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)