Abstract: Streptococcus thermophilus Q4F8 is a mutant strain producing extracellular polysaccharide, which was obtained through mutagenesis screening. In order to realize industrial application, its fermentation process was optimized to improve the yield of extracellular polysaccharide. In this study, GM17 was taken as the basic medium and the medium components and culture conditions were optimized through the combined use of one-factor-at-a-time method, Plackett-Burman design (PBD) and response surface methodology based on Box-Behnken design (BBD). The results showed that the influence of pH, culture temperature and lactose concentration on the synthesis of exopolysaccharides by S. thermophilus decreased in that order. The optimum process parameters were as follows: pH 6.90, 42 ℃, and 1.51% lactose. Under the optimized conditions, the yield of extracellular polysaccharide was 191.47 mg/L, a 2.3-fold increase compared to that of the original strain (P < 0.01). The extracellular polysaccharide of mutant Q4F8 had good antimicrobial activity. This strain holds a great potential as a functional starter culture.

Key words: Streptococcus thermophilus, exopolysaccharide, Plackett-Burman, Box-Behnken, fermentation process