Source of Antioxidant Activity of Soybeans and Soy Products

ABSTRACT Soybeans, defatted soy flour, soy protein concentrates, and soy isolates possess appreciable antioxidant activity in lipid‐aqueous systems. The antioxidant properties of soybeans, defatted soy flour, and soy protein concentrates are due primarily to polyphenolic compounds. Antioxidant activity of soy protein isolates are also due to polyphenolic compounds but other compounds also exert a marked antioxidant effect. Polyphenolic antioxidants of soy were found to be isoflavones, chlorogenic acid isomers, caffeic acid, and ferulic acid. These compounds occurred primarily as glycosides. The isoflavone aglycones were identified as genistein (5,7,4′‐trihydroxyisoflavone), daidzein (7,4′‐dihydroxyisoflavone), and glycitein (7,4′‐dihydroxyd‐methoxyisoflavone. The compounds have antioxidant activity but not to the same extent as flavonoids with the dihydroxy configuration in either the α or β ring.