| The objective of this study was to investigate the effect of low moisture, high moisture, and repeated high moisture extrusion on protein-protein interactions in soy protein isolate. Six solvents combinations were used to extract soluble proteins in extrudates from low moisture, high moisture, repeated high moisture, and dead-stop extrusion studies. The protein solubility results were used to elucidate changes in chemical bonds before and after various extrusions. SDS-PAGE was applied to some soluble protein samples to examine the molecular weight distribution of protein subunits. SEM was also used to observe the microstructure and texture of extrudates from repeated high moisture extrusion. In both low moisture and high moisture extrusion, product temperature changes from 125 to 140°C did not cause significant difference on the amount of soluble proteins. However, the samples between low moisture and high moisture extrusion did show significant differences. The results of the repeated extrusion suggested that the chemical bonds that contributed to the texturization of the extrudates could be broken apart and reform by the extrusion process. The repeated extrusion resulted in a darker and more fibrous extrudate as shown in the microstructure. In the dead stop experiment, the results suggested that major changes in the network of polypeptide chains and protein solubility occurred in zone 3 and zone 4 of extruder barrel. The chemical bonds that contributed to the texturization of proteins were almost the same from zone 4 on to the end of the extruder. In addition, melted proteins were realigned by the directional shear force at the cooling die to form the fibrous structure. The reduction in protein solubility of extrudates from low moisture extrusion, high moisture extrusion and of samples taken from zone 3 and zone 4 from the dead stop study were probably due to the formation of three dimensional network of soy protein isolate polypeptide chains. As a result of extrusion, these polypeptide chains aggregated together and became less accessible to the solvents used to extract soluble proteins. The solvent system consisted of PBS+2-ME+Urea extracted most soluble proteins from all extrudates. This was followed by PBS+SDS+Urea. The results from SDS-PAGE of soluble proteins showed very little changes in the protein subunit molecular weight distribution and any changes in the protein subunits were very small. It appears that the major effect of extrusion was to disassemble soy proteins into protein subunits and then reassemble them together by disulfide bonds, hydrogen bonds, and noncovalent interactions resulting in fibrous extrudates. |
