Extraction of phenolics and anthocyanins from purple eggplant peels by multi-frequency ultrasound: Effects of different extraction factors and optimization using uniform design

•Multi-frequency ultrasound was firstly applied to extract TPC and TMA from eggplant.•UD Combined with PLS was firstly used to determine the optimal extraction conditions.•Impact of ultrasound in different frequencies and powers on TPC and TMA were studied.•The sound extraction efficiency of TFU was superior to non-ultrasound, SFU and DFU.•The application prospect of multi-frequency ultrasound in extraction field is great.