Abstract: Objective: To establish a colorimetric method to determine the antioxidant capacity of water-soluble compounds in functional food based on the reaction between 1,10-2 phenanthroline and Fe3+ to form yellow complexes. Methods: The hydrophilic antioxidants in functional foods were extracted with pH 7.4 PBS buffer solution (75 mmol/L) and allowed to reduce Fe3+ to Fe2+. Fe2+ was exposed to 1,10-2 phenanthroline to form a stable Fe2+-phenanthroline complex. The absorbance of the complex was determined on a spectrophotometer at 536 nm and used to calculate the antioxidant capacity of water-soluble compounds in functional food using Trolox, a water soluble vitamin E analog, as the reference standard. The accuracy, linearity, repeatability, recovery rate and limit of detection (LOD) of this method were studied, and its feasibility was also verified. Results: This method showed a good linearity over the Trolox concentration of 0.66－10.53 μmol/L (R2 = 0.9999). The LOD was 0.54 μmol/L. The replicability and repeatability was 2.7% and 3.3% relative standard deviation (RSD), respectively. The mean spike recovery was (99.9±4.6)% (n = 5). Conclusion: This method is applicable to determine the antioxidant capacity of water-soluble compounds in functional food.

Key words: functional food, antioxidant capacity, phenanthroline-Fe3+ method