The DPPH assay was based on the slightly modified method of Li et al. (2009). A 5 µL sample was reacted with 995 µL DPPH⋅ solution in the dark at room temperature for 30 min. Reaction time was determined by analysis of the kinetics curve, which showed when the reaction reached the plateau stage. GA was used as the standard compound. The calibration curve was acquired by plotting the DPPH⋅ scavenging percentage (Abs (515 nm)t/Abs (515 nm)0, where t equals the reaction time) against GA concentration. The amount of GA equivalents in each sample was calculated using the equation y (scavenging ratio) = 11.4118 x(GA equivalents content) – 0.0062 (r2 = 0.9992). Results were expressed as mg of GA equivalents per g of DW.