Measuring antioxidant capacity directly in emulsions: A new paper-based FRAP assay and comparison with its DPPH...

A paper-based Ferric Reducing Antioxidant Power (FRAP) assay was developed to monitor the antioxidant potential of emulsions without prior extraction. The assay successfully quantified the antioxidant capacity of two hydrophilic antioxidants - ascorbic acid and N-acetyl-L-cysteine - showing an initially linear concentration-response relationship (R² ≥0.934) and produced half-maximal effective concentration values consistent with literature. However, the assay exhibited significant limitations regarding lipophilic antioxidants and water-in-oil emulsions, a result attributed to the FRAP reagent's inherent hydrophilicity. The assay met official guidelines for precision and accuracy when analyzing quality controls but showed decreased accuracy and increased variability upon analysis of commercial emulsions. Nonetheless, its level of performance remains acceptable for cosmetic and food industry applications, where comparative analysis and stability profiling are often more relevant. Comparative studies showed that its DPPH counterpart is more versatile, working with a broader range of antioxidants and with both oil-in-water and water-in-oil emulsions, all while having fewer interferents. Method outline • Analysis of antioxidant capacity of emulsions is based on measurement of reflected light intensity by a smartphone camera • An extraction-free paper-based format has been validated employing an adapted FRAP methodology. • Limitations of the FRAP-based format when compared to its DPPH counterpart are presented, relevant to the hydrophobicity of the antioxidant under study and the emulsion type.