Physicochemical characteristics and bioactivities of the exopolysaccharide and its sulphated polymer from Streptococcus thermophilus GST-6

• The EPS produced by S. thermophilus in skim milk was extracted and sulphonated.
• The EPS was composed of hexasaccharide repeating unit of glucose and galactose.
• More uniform microstructure was obtained after sulphonation of the EPS.
• Sulphonation lowered the degradation temperature of the EPS.
• Antioxidant and antibacterial activities increased after sulphonation of the EPS.

Exopolysaccharide (EPS) produced by Streptococcus thermophilus GST-6 in skim milk was extracted and purified. The EPS was composed of glucose and galactose in a molar ratio of 1.80:1.03 with identical primary structure to the EPS from S. thermophilus ST1 reported previously. The purified EPS was sulphated at a sulphonation degree of 0.26 ± 0.03, and presence of sulphate groups in the sulphated EPS (SEPS) was confirmed. Microstructural studies demonstrated a porous web with coarse surface for the EPS while the SEPS appeared as stacked flakes with relatively uniform shapes. Sulphonation of the EPS slightly decreased its degrading temperature from 234.6 °C to 232.5 °C. The DPPH, superoxide and hydroxyl radicals scavenging activities of the EPS were significantly (P < 0.05) improved after sulphonation. The SEPS also showed stronger inhibitory activity than the EPS against Eschericia coli, Salmonella typhimurium and Staphylococcus aureus.