Optimisation of an ultrasound-assisted extraction method for the simultaneous determination of phenolics in rice grains

Fifteen phenolic compounds were determined in rice grains by ultrasound assisted extraction and ultra-performance liquid chromatography with photodiode array detection. Primarily, an UAE method has been developed and validated for the extraction of phenolics from rice grains. For the optimization, a Box-Behnken Design based on six factors including extraction temperature (10-70 °C), solvent composition (0-50% methanol in water), cycle (0.2-0.7 s^-1), ultrasound amplitude (30-70%), and solvent to sample ratio (2.5:1 to 5:1) was employed. Multi-response optimization (MRO) was performed to develop a simultaneous extraction method of 15 phenolics. The optimal UAE conditions were: extraction temperature 45 °C, time 25 min, cycle 0.4 s^-1, ultrasound amplitude 47%, solvent composition 80% methanol in water at pH 4.25, and sample to solvent ratio 1:5. Linearity, LODs, LOQs, precision and recovery were determined. In addition, the method was successfully applied to analyse a number of rice samples with different varieties.