Total phenolic compounds and antioxidant activity

The determination of total phenolic compounds was performed by the reaction with Folin-Ciocalteau reagent [8]. Quantification was performed based on the calibration curve of gallic acid (2.0–8.0 mg/L), and the results were expressed in mg gallic acid equivalent (GAE)/100 g sample.

The DPPH method was performed as described by Brand-Williams et al. [9]. The decrease in the absorbance of 100 μM DPPH• radicals (2.9 mL) dissolved in 80% methanol was evaluated at 515 nm, 30 min after the addition of each extract. The total antioxidant activity of peel and edible portion (on a dry weight basis) were expressed in μMol/g of TEAC.

The ferric reducing antioxidant potential (FRAP) assay was performed according to the method described by Benzie and Strain [10], based on the direct measurement of antioxidant (reducing) ability through the reduction of the complex Fe³⁺/tripyridyltriazine (TPTZ) to Fe²⁺ at acid pH (3.6). The absorbance was read at 620 nm using a UV–vis spectrophotometer during the monitoring period (2 h). The antioxidant potential of the peel and edible portion extracts was determined based on a calibration curve plotted using FeSO₄∙7H₂O at a concentration ranging between 500 and 2000 μM.