Abstract: This study aimed to systematically investigate the effects of different salt ions on the formation of transglutaminase (TG) cross-linked soybean isolate protein (SPI) gels and the underlying mechanisms. Our results showed that treatments with 0.1 mol/L Na+, K+ and Mg2+ induced the formation of SPI gels with high gel strength and water-holding capacity (WHC), and the gel strength of the treated groups was 2.43, 1.51 and 2.32-fold higher than that of the control group, respectively. According to rheological analysis, the storage modulus (G’) and loss modulus (G”) of the salt ion-treated groups were significantly higher than those of the control group, which exhibited a slight frequency dependence. Molecular docking analysis showed that salt ions might directly affect the structure of TG cross-linked SPI to improve its gel properties. Specifically, the surface hydrophobicity and fluorescence intensity of the three treated groups remarkably decreased, while the absolute value of the zeta potential and average particle size increased significantly, indicating that different salt ions could promote the aggregation of protein molecules, causing the spatial structure to become more compact. Fourier transform infrared spectroscopy (FTIR) revealed that the three salt ions could induce an increase in the content of β-folding in SPI gels and a decrease in the contents of α-helix and random coil, suggesting that the secondary structure tended to become ordered after treatment with salt ions. The results of low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) confirmed that salt ion treatments reduced the mobility of water in SPI gels, which was consistent with increasing the WHC. Besides, the microstructure of salt ion-treated SPI gels exhibited the morphological features of dense stacked aggregates. Therefore, Na+, K+ and Mg2+ have potential in enhancing the gel properties of TG cross-linked SPI gels during food processing.

Key words: soybean isolate protein; transglutaminase; salt ions; gel strength; protein secondary structure