Molecular interactions and functionality of a cold-gelling soy protein isolate

ABSTRACT:  A soy protein isolate (SPI) was thermally denatured at a critical concentration of 8% protein for 3 h at 95 °C, resulting in a powder that was readily reconstituted at ambient temperature and that 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 3 orders of magnitude less than the original sample. The viscosity of SPI heated at 3% protein was still nearly 2 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 isolate's cold gelling ability. Gelation was 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 2 orders of magnitude when treated with 8 M urea, and when combined with 10 mM DTT the gel viscosity 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.     

Interactions between soy protein isolate and xanthan in heat-induced gels: The effect of salt addition

The influence of xanthan and/or KCl addition on the properties of heat-induced soy protein isolate (SPI) gels at pH 3.0 was studied. Changes in protein solubility and subunit composition as well as in the mechanical properties, microstructure and water holding capacity of the gels were determined. The effect of KCl addition on each biopolymer solution was also investigated. The results indicated that SPI–xanthan gels prepared without KCl were mainly stabilized by non-covalent (H-bonding and hydrophobic) and SS bond interactions, whereas in gels containing KCl, electrostatic interactions were also involved in maintaining the gel structure. The β-7S subunit was probably the fraction electrostatically linked to the xanthan. The different values found for the mechanical properties after the addition of xanthan and/or KCl, were associated with the coarseness of the gel. Xanthan and KCl probably showed a synergistic effect on the Young modulus because KCl induced a conformation transition of the xanthan molecules.