Structural characterisation of the exopolysaccharide produced by Streptococcus thermophilus EU20

Streptococcus thermophilus EU20 when grown on skimmed milk secretes a high-molecular-weight exopolysaccharide that is composed of glucose, galactose and rhamnose in a molar ratio of 2:3:2. Using chemical techniques and 1D and 2D-NMR spectroscopy (¹H and ¹³C) the polysaccharide has been shown to possess a heptasaccharide repeating unit having the following structure: Treatment of the polysaccharide with mild acid (0.5 M TFA, 100 °C for 1 h) liberates two oligosaccharides; the components correspond to the repeating unit and a hexasaccharide equivalent to the repeating unit minus the terminal α-L-Rhap.