Effect of thermal treatments on anti-nutritional factors and antioxidant capabilities in yellow soybeans and green-cotyledon small black soybeans.

Background: The effects of different thermal processes on the anti-nutritional factors (tannins, phytic acids and trypsin inhibitors) and antioxidant capacity of yellow soybeans (YS) and green-cotyledon small black soybeans (GCSBS) were investigated. The soybean samples were subjected to roasting, microwaving and boiling as the thermal treatments.

Results: Sixty-minute boiling was most effective for eliminating the tannins in both YS and GCSBS, and also resulted in the maximal reduction of phytic acids. Trypsin inhibitors were most effectively reduced (76.92-97.91%) by microwaving, and boiling was least favourable for retaining total phenolic content. The total flavonoid content was significantly (P < 0.05) decreased by all thermal processes, and particularly by employing 60-min boiling. The total anthocyanin content of GCSBS was higher than that of YS, but significantly declined upon boiling (P < 0.05). Roasting did not significantly affect the DPPH free radical scavenging activity of GCSBS (P > 0.05). Microwaving increased the FRAP (ferric reducing antioxidant power) of GCSBS up to 6.07%.

Conclusion: Our raw GCSBS possesses much better antioxidant capacity than raw YS. Microwaving resulted in a substantial reduction of the trypsin inhibitors of our soybean samples. Dry thermal treatments caused slight increase of FRAP in GCSBS.