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.