Update:
The isoflavone concentrations in fresh frozen edamame and soybean oil samples were lower than the limit of detection. In the soybean samples, the concentrations of daidzein, genistein and glycitein were 30-100, 4-10 and 30-70 µg/g soybean, respectively. For the tofu samples, the concentrations of daidzein, genistein and glycitein were 2.58-20.98, 13.54-52.50 and 12.31-40.68 µg/g tofu sample, respectively. The amounts of daidzein, genistein and glycitein in soy yoghurt samples were 14.00-14.34, 11.50-11.80 and 12.53-13.03 µg/g sample weight, respectively. For soymilk samples, the concentrations of daidzein, genistein and glycitein were 8.76-10.02, 3-9.06 and 7.56-10.62 µg/mL soymilk, resepectively. Soybean had the greatest isoflavone content followed by tofu, soy yoghurt and soy milk samples. These results indicate the isoflavone concentrations of soy products, but the analysis should be redone using dry weight of soy samples, since the water content in some fresh soy products might significantly change the results. No isoflavones were detected in fresh frozen edamame or soybean oil samples. Isoflavones are metabolized during the later growth stages of soybean, so these compounds did not exist in the early stage edamame samples.
Different soy products showed various capacities in antioxidant activity (scavenging ABTS radicals). For solid samples, fresh frozen edamame had the greatest antioxidant activity (0.23 µmol trolox equivalent/g sample) followed by the extra soft tofu (0.18 µmol trolox equivalent/g sample). No detection of antioxidant activity in some of our soybean samples was attributed to mistakes during experiment processing. For liquid soy products, all the samples showed almost similar antioxidant activities.
For solid soy samples, soybean and edamame samples showed greater total phenolic content (TPC) values compared with the tofu products; this might be due to the lower water content in bean samples, especially for the dry soybean samples. In liquid soy products, no phenolic compounds could be detected from soybean oil samples due to the polarity of oil. Soy yoghurt and soymilk samples showed relatively greater but almost similar TPC values, because their chemical profiles are almost similar.
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This project evaluated the nutritional values of soy foods to see where soy can contribute to a healthy diet. Soybean had the greatest isoflavone content followed by tofu, soy yoghurt and soy milk. Isoflavones have benefits to heart health and bone health. Fresh frozen edamame had the greatest antioxidant activity followed by extra soft tofu. Intake of antioxidants is closely linked with reduction in the incidence of several chronic diseases, including cancer. For solid soy samples, soybean and edamame samples also showed greater total phenolic content (TPC) values compared with tofu products, and soy yoghurt and soymilk samples showed relatively greater but almost similar TPC values. TPC is believed to contribute to decreased risk of developing cancer.