Heat-induced Soy Protein Gels Research Articles

Effects of removal of non-network protein on the rheological properties of heat-induced soy protein gels

Effects of removal of non-network proteins by diffusion process on the storage modulus and loss modulus of soy protein isolate (SPI) gels and glycinin (11S) gels as a function of heating temperature and ionic strength were investigated. The composition of non-network proteins was determined by native-PAGE, non-reducing SDS-PAGE and reducing diagonal SDS–PAGE. Results showed that non-network proteins were composed of a majority of acid polypeptides (A), small amounts of AB subunits, soybean agglutinin, Bowman−Birk trypsin inhibitor, and lower amounts of α′, α and A3 polypeptides. The results further revealed that 11S gels had higher ratios of non-network proteins than SPI gels, due to the increased ratio of A polypeptides in 11S gel. In addition, the removal of non-network proteins from the gel was found to have no effect on the storage modulus, but on the other hand resulted in a decrease in the loss modulus, suggesting that the loss modulus of the gel network is closely related to non-network proteins. This study presents an approach to investigate the changes of storage modulus and loss modulus of globular gels in relation to the removal of non-network proteins, and provides valuable information on the composition and content of these proteins in gel network formed at various conditions.

Effect of temperature, ionic strength and 11S ratio on the rheological properties of heat-induced soy protein gels in relation to network proteins content and aggregates size

The effects of heating temperature, ionic strength and 11S ratio on the rheological properties of heat-induced soy protein gels, particularly in relation to network proteins content and aggregates size were investigated. Results revealed that non-network proteins consisted of undenatured glycinin (11S) AB subunits, high levels of A and A3 polypeptides but low levels of β-conglycinin (7S) α and α′ subunits. Results further showed that a positive correlation between storage modulus (G′) and network proteins ratio of gels that were formed at different temperatures. Additionally, the G′ values of gels initially increased with increasing NaCl concentration from 0 to 0.25 M, but decreased with further increasing ionic strength from 0.25 to 0.50 M. This trend was consistent with the variation of heat-induced protein aggregates size in soy protein solution at the same ionic strength. Higher 11S/7S ratio resulted in higher storage modulus values of soy protein gels due to the formation of larger and compacter aggregates mainly by B polypeptides via hydrophobic bonds.

Effect of 7S/11S ratio on the network structure of heat-induced soy protein gels: a study of probe release

The effect of 7S/11S ratio on the soy gel network will be uncovered by probe diffusion kinetics.

Release behavior of non-network proteins and its relationship to the structure of heat-induced soy protein gels.

Heat-induced soy protein gels were prepared by heating protein solutions at 12%, 15% ,or 18% for 0.5, 1.0, or 2.0 h. The release of non-network proteins from gel slices was conducted in 10 mM pH 7.0 sodium phosphate buffer. SDS-PAGE and diagonal electrophoresis demonstrated that the released proteins consisted of undenatured AB subunits and denatured proteins including monomers of A polypeptides, disulfide bond linked dimers, trimers, and polymers of A polypeptides, and an unidentified 15 kDa protein. SEC-HPLC analysis of non-network proteins revealed three major protein peaks, with molecular weights of approximately 253.9, 44.8, and 9.7 kDa. The experimental data showed that the time-dependent release of the three fractions from soy protein gels fit Fick's second law. An increasing protein concentration or heating time resulted in a decrease in diffusion coefficients of non-network proteins. A power law expression was used to describe the relationship between non-network protein diffusion coefficient and molecular weight, for which the exponent (α) shifted to higher value with an increase in protein concentration or heating time, indicating that a more compact gel structure was formed.

Concentration dependence of dynamic moduli of heat-induced soy protein gels

The concentration dependence of dynamic moduli of soy protein gels was studied for different protein preparations (soy protein isolate), purified glycinin and a β-conglycinin rich fraction) at various pHs and salt concentrations. The concentration dependence of the storage modulus of glycinin and β-conglycinin gels was similar to that of SPI gels. For SPI, the critical protein concentration for gelation was estimated to be between 3 and 5% at pH 7.6 (0.2 M NaCl), and between 6.5 and 8% at pH 7 (0 M NaCl), and 0% at pH 5.2 and pH 3.8 (0.2 M NaCl). At the high pHs the critical protein concentration decreased with the stage in the heating cycle. Relating the experimental data to a fractal model, both rheological and permeability measurements resulted in a consistent value for the fractal dimensionality D f (=2.3) for SPI gels at pH 3.8 and 0.2 M NaCl. At pH 5.2 and 7.6 (0.2 M NaCl), and pH 7 (0 M NaCl), the concentration dependence of the modulus could not be analysed theoretically. Likely this is due to rearrangements in the spatial structure of the network starting directly after a percolating network was formed. It causes the concentration dependence to depend on ageing time.

Structural Properties of Heat-Induced Soy Protein Gels As Affected by Ionic Strength and pH

Hydration properties of acidic soy protein gels, prepared with different salt solutions, were studied. The type of bonds that stabilize gel structure and the nature of protein species that make up and stabilize such structure were also investigated. The microstructure of gels was evaluated by scanning electron microscopy (SEM) and water-holding capacity (WHC) assays. The stability and nature of protein fractions of gel matrices were analyzed by solubility measurements and sodium dodecyl sulfate−polyacrylamide gel electrophoresis. The WHC of gels prepared with NaCl and CaCl2 decreased with increasing salt concentration. This fact suggested, as was corroborated by gel SEM, that at high ionic strength a more open matrix was formed. The structure of acidic gels, stabilized by noncovalent bonds, changed with NaCl addition. Both 7S and 11S globulin subunits participated via hydrophobic interactions to the stability of pH 2.75 gels. At pH 3.50 the gel matrix was stabilized by hydrophobic interactions among β-con...