The exopolysaccharides produced by Streptococcus thermophilus Rs and Sts have the same repeating unit but differ in viscosity of their milk cultures

The polysaccharides produced by Streptococcus thermophilus Rs and Sts in skimmed milk consist of -Gal and -Rha in a molar ratio of 5:2. Linkage analysis and 1D/2D NMR (¹H and ¹³C) studies revealed that both polysaccharides have the same branched heptasaccharide repeating unit:

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Remarkably, the two strains differ in their effects on the viscosity of stirred milk cultures. The milk culture of S. thermophilus Rs is non-ropy and affords 135 mg/L polysaccharide with an average molecular mass of 2.6×10³ kDa. In contrast, the milk culture of S. thermophilus Sts is ropy and produces 127 mg/L polysaccharide with an average molecular mass of 3.7×10³ kDa. Permeability measurements of non-stirred milk cultures of both strains suggest that both strains have a similar effect on the protein–polysaccharide network. Therefore, the only clear difference between both strains, which may cause the difference in ropiness of the milk cultures, is the difference in molecular mass of the polysaccharide.