Emerging evidence supports that a number of health-promoting substances have been identified in rice (Oryza sativa) including phenolic compounds. The development of single optimized extraction of phenolic compounds from plant may complicate due to the structural diversity as having multiple hydroxyl groups which results the polarities of these compounds vary significantly. Therefore, an optimised extraction method that effectively extracts phenolics from rice is needed prior to the reliable determination of these compounds in the grains.
A new ultrasound-assisted extraction (UAE) of phenolics has been optimized and validated. A three-level, six-factor Box-Behnken design in conjunction with Multi-responses optimisation was employed to evaluate the significances of studied factors (solvent composition, temperature, solvent-solid ratio, pH, amplitude and cycle) on the extraction yields. Subsequently, extraction kinetic was studied to confirm high recovery of phenolics extracted from the matrix. The levels of phenolics were quantified by Ultrahigh Performance Liquid Chromatography coupled with Photo-diode Array detector (UPLC-PDA) in less than 5 min.
The optimal UAE condition was as follows: extraction time 25 min, ultrasound amplitude 47%, cycle 0.4 s-1, extraction temperature 45oC, pH 5.5, extraction solvent 80% MeOH in water and solvent-solid ratio 5:1. A complete validation of the method was performed and presented agreeable linearity (correlation coefficients between 0.998 and 0.999) with high precision (in average, expressed as CV, repeatability and intermediate precision were lower than 3 and 4.5% respectively). The method was successfully applied in the extraction of a number of real samples in order to evaluate the applicability. Hence, it can be concluded that UAE under optimum conditions can be considered as a powerful tool for the determination of phenolic compounds from a wide variety of rice grains.