Potential Value of Bioactive and Enzymatic Antioxidant Compounds in Grapefruit (Citrus × Paradisi Macf.) Varieties

Growing concern about the safety of commonly used synthetic antioxidants has increased the attention toward natural antioxidants that occur as secondary metabolites in plants. The aim of this study was to investigate the bioactive composition from the fruit pulp and juice of nine varieties of grapefruit (Citrus × paradise Macf.). Total antioxidant activity performed using DPPH (2,2-diphenyl-1-picrylhydrazyl) and FRAP (ferric reducing antioxidant power) assay revealed ‘Ray Ruby’, ‘Rio Red’, and ‘Ruby Red’ as promising varieties with strong antioxidant activity. Pearson’s correlation analysis showed that the total phenols had a significant effect on the antioxidant activity of the grapefruit, as indicated by a positive correlation with the DPPH assay (r = 0.494^**). The TPC (total phenolic content) showed significant differences (77.66-100.32 mg GAE/100 ml) between the grapefruit varieties ( p<0.5). The two grapefruit varieties ‘Rio Red’ and ‘Ray Ruby’ had relatively higher TPC (100.32/100 ml) and 97.68/100 ml, respectively. Naringin, which is responsible for the bitter taste in grapefruit, is found in lower concentrations in deeply red grapefruit varieties like ‘Ruby Red’, ‘Rio Red’, and ‘Star Ruby’, making them more effective in increasing the acceptability of grapefruit among consumers. Heat map analysis based on bioactive composition clustered the genotypes representing higher antioxidant potential into a single cluster ‘A’ (Star Ruby, Rio Red, Ray Ruby). The genotypes ‘Rio Red’ and ‘Ray Ruby’ exhibited higher enzymatic activity (catalase, peroxidase and ascorbate peroxidase), which was effective in reducing the hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) content. The results showed that grapefruit pulp contain phenolic compounds and flavonoids, as well as antioxidant enzymatic activity. ‘Ray Ruby’, ‘Rio Red’, and ‘Ruby Red’ identified as promising genotypes with the optimum level of both enzymatic and non-enzymatic antioxidant compounds.