Developmental changes in phenolic compounds, antioxidant capacity and enzymes activity in skin of 'El-Bayadi' table grapes

In two successive seasons, changes in phenolic compounds, antioxidant capacity and enzymes activity in skin of `El-Bayadi' table grape skin during development were evaluated. The growth of berries followed a typical double sigmoid curve, and veraison stage corresponded to the inception of the slow growth phase. Total phenols concentration gradually decreased during berry development reaching 0.67 and 1.34 g kg(-1) at ripening stage for 2014 and 2015 season, respectively. Total flavonoids concentration gradually decreased during growth with a slight increase at ripening stage reaching 0.59 and 0.38 g kg(-1) for 2014 and 2015 season, respectively. Total phenols concentration showed higher values in 2015 than 2014 season, in contrast to total flavonoids. Both trans-resveratrol and trans-picied concentrations were highest early in the season, but gradually decreased during growth with an increase during ripening, representing 0.28 and 0.26 mg kg(-1) for trans-resveratrol, and 1.39 and 1.12 mg kg(-1) for trans-pitied, for 2014 and 2015 season, respectively. The level of trans-pitied was much higher than that of trans-resveratrol during growth and ripening in both seasons. Antioxidant capacity gradually increased (lower IC50 values of DPPH and ABTS) during growth and ripening. Peroxidase (POD) activity gradually increased to a peak at veraison with a slight fluctuation during ripening. While, polyphenoloxidase (PPO) activity initially increased, slightly fluctuated and then decrease during ripening. Polygalacturonase (PG) activity gradually increased to a maximum at 20 days after veraison and, then decrease at ripening. While, xylanase activity was highest early in the season, but sharply decreased during growth and, then increased during veraison and ripening.