2.5. Measurement of total phenolics
Total phenolic content was determined by the modified method of Folin–Ciocalteu (Singleton and Rossi, 1965). Frozen peel tissues (3.0 g) from eight fingers were dipped into 30 mL of methanol and ultrasonic-assisted extraction under 50 °C for 30 min to optimize extraction. They were then centrifuged at 25 °C for 20 min at 10,000 × g, and the precipitate was dipped into 30 mL of methanol, two times; in combination with the supernatant. The reaction mixture consisted of 0.5 mL of the methanol extracts, 5 mL of distilled water and 0.5 mL of the Folin–Ciocalteu reagent. After 8 min, 1.5 mL of saturated 20% sodium carbonate solution was added. The mixture was thoroughly mixed, and allowed to stand for 30 min at room temperature and the absorbance was measured at 760 nm. Each measurement was repeated three times and aqueous solutions of gallic acid (between 0 and 100 mg L−1) were used for making a standard curve. Results were expressed as mg of gallic acid equivalents (GAEs) per 100 g of fresh weight (mg of GAEs/100 g of FW, fresh weight)