Optimization of the extraction process of polyphenols from Allium cepa using response surface methodology and assessment of its antioxidant and lipid-lowering action

Non-alcoholic fatty liver disease (NAFLD) development and progression is closely associated with oxidative stress (OS). The main objective of this research was to optimize the extraction process of Allium cepa polyphenols using a response surface methodology (RSM) and evaluate its antioxidant and lipid-lowering activity correlated with NAFLD. First of all, ultrasound-assisted extraction (UAE) process of the polyphenols from A. cepa was optimized using a three-factor-three-level Box-Behnken Design (BBD) combined with single factor experiment, among which, the investigation factor was total polyphenol content (TPC) from A. cepa measured by Folin-ciocalteu reagent. Then, the quantification of the main polyphenols from A. cepa was undertaken using high performance liquid chromatography (HPLC). Afterwards, the antioxidant activity of A. cepa extract (ACE) was evaluated using 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylrylhydrazyl (DPPH) free radicals. In addition, the in vitro lipid-lowering action was investigated using hepatic steatosis model in HepG2 cells induced by free fatty acid (FFA). As a result, the optimal extraction conditions were extraction time of 30 min, extraction temperature of 53.42 ℃, 1:70 solid-to-solvent ratio and 64.5% ethanol. Under these conditions, the measured TPC was 17.0819 mg/g GAE, which was in agreement with the predicted value, while TPC was 17.2449 mg/g GAE with relative error of 0.96%. Furthermore, HPLC analysis showed that the main polyphenols existed in ACE were isoquercitrin (5.07 mg/g DW) and quercetin (2.03 mg/g DW). In addition, Antioxidant activity showed that the IC 50 values of ACE against ABTS and DPPH free radicals were 84.19 ± 0.03 μg/mL and 171.2 ± 0.02 μg/mL, respectively. Moreover, the in vitro lipid-lowering effect exhibited that ACE reduced the levels of TC and TG ( P