Optimization of ultrasound-assisted extraction conditions for phenolics, antioxidant, and tyrosinase inhibitory activities of Vietnamese brown seaweed (Padina australis)

This study aimed to determine the optimum ultrasound-assisted extraction (UAE) conditions for obtaining the highest yields of phenolics and antioxidants from Padina australis. The effects of extraction variables including temperature (40–60°C), extraction time (50–80 min), ethanol concentration (0%–60%) and sample-to-solvent ratio (1–5 g/100 mL) on the total phenolic content (TPC), DPPH radical scavenging activity (DRSA) and ferric reducing antioxidant power (FRAP) were determined and optimized using Box-Behnken design in conjunction with response surface methodology (RSM). The extraction temperature, ethanol concentration and sample-to-solvent ratio significantly affected TPC and DRSA, while extraction time had no impact on TPC and antioxidant activity of P. australis within the tested ranges. The optimal UAE conditions were determined to be ultrasonic temperature of 60°C, ultrasonic time of 60 min, solvent concentration of 60% (v/v) aqueous ethanol and sample-to-solvent ratio of 1 g/100 mL. The developed mathematical models were found to be reliable predictors for TPC, DRSA and FRAP, and the predicted values fit well with the experimental data (R2 = 0.86–0.96). The extract was then fractionated to generate n-hexane, ethyl acetate and aqueous fractions. Of these fractions, ethyl acetate fraction was found to possess the highest values of TPC (807.20 mg GAE/g), DPPH (1,417.01 mg TE/g), FRAP (615.07 mg TE/g dry fraction) and the strongest tyrosinase inhibitory activity (29.90 mg AAE/g). Practical applications: The study employed ultrasound-assisted extraction technique, an advanced extraction method, and RSM method for maximizing the yields of phenolics and antioxidants from P. australis. In addition, the tyrosinase inhibitory activity of P. australis extract and its fractions, for the first time, was also investigated. The results suggested the optimal UAE conditions to recover phenolics and bioactive compounds with high antioxidant and tyrosinase inhibitory properties from P. australis. Moreover, this study establishes that the ethyl acetate fraction derived from P. australis was a rich source of phenolics and antioxidants; thus, it could be further isolated and purified to obtain individual bioactive compounds for the utilization in the food and cosmeceutical industries. © 2021 Wiley Periodicals LLC.