Effects of steaming on soybean proteins and trypsin inhibitors

Steaming of defatted soybean flakes decreases both protein solubility and trypsin inhibitor (TI) activity as measured by nitrogen solubility index (NSI) and TI assay, respectively. NSI can be Iowered from 80–90, in raw flakes, to 10–20 with sufficient steaming. Concurrently, about 90% of TI activity is destroyed. Differential scanning calorimetric (DSC) analysis of raw soy flour demonstrated transition temperatures (T max ) of 58.2, 72.8 and 96.4°C for TI, 7S and 11S proteins, respectively. Lowering the NSI to 42.4 by steaming caused the TI transition to disappear, even though half of the TI activity remained. DSC analysis of steamed soybean flakes revealed that undenatured 7S and 11S storage proteins remain, even when the NSI is decreased to 10.