A new microplate procedure for simultaneous assessment of lipophilic and hydrophilic antioxidants and pro-oxidants, using crocin and β-carotene bleaching methods in a single combined assay: Tea extracts as a case study

β-Carotene and crocin bleaching reactions are the basis of two methods extensively used to quantify antioxidant and pro-oxidant activities. They are appropriate for lipophilic and hydrophilic matrices, respectively, and can provide useful complementary information in the study of complex natural extracts containing components with variable degrees of polarity. In this regard, a microplate procedure (Carotene Combined Bleaching) is proposed that enables the combination of both methods in a single, informative and less expensive method which is also faster to carry out. As an illustrative model, the method was applied to test a set of commercial lipophilic and hydrophilic antioxidants and some predictable pro-oxidant agents. Afterwards, as a food compound case study, the antioxidant activity of five types of tea extracts (Green, Blue, White, Black and Red) were characterized and their equivalent potential activity was calculated using commercial antioxidants on the basis of the new procedure developed in this research. The activity of the tea extracts decreased in the following order: (a) In a predominantly lipophilic environment: White > Black > Red > Blue > Green tea extracts; and (b) In a predominantly hydrophilic environment: Green > Red > White > Black > Blue tea extracts