Physicochemical, rheological and digestive characteristics of soy protein isolate gel induced by lactic acid bacteria

Influence of lactic acid bacteria (LAB) on gel characteristics of fermentation-induced soy protein isolate (SPI) gel, including hardness, water holding capacity (WHC), pH, solubility, turbidity, particle size, zeta potential, surface hydrophobicity, free sulfhydryl, microstructure, rheological properties and in vitro gastrointestinal digestion was evaluated. First, fermentation increased the turbidity and average particle size, which was supported by a lower degree of solubility and zeta potential absolute value. The surface hydrophobicity was reduced after lactic acid bacteria fermentation, and a similar phenomenon occurred on the free sulfhydryl groups. It was found that fermentation resulted in SPI denaturation, structural changes, and aggregates formation, which were the foundation of gel formation. Subsequently, fermentation-induced SPI gels depended on the lactic acid bacteria used for fermentation, showing the specificity of gel characteristics. Among them, Lactobacillus acidophilus (L. acidophilus), Lactobacillus plantarum (L. plantarum), Lactobacillus casei (L. casei), and mixed strains of Lactobacillus bulgaricus (L. bulgaricus), and Streptococcus thermophilus (S. thermophilus) (1:1) exhibited the high coagulation ability, which could enhance the hardness and water holding capacity of the gels. L. plantarum-G had the lowest solubility and free sulfhydryl content and the highest turbidity, particle size, and zeta potential. Scanning electron microscope (SEM) results showed that L. casei-G and L. plantarum-G exhibited a dense and uniform three-dimensional network. The rheological properties showed that the earliest gel point and the highest storage modulus (G') and loss modulus (G") were acquired by L. casei-G, whereas the G' curve of L. lactis-G was still in the growth stage, and no platform was found in the growth curve. The ability of L. lactis to form gels was weaker. At the same time, the G' and G" of all LAB fermentation-induced gels showed strong frequency dependency. Digestion of gels was much slower than that of the soy protein isolate solution. The peptide content increased significantly during the digestion process, but, the average particle size decreased (p < 0.05). The digestion could be influenced by the microstructure of the fermentation-induced gels caused by the immobilisation of the protein in the network and the steric obstruction, including the entrance of the enzyme and release of the peptide. The current work has explored the potential of applying starters in fermented gels by performing texture, aggregation, rheological and digestive properties analysis. Our objective is to identify cultures that are suitable for developing novel functional fermentation-induced gels.