Effects of Aqueous Extraction Factors on Commercial Protease Inhibitory Activities, Phenolic Acid Profiles and Chemical Properties of Hom Mali-105 Rice (Oryza sativa L.) Bran Extract

De- oiled rice bran (DRB) is a by-product generated in a large amount from oil extraction processing. The DRB contains a high content of protein and is rich in phenolic components. Phenolic compounds were noticed to exhibit inhibitory activity against protease to some extent in protein hydrolysate production from rice bran. Therefore, this investigation aims to study the factors effecting the extraction of phenolic compounds from DRB and the inhibitory activities of the extracted phenolics against commercial proteases. The aqueous extraction process was employed for extraction of phenolic compounds from DRB. Three extraction factors, including time (30-180 min), temperature (60-80 °C), and pH level (2-10) were used using completely randomized design (CRD). Total phenolic content (TPC) and profiles, total flavonoids content (TFC), and antioxidant activities of all extracts were analyzed. Additionally, the commercial proteases (G6 and GN) inhibitory activities of phenolic extracts were determined. The optimum extraction time, temperature and pH that providing highest total phenolic content, flavonoid content antioxidant activities and protease inhibitory activities were observed at 150 min, 70°C and pH 10, respectively. TPC and TFC values were in the ranges of 2.50 to 5.76 mg GAE/g rice bran and 1.79 to 4.42 mg RTE/g rice bran, respectively. DPPH, ABTS and FRAP values were in the ranges of 0.44 to 0.82 mg GAE/g rice bran, 2.56 to 5.14 mg TE/g rice bran, and 4.40 to 8.10 mg TE/g rice bran, respectively. The main phenolic acids were ferulic acid, p-coumaric acid, protocatechuic acid, vanillic acid, sinapic acid, and p-hydroxybenzoic acid. The protease inhibitory activity of the DRB extract was 42% on protease G6 and 71% on protease GN. This result suggests that the phenolics and flavonoids from rice bran could exhibit protease inhibitory activities to some extent and these compounds might affect the hydrolysis process of protein during protein hydrolysate production.