Chemical cross-linking and molecular aggregation of soybean protein during extrusion cooking at low and high moisture content

Soybean protein isolate (SPI) was extruded using a co-rotating twin-screw extruder at low (28%) and high (60%) moisture content. Dead-stop operation was employed to obtain samples from different zones of the extruder. The protein solubility of extrudate was analyzed based on 8 different combinations of chemical bond-breaking solutions, which contained only phosphate buffer (P); two reagents, i.e. sodium dodecyl sulfate (SDS) in P (P + S), urea in P (P + U) and 2-mercaptoethanol (2-ME) in P (P + M); three reagents, i.e. (P + S + U), (P + U + M) and (P + M + S); four reagents, i.e. (P + S + U + M). Based on the protein solubility in the combined extraction solutions, each specific chemical bond and their interactions responsible for supporting the extrudate structure were further analyzed. The aggregation of protein subunits in extruded sample was investigated by using SDS-PAGE electrophoresis. The results showed that hydrophobic interactions, hydrogen bonds, disulfide bonds and their interactions collectively hold the structure of extrudate; and the importance of non-covalent bonds outweighs covalent bonds. Increasing feed moisture content could increase the interactions between disulfide bonds and hydrogen bonds and between disulfide bonds and hydrophobic interactions (p < 0.05), reduce the degree of aggregation and the difference in protein–protein interactions and protein subunits among different zones within the extruder.