Freeze-dried ABTS+ method: A ready-to-use radical powder to assess antioxidant capacity of vegetable oils

A new approach to 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS+) assay was conducted. ABTS+ radical solution in water was obtained by using classical method of incubating ABTS with potassium peroxodisulphate. The resulting radical solution was freeze-dried and tested for its radical efficiency by comparing freshly prepared ABTS+ (FP-ABTS) via using standard antioxidants. A high correlation was observed between the results obtained from FP-ABTS and freeze-dried ABTS (FD-ABTS) assays. When it was stored in the freezer, FD-ABTS powder was found to keep its initial activity to a great extend over one month. For the measurement of antiradical capacity of oil samples, FD-ABTS was dissolved in methanol and diluted to desired concentrations with a chloroform–methanol mixture. Linearity and accuracy of the method was confirmed by measuring the antiradical capacity of stripped sunflower oil spiked with different concentrations of standard antioxidants. Antiradical capacity of 22 oil samples including some commonly consumed vegetable oils was also tested by the FD-ABTS method.