Application of Far-Infrared and Subcritical Water for Recovering Antioxidant Compounds from Rice Hulls

Far-infrared (FIR) irradiation and subcritical water were applied for recovering antioxidant compounds from rice hulls (RHs). After 30 min of FIR treatment at 100°C, the radical scavenging activity (RSA) and total phenol contents (TPC) of RH extracts increased from 47.74% to 79.63% and from 0.12 mM to 0.19 mM, respectively, compared to control. The inhibition of lipid peroxidation in extracts was also increased from 41.07% to 47.96%. According to the GC-MS analysis, more phenolic compounds (p-coumaric acid, 3-vinyl-1-oxy benzene, p-hydroxy benzaldehyde, vanillin, p-hydroxy benzoic acid, and 4,7-dihydroxy vanillic acid) were detected in FIR-irradiated RH extract. On the other hand, subcritical water (SCW) is hot water under pressure sufficient to maintain water in the liquid state. The TPC of RHs extract increased from 0.165 mM of extraction at 25°C for 10 min to 0.270 mM of 200°C for 60 min, and the RSA of RHs extract increased from 8.40% of extraction at 25°C for 10 min to 79.33 of 200°C for 60 min. The effect of SCW extraction temperatures and treatment periods on TPC and RSA showed almost same trends. Our results support that FIR irradiation or SCW extraction could be an excellent method for recovering antioxidant materials from biowaste such as RHs.