Response surface methodology (RSM) was used to estimate the optimum extraction parameters, in which the antioxidant
activity (AA) of the extract from the defatted marigold residue was the strongest. The AA of marigold extracts was
investigated by the radical scavenging activity assays ((2,2-azinobis-(3-ethylbenzothiazolin-6-sulfonic acid) (ABTS)
and 1,1-diphenyl-2-picrylhydrazyl (DPPH)). Results demonstrated that AA was significantly affected by the content
of total phenolics (TP) and total flavonoids (TFA) (p < 0.0001). The optimal extraction parameters of marigold extracts
for the highest AA by ABTS method were ethanol concentration of 79.7%, extraction temperature of 74.2 ◦C and
time of 8.1h, and by DPPH assay with 89.3% of ethanol concentration at 81.5 ◦C for 11.1h, AA values were 2.42 and
1.86 mmolTE/g, respectively