Antioxidant activity of commercial wild rice and characterization of phenolic compounds by HPLC-DAD-ESI-MS/MS.

摘要

Wild rice (Zizania) as the only cereal native to North America played an important role in Americans' life in history. Due to the nutritional value and unique taste, it is currently used in a wide range of gourmet food products. In order to signify the potential health benefits to be derived from consumption of this cereal grain, the antioxidant capacities of eleven commercial wild rice samples (raw, mixed, and processed) were investigated in this study and their phenolic profiles were explored using HPLC-DAD-ESI-MS/MS techniques. Two solvent systems, aqueous methanol and acidic acetone, were involved in the extraction of phenolic compounds. The total phenolic contents (TPC) and antioxidant activities (AOA) of these two extracts were determined by Folin-Ciocalteau assay, DPPH photometric method and oxygen radical absorbance capacity (ORAC) assay. The results showed that the methanol extracts and acetone extracts contained comparable levels of TPC. The higher DPPH radical scavenging ability was found in acetone extracts, whilst methanol extracts showed greater ORAC values. Wild rice, compared with white rice (control sample), was more effective as an antioxidant. Significant differences (p0.05) in antioxidant activities were detected among raw, mixed and processed samples. Several phenolic acids and their derivatives were detected in alkaline hydrolysates of methanol extracts and the remaining residues. Ferulic acid and sinapic acid were found in the most quantities, mainly occurring in insoluble bound form. Other monomeric phenolic acids detected in wild rice consisted of p-coumaric, vanillic, syringic, and p-hydroxybenzoic acids, along with two phenolic aldehydes (vanillin and p-hydroxybenzaldehyde). Phenolic acid dehydrodimers are cell wall bound and only appeared in the insoluble fractions featured by diferulic acids (DFA) and disinapic acids (DSA). The isomers of DFA included 8-8', 5-5', 8-O-4', and 8-5' (benzofuran form) coupled dimers, with 8-0-4' as the predominant form. DSA only appeared as 8-8' coupled with the noncyclic isomers in most quantities. The antioxidants identified in acetone extracts were flavonoid glycosides (di-glucosyl apigenin, glucosyl-arabinosyl apigenin, and di-arabinosyl apigenin) and flavan-3-ols (catechin, epicatechin and oligomeric procyanidin). The above identified phenolic acids and flavonoids made significant contribution to the antioxidant activity of wild rice.