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 þ S), urea in P (P þ U) and 2-mercaptoethanol (2-ME) in P (P þ M); three
reagents, i.e. (P þ S þ U), (P þ U þ M) and (P þ M þ S); four reagents, i.e. (P þ S þ U þ 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 proteineprotein interactions and protein subunits among different
zones within the extruder.