Exploration of the key factors influencing the viscosity of exopolysaccharides produced by Streptococcus thermophilus in milk fermentation through comparative studies

Fermented milk viscosity is strongly influenced by exopolysaccharides (EPS) produced by lactic acid bacteria (LAB), although the exact correlation remains unclear. This study analyzed the relationship between EPS yield and fermented milk viscosity across 24 Streptococcus thermophilus strains. Two phenotypically distinct strains, S6-13 and S4-2-8, differing in EPS yield and viscosity, were selected for further comparison of EPS production, molecular characteristics, and genetic determinants. A moderate correlation was observed between EPS yield and viscosity, indicating that yield alone does not determine viscosity. At equivalent concentrations, EPS-1 from S6-13 exhibited significantly higher viscosity than EPS-3 from S4-2-8. Both EPS were composed of glucose, galactose, and N-acetylglucosamine and lacked a triple helix structure. However, EPS-1 exhibited a higher molecular weight (8.4 × 10⁶ Da) and branched chains, while EPS-3 was unbranched with a molecular weight of 1.3 × 10⁶ Da. These findings suggested that molecular weight and degree of branching are critical factors influencing viscosity. Genomic analysis revealed that the higher EPS production in S6-13 may be associated with an increased number of glycosyltransferase and carbohydrate-related regulatory (crr) genes. Moreover, differences in the eps gene cluster may have contributed to the observed variation in molecular weight and branching patterns.