Functional Components in Soybean Cake and Their Effects on Antioxidant Activity

Abstract

The antioxidant activities of four fractions of isoflavones from soybean cake were evaluated and compared with those of ISO-1 and ISO-2 fractions, five isoflavone standards, and mixtures of two or four isoflavone standards, as well as four commercial antioxidants, using DPPH, TEAC, reducing power, metal ion chelating, conjugated diene, and TBARS assays. Both malonylglucoside and glucoside fractions were isolated using preparative chromatography with Diaion HP-20 as adsorbent, whereas acetylglucoside and aglycone fractions were separated with silica gel as adsorbent. The other two fractions, ISO-1 and ISO-2, were soybean cake extracts containing 12 isoflavones for the former and a combination of 4 fractions for the latter. Both acetylglucoside and ISO-1 fractions exhibited the highest efficiency in scavenging DPPH free radicals, whereas all six fractions were effective in inhibiting conjugated diene formation. However, a low reducing power was observed for all six fractions and isoflavone standards. The aglycone fraction and genistein standard showed a pronounced increase of TEAC value and a moderate decrease of TBARs value. For chelating metal ions, both ISO-1 and ISO-2 fractions were the most efficient. Overall, the isoflavone fractions showed a better antioxidant activity than the isoflavone standards, probably caused by the presence of some other functional components such as saponin, flavonoid, and phenolic compounds in soybean cake.