Determination of Iron (III) Reducing Antioxidant Capacity for Manuka Honey and Comparison with ABTS and Other Methods

Abstract

Aims: Applying multiple assays with trolox as the sole reference compound is a recent AOAC proposal to improve the reliability of total antioxidant capacity determinations. The aim of this study was to evaluate, iron (III) reducing antioxidant capacity (iRAC) for Manuka honey samples and comparisons with ABTS and other well-known assays. Study Design:  In-vitro, laboratory-based study. Place and Duration of Study: School of Biomedical Sciences, Faculty of Life and Health Sciences, Ulster University, Cromore Road, Coleraine, BT52 1SA, UK; September 2015-May 2016. Methodology: Manuka honey rated Unique Manuka Factor (UMF) 5+, 10+, 15+, 18+ and a non-rated (NR) sample were analysed using five assays for total antioxidant capacity namely, iRAC, ABTS, DPPH, FRAP, and Folin assays. Values for total antioxidant capacity were normalized as Trolox Equivalent Antioxidant capacity (TEAC) for comparison within and between assays. Results: The TAC were correlated for all methods (R2 = 0.83-0.99) and also correlated with the total phenols content. Actual TEAC value for a given honey ranged by 21-70-fold depending on the assay method with the following general order of increase; DPPH < FRAP (pH 3.6) < iRAC (pH 7.0) < ABTS (pH7) < Folin (pH ~11). The trends in TAC values are discussed alongside of TEAC values for 50 food items and some challenges for comparing different antioxidant methods are highlighted. Conclusion: Total antioxidant capacity of Manuka honey changes in a regular manner probably affected by assay pH. The findings are important for attempts to standardize antioxidant methods as currently applied to foods, beverages and dietary supplements.  Further research is recommended to examine the effect of normalizing antioxidant methods for solvent composition and pH.