Content composition and antioxidant activity of isoflavones in commercial and homemade soymilk and tofu

BACKGROUND: Isoflavones, found in soymilk and tofu, are one of the phytochemicals in soy-based products that may promote good health. Homemade tofu and various homemade soymilk samples were made using different soaking, grinding, and cooking methods. The homemade samples were compared to commercial tofu and soymilk for total isoflavone content and composition as well as their antioxidant capacity. All samples were freeze-dried and extracted with a 58% acetonitrile solution which was subsequently used to determine the isoflavone content by reverse-phase high-performance liquid chromatography. The antioxidant activity of extracts was determined using a modified 2,2-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) method and total antioxidant capacity was reported as ascorbic acid equivalents. RESULTS: The total isoflavone, aglycone, and antioxidant levels were significantly higher in homemade soymilk and tofu (1571 mu g) than in commercial samples. Homemade soymilk made by the extended boiling method yielded the highest total isoflavone (2567 mu g) and glucoside (1525 mu g) content. A strong positive correlation was observed between the total isoflavone, aglycone conjugates, and genistein series concentration and antioxidant capacity of soymilk. CONCLUSION: Increased moist heating time yielded the highest concentration of total isoflavones as well as aglycone conjugates and the genistein series. Increasing the duration of boiling can increase the isoflavone content of both homemade and commercial soymilk and tofu. (c) 2007 Society of Chemical Industry.