A comprehensive evaluation of chelating ability of olive phenolic compounds and method development of Ferrozine assay

Abstract: Natural phenolic compounds such as olive phenolics perform antioxidant activities but are still limitedly used for substituting EDTA which are synthetic chelators for preserving fatty foods. And, there is limited data regarding the chelation ability of olive phenolics, and few reports can be found for the Ferrozine-iron assay under pH conditions.
Chemical standards of olive phenolics were analyzed for iron chelation ability by Ferrozine assay in pH 3.78 and 6.81 within concentrations between 0.04 and 0.5 mM. EDTA and phytic acid were used for comparison.
Limited changes were observed for standard curves at different pH with R2 >0.9969. High chelation olive phenolics were determined to include phytic acid 2.103 mM Iron(II)/ mM molecule or ‘iron(II) ion per molecule’, hydroxytyrosol 1.133, verbascoside 1.047, caffeic acid 0.9942, gallic acid 0.9708, rutin 0.7015, luteolin 0.5767 and oleuropein 0.2625 at pH 6.81. However, at pH 3.78, almost all of the chelating ability diminished to zero, while EDTA maintains strong chelations that are more than 1.879 at both pHs. Less than 0.1245 to zero ‘iron ion per molecule’ of chelation abilities were observed for apigenin, vanillin, apigenin-7-glucoside, o-coumaric acid, vanillic acid, cinnamic acid, p-coumaric acid, ferulic acid, 4-hydroxyphenylacetate and tyrosol at both pHs. At pH 6.81, the R2 of linear relationship by chelation ability vs. molecular weight was 0.0476; while better and higher R2 of linear relationship was observed to be 0.5123 by chelation ability vs. the number of potential chelation sets such as phenolic hydroxyl groups and carbonyl groups.
This study provides chelation data of olive phenolics for potential application in foods at different pH conditions as natural additives. The developed assay with a standardized unit of ‘iron ion per molecule’ could be used for chelation comparisons across different experiments.