Antioxidant Activity of Foods: Development of Singlet Oxygen Absorption Capacity (SOAC) Assay Method.

Recently a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants (AOs) and food extracts was proposed. Singlet oxygen (1O2) quenching rates (kQ) and the relative SOAC values were measured for many carotenoids and phenolic AOs in ethanol/chloroform/D2O (50 : 50 : 1, v/v/v) solution at 35ºC using UV-vis spectrophotometry. It has been clarified that the SOAC method is useful to evaluate the 1O2-quenching activity of lipophilic and hydrophilic AOs having 5 orders of magnitude different rate constants (kQ). Measurements of the kQ and SOAC values were also performed for 39 kinds of food extracts. The results indicate that the SOAC method is useful to evaluate the 1O2-quenching activity of food extracts having two orders of magnitude different kQ values. Further, the kQ values for the reaction of 1O2 with 8 carotenoids and 8 vitamin E homologues were measured in an aqueous Triton X-100 (5.0 wt %) micellar solution (pH 7.4). Results obtained demonstrate that the kQ values of AOs in homogeneous and heterogeneous solutions vary notably depending on (i) polarity (dielectric constant (ε)) of the reaction field between AOs and 1O2, (ii) local concentration of AOs, and (iii) mobility of AOs in solution. Measurements of kQ and SOAC values in a micellar solution may be useful for evaluating the 1O2-quenching activity of AOs in biological systems. Furthermore, measurements of the SOAC values were performed for 32 kinds of food extracts using a microplate reader. The SOAC assay method was validated by inter-laboratory validation study due to 14 laboratories.