Effects of Bacterial Cellulose Combined pH Shifting Treatment on Gel Characteristics and Microstructure of Soy Protein Isolate

This study aimed to investigate the effects of bacterial cellulose combined pH shifting treatment on the gel properties and microstructure of soy protein isolate, and to elucidate the underlying mechanisms, so as to provide the theoretical support for the interactions between insoluble polysaccharides and protein molecules.

Soy protein isolate-bacterial cellulose composite systems were prepared with varying ratios of bacterial cellulose, with and without pH shifting treatment. Then, the effects of bacterial cellulose combined with pH shift treatment the gel properties, rheological characteristics, and microstructure of the soy protein isolate composite systems were study.

After the bacterial cellulose combined with pH shifting treatment, the particle size distribution analysis revealed a new composite peak around 4 145 nm, indicating an increase in the particle size of the soy protein isolate composite system. Additionally, as the ratio of bacterial cellulose increased, the particle size further rose to approximately 4 801 nm. The turbidity of the soy protein isolate with bacterial cellulose combined pH shifting treatment significantly increased, and this turbidity also rose with the higher addition of bacterial cellulose. Notably, the surface hydrophobicity of the group with bacterial cellulose combined pH shifting treatment was significantly enhanced (P<0.05). The visual appearance of the composite gel indicated that both the group with bacterial cellulose addition and the group with bacterial cellulose combined pH shifting treatment exhibited a smooth surface and good elasticity following the thermal process. The gel strength and rheological properties of the group with bacterial cellulose combined pH shifting treatment (15:1) showed significant improvement (P<0.05), with values increasing from 21.49 g and 0.93 Pa to 129.16 g and 556.2 Pa, respectively (P<0.05). Furthermore, the content of the β-sheet structure in the secondary structure increased significantly from 39.58% to 42.05% (P<0.05). Paraffin section results indicated that the bacterial cellulose physically filled the protein gel network, showing a distinct boundary with the soy protein isolate. In contrast, the gel network of the composite system treated with bacterial cellulose combined pH shifting treatment was uniformly distributed. Scanning electron microscopy (SEM) analysis showed that the soy protein isolate exhibited a porous structure with numerous aggregated formations, while the group with bacterial cellulose addition demonstrated extensive areas of accumulation. In the group bacterial cellulose combined pH shifting treatment, the filamentous bacterial cellulose was found to be embedded within the soy protein isolate, resulting in a tightly cross-linked protein structure. Laser confocal microscopy results indicated the presence of numerous pore structures in the control group, while the addition of bacterial cellulose did not reduce these pore structures and exhibited a phase-separated microstructure. However, after the pH shifting treatment, a tighter intertwined connection was formed between the soy protein isolate and bacterial cellulose, leading to a microstructure transformation from phase separation to phase uniformity, accompanied by a decrease in pore structure.

The combination of bacterial cellulose with pH shift treatment could improve the gel properties of soybean protein isolate, and promote the phase separation structure to a uniform phase structure, which made the microstructure of the composite gel system more uniform and dense.