Inability of total antioxidant activity assays to accurately assess the phenolic compounds of date palm fruit (Phoenix dactylifera L.)

Background: Phenolic compounds in date palm fruit (dates) contribute to the color, antioxidant activity, and nutritional value of the fruit. This study aimed to compare the accuracy of the Folin–Ciocalteu, 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS^.+), ferric reducing antioxidant power (FRAP), and 1,1-diphenyl-2-picrylhydrazyl (DPPH) methods for measuring the total antioxidant activity of date fruit extracts. Methods: A total of 26 date varieties from the United Arab Emirates and Pakistan that varied in color and shape were analyzed in terms of their total phenolic content and antioxidant activity using the Folin–Ciocalteu, ABTS^.+, FRAP, and DPPH methods. Two extracts were obtained from each date variety to study the soluble and insoluble phenolic fractions. Results: The total phenolic content varied 46–397 mg GAE/100 g fresh weight (FW) using the Folin–Ciocalteu method, 0.9–4.3 μmol TE/100 g FW using ABTS^.+, 355–2421 μmol TE/100 g FW using FRAP, and 0.0–1.8 mg/mL using DPPH. These results suggest large variations in total phenolic content and antioxidant activity among the date varieties. However, the results of the four total antioxidant activity assays were not correlated with each other and exhibited large disparity. Conclusions: These findings put in question the ability of these total antioxidant activity assays to accurately assess the phenolic compounds of dates.