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

Abstract

In this work, multi-frequency ultrasound (working modes for the single-, dual- and tri-frequency in simultaneous ways) was applied to extract bioactive compounds from purple eggplant peels. The single-factor experiments were performed by varying six independent variables. A six-level-five-factor uniform design (UD) was further employed to evaluate the interaction effects between different factors. It was found that extraction temperature and extraction time significantly affected the total phenolic content (TPC), whereas the total monomeric anthocyanins (TMA) was mainly influenced by ethanol concentration, extraction temperature and solid-liquid ratio. Based on partial least-squares (PLS) regression analysis, the optimal conditions for TPC extraction were: 53.6 % ethanol concentration, 0.336 mm particle size, 44.5 °C extraction temperature, 35.2 min extraction time, 1:43 g/mL solid-liquid ratio, and similar optimal conditions were also obtained for TMA. Furthermore, the TPC and TMA extraction were investigated by ultrasound in different frequencies and power levels. Compared with single-frequency (40 kHz) and dual-frequency ultrasound (25 + 40 kHz), the extraction yield of TPC and TMA with tri-frequency ultrasound (25 + 40 + 70 kHz) increased by 23.65 % and 18.76 % respectively, which suggested the use of multi-frequency ultrasound, especially tri-frequency ultrasound, is an efficient strategy to improve the TPC and TMA extracts in purple eggplant peels.

Keywords: Monomeric anthocyanins; Parameter optimization; Phenolic content; Ultrasound extraction; Uniform design.

Graphical abstract

Fig. 1

Effects of (A) solvent type, (B) ethanol concentration, (C) extraction temperature, (D) particle size, (E) extraction time and (F) Solid-liquid ratio on the extraction yield of bioactive compounds from purple eggplant peels.

Fig. 2

The SRC values of each factor or factor combination when the responses were TPC yield (A) and TMA yield (B) from purple eggplant peels.

Fig. 3

The influence of ultrasound power on the extraction of TPC and TMA at 40 kHz and under other optimal conditions.

Fig. 4

The influences of multi-frequency ultrasound (A: Single-frequency ultrasound (SFU); B: Dual-frequency ultrasound (DFU); C: Tri-frequency ultrasound (TFU)) on the extraction of bioactive compounds from purple eggplant peels under other optimal conditions.