Significance of FRAP, DPPH, and CUPRAC assays for antioxidant activity determination in apple fruit extracts

Thirteen apple cultivars were analyzed for their total phenolic content, total flavonoids, anthocyanins, ascorbic acid in methanolic extracts of both peel and cortex fractions. Three in vitro assays (FRAP, DPPH, and CUPRAC) were used to determine the antioxidant activity. Concentration of the phytochemicals studied varied greatly between the apple peel and the cortex region. Peels showed ~ 2.8 times higher total phenolic content and ~ 2.68% higher flavonoid content than the cortex. Principal component analysis could successfully explain 76.86% and 84.27% variability in the antioxidant determinants (antioxidants/assays) in the peel and cortex region of apple cultivars, respectively. Major contributor for antioxidant activity in both apple peel and cortex was total flavonoid content. Cultivars 'Well Spur' and 'Oregon Spur II' were found to be substantially rich in these two antioxidants. The antioxidant activity was best expressed by the in vitro FRAP assay in both the fractions. Non-hierarchical K-medoids clustering reflected the presence of an antioxidant/ assay protocol apart from the antioxidant/assay we considered in this study that needs further exploration to get full spectra of antioxidant profile across apple genotypes. Based on multivariate analysis and the concept of RACI, the FRAP antioxidant assay is recommended for determining antioxidant activity in apples.