Structural changes of exopolysaccharide of Lactobacillus helveticus MB2-1 during fermentation and its effect on fermented milks quality

The primary objective of this study was to investigate the influence of exopolysaccharides (LHEP), produced by Lactobacillus helveticus MB2-1, on the quality characteristics of fermented milk. The structure, conformational properties of LHEP, and the structural quality characteristics of fermented milk at different fermentation points (ranging from 1 to 8 h) were dynamically monitored. The results revealed no statistically significant variations in the monosaccharide composition among the eight LHEP samples analyzed. These samples all have a total sugar content over 90 % in terms of chemical composition, exhibiting high purity, with negligible protein content. The Fourier transform infrared (FT-IR) spectra of the LHEP samples exhibited variations as the fermentation time increased. Microscopic observation revealed that the morphology of the LHEP samples became progressively rougher and more fragmented, resulting in enhanced solubility. Conformational analysis of the samples consistently showed soft and irregular shapes. Notably, the mean-square radius of gyration (R_g) and hydrodynamic radius (R_h) initially decreased and subsequently increased with prolonged fermentation, potentially correlated with the decreasing pH condition of the fermented milk. These changes coincided with the structural alterations observed in the FT-IR analysis, confirming that conformational changes in LHEP influenced the spectroscopic structural characteristics. Moreover, as the fermentation time increased, both the LHEP samples and L. helveticus MB2-1 fermented milk exhibited enhanced rheological properties, along with progressively improved textural characteristics of the fermented milk. This further substantiated that the structural transformation of LHEP derived from L. helveticus MB2-1 played a crucial role in influencing the textural quality characteristics of the fermented milk. In all, the findings contributed a theoretical foundation for promoting the application of LHEP produced by L. helveticus MB2-1 in the field of food processing.