Optimization of microwave-assisted extraction of polyphenols from Quercus bark

Abstract

Box–Behnken response surface methodology was used to investigate and optimize microwave assisted extraction (MAE) (treatment power and time) of antioxidant compounds, especially polyphenols, from bark extracts. Moreover, the effects of solvent composition (methanol and ethanol) particle size, and pH conditions were evaluated and optimized. In addition, the effects of MAE were compared to conventional solvent extraction at room temperature and reflux extraction (at 100°C). The results indicated that all process variables had a significant effect on polyphenol and antioxidant recovery, except for methanol (p>0.05). For instance, TPC and antioxidant activity were increased when higher power and longer treatment time were used. Optimum MAE conditions were obtained with a particle size 0.5×1×0.3 cm3, energy (power 45W and irradiation time 60min), pH 10.75, 33% of ethanol and 0.38% of methanol. Verification of predictive model showed that the model was suitable for the optimization of phenolic compounds from bark. Under optimized conditions, extraction efficiencies were increased by 3 times and 2 times respectively for total phenolic content and antioxidant recoveries, as compared to conventional methods that use the same parameters but without MAE. Moreover, the determination of phenolic profile was performed by LC–MS. The LC analysis results showed that MAE increased the content of (+)-catechin, (−)-epicatechin, (−)-epigallocatechin, syringic acid, p-coumaric acid, sinapic acid and naringenin compared to control extraction sample at the equivalent diffusion time conditions and temperature.