Comparative evaluation of the green extraction techniques in peanut skin: characterization of bioactive compounds, antioxidant activity and antimicrobial properties of SC-CO2 extract

In this study, the maceration (MAC), microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE) and supercritical carbon dioxide (SC-CO2) extraction methods were compared for the extraction from peanut skin. Various method parameters were examined by one-factor experiments (ethanol concentration, extraction time and solvent/material ratio, microwave power, extraction temperature and pressure). The total phenolic content (TPC) of peanut skin was measured and the antioxidant activity was assessed by 2,2-diphenyl-1-picrylhydrazyl. (DPPH) and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS) assays. Among the methods, SC-CO2 was most effective for peanut skin, achieving a TPC of 99.39 mg GAE/g and a DPPH activity of 776.35 mu mol TE/g, while MAC showed a similarly high TPC (96.06 mg GAE/g) but lower antioxidant capacity. SC-CO2 extraction was selected as the optimal technique due to its effectiveness in extracting bioactive compounds. The SC-CO2 extract was freeze-dried and then the lyophilized extract was solved in water and ethanol (50%, v/v) at 20 mg/mL concentration to obtain aqueous and ethanolic extracts for further characterization. The lyophilized extract showed low water activity (0.23), low moisture (8.78%) and low solubility (15.7% and 20.10% in water and ethanol (50%, v/v)). LC-DAD-ESI-MS/MS analysis evaluated the phenolic profile of extracts and similar profiles were observed in aqueous and ethanolic extracts. However, most peaks were detected at lower concentrations in aqueous extract. The phenolic profile was dominated largely by proanthocyanidins, particularly A-type procyanidins. Based on the FTIR spectra of the lyophilized, aqueous and ethanolic extracts, the following classes of compounds were confirmed: phenols, amides, esters, amino acids, carboxylic acids, alcohols, polysaccharides, lipids, carbohydrates, and flavonoids. The antimicrobial activity of the extracts was detected against S. aureus, B. subtilis, and K. pneumoniae, but they were ineffective against E. coli.