Antioxidant activity of the natural flavonoids luteolin and catechin, which shows inconsistent comparative results in literature, was extensively evaluated as affected by the 1,4-pyrone moiety and 3-OH group. Catechin and luteolin were compared to the synthetic antioxidant-butylated hydroxytoluene for their 2,2-diphenyl-1-picrylhydrazyl (DPPH● ) scavenging activity in polar protic and aprotic solvents and ferric reducing-antioxidant power (FRAP). Moreover, their effect on lipid oxidation kinetics of canola and olive oil triacylglycerols as well as their oil-in-water (O/W) emulsions, in addition to oil stability indices (OSIs), was evaluated. In the DPPH● assay, catechin's 3-OH group led to lower IC50 values than luteolin's 1,4-pyrone moiety in acetone (3.4 µM compared with 9.4 µM), while there was no significant difference in methanol (IC50 = approximately 18.3 µM) or tetrahydrofuran (IC50 = approximately 27.2 µM). The FRAP test indicated a higher reducing power for catechin than for luteolin (689.4 µM compared with 573.1 µM). The antioxidants showed various performances in the oil triacylglycerols and their O/W emulsions due predominantly to the interfacial phenomena. A better OSI was found for catechin than for luteolin (20.0 to 52.7 hr compared with 3.5 to 4.2 hr) in Rancimat test.
Keywords: antioxidant activity; catechin; interfacial phenomena; luteolin.
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