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 (k_Q) 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 (k_Q). Measurements of the k_Q 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 k_Q values. Further, the k_Q 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 k_Q 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 k_Q 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.