Optimization of extraction yield and antioxidant properties of Brassica oleracea Convar Capitata Var L. leaf extracts

•The total extract yield of phytochemicals in Brassica oleracea leaves was increased with increase in solvent polarity.•The extracts in high polarity solvents shoed stronger antioxidant potential against metals and free radicals.•The extracts in low polarity solvents were found to be effective in protection against lipid peroxidation.•The optimum value of solvent polarity was 2.8 D for total extract yield and 4.4 D for antioxidant potential.

Effects of extraction time and solvent polarity on antioxidant properties of Brassica oleracea leaves were optimized by response surface methodology using a central composite design. Five extraction times (12, 24, 36, 48 and 60 h) and solvent polarities (dipole moment - hexane: 0.0, diethyl ether: 2.80, ethyl acetate: 4.40, methanol: 5.10 and water: 9.0 D) were selected for optimization. Response surface analysis of data showed a significant increase (p < 0.05) in extract yield and antioxidant potential, based on total phenolic acids, reducing abilities and free radical scavenging capacities, in response to an increase in extraction time and solvent polarity. The optimal response was obtained using relatively polar solvents (4.40-9.00 D) and prolonged extraction times (50-60 h). This suggests that most of the phytochemical constituents of B. oleracea leaves are polar and possess strong antioxidant potential.