Phenolic Compounds and Antioxidant Activity of Rice Straw Extract

Agricultural wastes cause a serious environmental problem in Egypt. Utilization of these wastes by an environmentally friendly way is a very important issue. The objective of this study was to utilize rice straw into high-value products. Antioxidant activity, total soluble phenols, and flavonoids were evaluated in ethyl acetate extract prepared from rice straw after alkaline hydrolysis. Total phenols were found to be 221.6 mg gallic acid equivalents (GAE), while total flavonoids were 4.9 mg rutin equivalents (RE). The extract exhibited strong antioxidant activity measured by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging method and its IC50 value was 0.4 mg/ml. Furthermore, the ethyl acetate extract possessed high antioxidant activity assayed by β-carotene bleaching method and its value of lipid peroxidation inhibition (LPI) was 75.4%. It also exhibited high reducing power and its IC50 value was equal to 0.06 mg/ml. HPLC analysis indicated that this extract contained seven phenolic acids including; protocatechuic, caffeic, syringic, p-coumaric, ferulic, rosmarinic and cinnamic, in addition to two flavonoids; qurecetin and kaempferol. Results indicated that ferulic and p-coumaric acids were the major soluble phenolic acids in rice straw, and their concentrations were 3.9 and 2.9 mg/g DW. It can be summarized that the strong antioxidant activity of ethyl acetate extract, prepared from rice straw, was highly correlated with its high level of phenolic compounds.