Characterization of phenolics, betacyanins and antioxidant activities of the seed, leaf, sprout, flower and stalk extracts of three Amaranthus species

•Total and individual phenolics differed among parts of three Amaranthus species.•Total individual betacyanins differed among parts of three Amaranthus species.•Amaranthus hypochondriacus leaf showed the highest antioxidant activities.•The antioxidant activities positively correlated with the phytochemical content.