric acid content on the total phenol content, total anthocyanin content, hue, color intensity, DPPH and ABTS radical scavenging activities. Under the optimum conditions (microwave time of 46 s, citric acid addition of 273 mg/kg), the total phenol content, total anthocyanin content, the DPPH and ABTS radical scavenging activities reached 4.24 mg GAE/mL, 3.29 mg C3G/mL, 4.59 mg TE/mL and 11.90 mg TE/mL, respectively. Subsequently, the mulberry juice was processed with ultrasound of different frequencies. It was found that low-frequency ultrasonic treatment (25 kHz) could significantly reduce the loss of total phenolic and anthocyanin monomers and improve the antioxidant capacity of mulberry juice during storage for five weeks. Overall, mulberry juice with microwave-ultrasonic pretreatment is a natural antioxidant.