Structure-activity relationship of exopolysaccharides produced by Limosilactobacillus fermentum A51 and the mechanism contributing to the textural properties of yogurt

Exopolysaccharides (EPS) produced by lactic acid bacteria considerably affect the texture of yogurt. However, the mechanism underlying the improvement in texture is not well understood. In this study, the structure-activity relationship of the neutral polysaccharide, EPS-1, produced by Limosilactobacillus fermentum A51 (L. fermentum A51), and the mechanism underlying the formation of high-quality yogurt texture were investigated. The results showed that EPS-1 effectively filled the three-dimensional network structure of the casein (CAS) clusters and improved the textural properties of yogurt. Structural analysis demonstrated that EPS-1 was composed of fucose, galactose, glucose, mannose, and galacturonic acid. In addition, EPS-1 exhibited a crystalline structure (2θ = 19.98°) and possessed good thermal stability (223.8 °C) and gelling properties. Furthermore, porous and regular sheet-like structures were observed in the microstructure of EPS-1 using scanning electron microscopy (SEM). Moreover, X-ray diffraction, SEM, confocal laser scanning microscopy (CLSM), and particle size analysis confirmed the unique structure-activity relationship of EPS-1, which interacted with CAS clusters to form stable CAS-EPS-1 complexes. Fourier transform infrared spectroscopy and zeta potential analysis demonstrated that the complexes were formed via hydrogen bonding (N-H and O-H) and electrostatic interactions (amide bond). Molecular docking revealed that EPS-1 was bound to active residues, Ile27, Ser30, Glu20, Glu26, Val23, and Leu21, of α-CAS via hydrogen bonds. These findings offer new insights regarding how EPS improves the texture of yogurt and indicate that L. fermentum A51 can be used for improving the textural properties of dairy products.