Abstract

In this study, the effect of extraction conditions on the antioxidant activity of Vernonia amygdalina Del. (Asteraceae) was evaluated by Response surface methodology and central composite design (RSM-CCD) to predict the content of phenolic compounds with maximum antioxidant activity. Total phenol and flavonoid contents were determined by spectrophotometry method, especially the flavonoid content was identified by HPLC-DAD system. The antioxidant activity was estimated by the DPPH and the FRAP method. Results showed that extracting time, extracting temperature and solvent-to-material ratio had a significant effect on phenolic content (p < 0.001). The interactions between the three factors were also found to be significant at 0.05 level of probability. After re-estimating predicted variables on the experiment, we found that the polyphenol content was 137.15 ± 1.36 mg gallic acid /g dry weight (dw), the flavonoid content was 96.78 ± 1.39 mg quercetin/g dw, the total antioxidant activity was 1.95 ± 0.09 mg ascorbic acid/g dw and iron reduction activity was 5.90 ± 0.12 mg FeSO4/g dw at optimum conditions of 34.82 hours at 53.09 °C with solvent to material ratio is 43.64 (ml/g). The correlation coefficients were greater than 0.995 observed between the predicted and actual values for the response variables, which are evidences that the regression model can represent the experimental data well. HPLC showed that leaves contain at least six flavonoids, two of which are apigenin and luteolin. The flavonoids apigenin and luteolin were identified in the extract from Vernonia amygdalina with high levels of apigenin (2.72 mg/g dw), luteolin (3.76 mg/g dw).