Effect of high intensity ultrasound on transglutaminase-catalyzed soy protein isolate cold set gel

The effects of high intensity ultrasound (HIU, 105–110 W/cm² for 5 or 40 min) pre-treatment of soy protein isolate (SPI) on the physicochemical properties of ensuing transglutaminase-catalyzed soy protein isolate cold set gel (TSCG) were investigated in this study. The gel strength of TSCG increased remarkably from 34.5 to 207.1 g for TSCG produced from SPI with 40 min HIU pre-treatment. Moreover, gel yield and water holding capacity also increased after HIU pre-treatments. Scanning electron microscopy showed that HIU of SPI resulted in a more uniform and denser microstructure of TSCG. The content of free sulfhydryl (SH) groups was higher in HIU TSCG than non-HIU TSG, even though greater decrease of the SH groups present in HIU treated SPI was observed when the TSCG was formed, suggesting the involvement of disulfide bonds in gel formation. Protein solubility of TSCG in both denaturing and non-denaturing solvents was higher after HIU pretreatment, and changes in hydrophobic amino acid residues as well as in polypeptide backbone conformation and secondary structure of TSCG were demonstrated by Raman spectroscopy. These results suggest that increased inter-molecular ε-(γ-glutamyl) lysine isopeptide bonds, disulfide bonds and hydrophobic interactions might have contributed to the HIU TSCG gel network. In conclusion, HIU changed physicochemical and structural properties of SPI, producing better substrates for TGase. The resulting TSCG network structure was formed with greater involvement of covalent and non-covalent interactions between SPI molecules and aggregates than in the TSCG from non-HIU SPI.