Trypsin inhibitor activity, phenolic content and antioxidant capacity of soymilk as affected by grinding temperatures, heating methods and soybean varieties

Two soybean varieties (yellow and black) were ground under three temperature regimes (cold, ambient and hot) and then the raw soymilk underwent three thermal treatments (traditional stove cooking, one-phase ultra-high temperature (UHT) processing, and two-phase UHT processing). Trypsin inhibitor activity (TIA), chymotrypsin inhibitor activity (CIA), total phenolic content (TPC), total flavonoid content (TFC), condensed tannin content (CTC), DPPH free radical scavenging activity, and oxygen radical absorbing capacity (ORAC) were significantly affected by the above factors and their interactions. For raw yellow soymilk, hot grinding generated 64 and 69% lower TIA than cold and ambient grinding, respectively. For raw black soymilk, hot grinding generated 40 and 43% lower TIA than cold and ambient grinding, respectively. After heating process, the advantage of hot grinding was reduced. By suppression of the development of lipoxygenase-induced hydroperoxides, hot grinding preserved the most sulfhydryl and disulfide groups. Stove cooking following ambient grinding resulted in the lowest CIA. Except TFC, hot grinding recovered the highest phenolic content and antioxidant capacity. CTC from hot grinding was about twice that from ambient grinding. Black soymilk contained significantly higher antioxidant capacity than yellow soymilk and UHT treatments showed advantage over traditional stove cooking in the preservation of phenolics.