Response surface optimisation of antioxidant extraction from kinema, a Bacillus-fermented soybean food

The objective of this work was to optimise different parameters (solvent concentration, time and temperature) for antioxidant extraction from kinema, demonstrated by total phenol content (TPC) and DPPH-scavenging activity (DSA), using response surface methodology. A central composite design was performed to determine the effect of solvent concentration (methanol, 30–100%), temperature (30–60 °C) and time (30–150 min) on the TPC and DSA of the extract. The solvent concentration and temperature had the most significant (p < 0.05) effect. The optimum conditions for the TPC extraction and DSA were 100% methanol, 50 °C and 30 min, in which 140 mg of gallic acid equivalent (GAE) g−1 lyophilised extract and 52% DSA were predicted, while 135 mg GAE g−1 lyophilised extract and 56% DSA were experimentally obtained. No significant difference (p < 0.05) was found between the experimental and predicted values of the response variables.