Submitted to: Journal of Food Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/28/2007
Publication Date: 1/1/2008
Citation: Cramp, G., Kwanyuen, P., Daubert, C. 2008. Molecular Interactions and Functionality of a Cold-Gelling Soy Protein Isolate. Journal of Food Science 73:E16-E24. Interpretive Summary: A thermally modified soy protein isolate (SPI) that had been prepared at 8% protein demonstrated improved heat stability and cold-set gel functionality while thermally modified SPI at lower concentration at 3% protein did not. This suggested that heat denaturation at low protein concentration limited network formation impacting viscosity parameters of the system. Treatment of SPI with denaturants and modifiers established the role of disulfide bonds in network formation. Additional treatment with urea combined with reducing agent suggested that hydrophobic interactions played a primary role in gel strength after disulfide bonds formation. Therefore, the need for a higher protein concentration during thermal modification indicated that the critical disulfide bonds are intermolecular.
Technical Abstract: A thermally modified, readily reconstituted, soy protein isolate (SPI) that had been prepared at 8% protein for three hours at 95°C demonstrated improved heat stability and cold-set gel functionality when compared to a control SPI. When SPI was heated at 3% protein equivalently, prior to reconstitution to 8% protein, the final viscosity was about three orders of magnitude less than the original sample. The viscosity of SPI heated at 3% protein was still nearly two orders of magnitude less than the original sample after both samples were reheated at 8% protein. These results suggested that heat denaturation at low protein concentrations limited network formation even after the protein concentration and interaction sites increased, impacting the viscosity parameters of the system. Gelation was entirely prevented upon treatment of SPI with iodoacetamide, which carbaminomethylated the cysteine residues, establishing the role of disulfide bonds in network formation. The viscosity of the 8% protein dispersion was also reduced by two orders of magnitude when treated with 8M urea, and when combined with 10mM DTT the viscosity of the gel was decreased by another order of magnitude. These results suggested that hydrophobic interactions played a primary role in gel strength after disulfide bonds form. The need for a higher concentration of protein during the heating step indicated that the critical disulfide bonds are intermolecular. Ultimately, the functionality produced by these protein-protein interactions produced a powdered soy protein isolate ingredient with consistent cold-set and thermal gelation properties.