Optimization of antioxidant activity of soursop (<i>Annona muricata</i> L.) leaf extract using response surface methodology

Free radicals are inhibited by antioxidant compounds, which can be naturally sourced from soursop (<i>Annona muricata</i> L.). Extraction processes affect levels of antioxidant compounds in the extract. Therefore, the present study aimed to evaluate the optimal extraction conditions for soursop leaves using maceration and pressing methods and determine the content of the flavonoid compound rutin in the optimized extract. The analysis was conducted to achieve maximum levels of 2.2-diphenyl 1-picrylhydrazyl (DPPH), cupric ion reducing antioxidant capacity (CUPRAC) and ferric reducing antioxidant power (FRAP), as well as total flavonoid content (TFC) and total phenolic content (TPC), along with identification and determination of the flavonoid compounds in the optimized extract. Response surface methodology with Box-Behnken model was applied, considering three extraction variables, including duration (10-40 min), crude drug:solvent ratio (1:3-1:10) and solvent concentration (70-96% ethanol). Determination of antioxidant activity using the CUPRAC, FRAP and DPPH methods, as well as TPC and TFC levels, was carried out using ultraviolet-visible spectrophotometry. Identification and determination of flavonoid compounds in optimized extract was performed by high-performance liquid chromatography. The optimal extraction of soursop leaves by maceration and pressing was obtained at extraction duration of 10 min, crude drug:solvent ratio of 1.0:6.5 and a solvent concentration of 96%. These conditions produced TPC and TFC levels, DPPH, CUPRAC, and FRAP values 119.388±14.057 mg gallic acid equivalent/g, 91.212±4.796 mg quercetin equivalent/g and 189.095±15.931, 162.121±11.076 and 204.679±5.164 mg ascorbic acid equivalent antioxidant capacity/g respectively. In general, the flavonoid and phenol content in ethanol soursop leaf extract was associated with antioxidant activity of DPPH, CUPRAC and FRAP. Additionally, the optimized extract of soursop leaves contained a rutin compound (11.52±1.06 mg/g).