Eggplant Peels as a Valuable Source of Anthocyanins: Extraction, Thermal Stability and Biological Activities

Abstract

This study aimed to use eggplant peels as a potential source of anthocyanins with biological activities. Two different extraction methods were tested in order to obtain extracts with a high anthocyanin content. The selected methods were the solid-liquid extraction (SLE) and ultrasound-assisted extraction (UAE) methods. For each method, two concentrations of ethanol (EtOH) were used, while varying the extraction time and temperature. Based on the results, the extracts obtained by SLE using EtOH 96% after 30 min of extraction at 50 °C showed the highest anthocyanin concentration. The UAE allowed the best results with EtOH 96% after 30 min at 25 °C. Both selected extracts showed similar chromatographic profiles, with delphinidin 3-O-rutinoside as the major anthocyanin, but in a higher concentration in UAE. The extracts also presented inhibitory activity against lipoxygenase (LOX), lipase, and α-amylase, thus suggesting a possible involvement in reducing the risk of various disorders. The first order kinetic model was used to predict the changes that can occur in the anthocyanin content and antioxidant activity from the eggplant peel extract. The calculated kinetic and thermodynamic parameters confirm the irreversible degradation of phytochemicals.

Keywords: anthocyanins; bioactive compounds; biological activity; eggplant peels; extraction methods; thermal degradation.

Figure 1

Chromatographic profile of eggplant peel extracts obtained by SLE with EtOH 96% for 30 min at 50 °C (a) and UAE with EtOH 96% for 30 min at 25 °C (b): Peak 1—delphinidin 3-O-rutinoside-5-glucoside; Peak 2—delphinidin 3-O-glucoside; Peak 3—delphinidin 3-O-rutinoside; Peak 4—cyanidin 3-O-rutinoside; Peak 5—petunidin 3-O-rutinoside.

Figure 2

Chemical structure of the anthocyanin glycosides identified in the eggplant peel extracts: delphinidin 3-O-rutinoside-5-glucoside (a); delphinidin 3-O-glucoside (b); delphinidin 3-O-rutinoside (c); cyanidin 3-O-rutinoside (d), and petunidin 3-O-rutinoside (e). [26].

Figure 2

Chemical structure of the anthocyanin glycosides identified in the eggplant peel extracts: delphinidin 3-O-rutinoside-5-glucoside (a); delphinidin 3-O-glucoside (b); delphinidin 3-O-rutinoside (c); cyanidin 3-O-rutinoside (d), and petunidin 3-O-rutinoside (e). [26].

Figure 3

Degradation of total anthocyanin content—TAC (a) and antioxidant activity (b) in eggplant peel extract treated at different temperatures; D3G = delphinidin 3-O-glucoside; TE = Trolox Equivalent; dw—dry weight.