Study On Optimization Of Exopolysaccharide And Characteristics Of Streptococcus Thermophius ST1

There are kinds of lactic acid bacterias which can ferment carbohydrates to produce large amounts of exopolysaccharides and lactic acid,including Lactobacillus, Leuconostoc, Streptococcus and so on.They secrete the extracellular polysaccharide, capsular polysaccharide adhesion formation in the cell surface, or in the form of mucilage polysaccharides present in the cells surrounding the medium, EPS will help to improve the viscosity and texture. Study found that EPS can decrease cholesterol, immune regulation,antitumor and other functions.Effect of the initial pH of growth medium, temperature, supplementation with carbon source and nitrogen source on the EPS production by streptococcus thermophilus ST1 was studied when the organism was grown in 10% reconstituted skim milk under different cultural conditions. The optimal temperature and initial pH for the EPS production were determined to be 42℃and pH 6.5. S. thermophilus ST1 was found to produce more EPS with 2%(w/v) sucrose than with other carbon sources tested. Supplementation of skim milk with 0.5%(w/v) WPC was found to increase the EPS production significantly. Under the optimal cultural conditions, i.e. pH 6.5,42℃with 2%(w/v) sucrose and 0.5%(w/v)WPC,135.80mg/l of EPS was produced by S. thermophilus ST1. The EPS was isolated and purified to homogeneity by anion exchange and gel filtration chromatography. Monosaccharide analysis of the purified EPS sample showed that the EPS of S. thermophilus ST1 was composed of glucose and galactose in a molar ratio of 2:1. S. thermophilus ST1 could serve as a candidate for potential application in fermented dairy products.In this paper, the rheological properties of exopolysaccharides(EPS) produced by S. thermophilus ST1 isolated from the natural fermented products were studied.The results showed that the EPS solution was a typical non-Newtonian pseudoplastic fluid, and flow behavior of the solution was affected by concentration, temperature, shearing time, and pH. The viscosity of EPS solutions increased when the concentration was increased, The higher the concentration was, the more serious the phenomenon of shear thinning was;the viscosity of EPS solutions had little change with good temperature resistance at 10-80℃;under the action of shearing time 0-1000s, the solution viscosity had been in a downward trend;the viscosity at pH4 EPS solution was relatively stable, the viscosity at pH7 and pHIO solution was greatly decreased with increasing shear rate.The viscosity of skim milk was significantly increased by adding 1% EPS.However,with the shear rate increased, the effect to skim milk solution viscosity was smaller.In this study, the morphologic characteristics of a slime-producing strain, S. thermophilus ST1 were studied. By crystal violet staining, the strain grown on a full-fat milk solid medium was observed by microscopy to be globular or oval without capsules on the bacterial surface, and the bacteria cells were present in chains. The effects of pH, temperature,supplementation with carbon sources and whey protein concentrate (WPC) on the viscosity of fermented skim milk by S. thermophilus ST1 were studied when the strain was grown in 10% reconstituted milk. According to the results of the apparent viscosity and titration acidity of the fermented samples, the optimized cultural conditions for slime production by S. thermophilus ST1 were pH6.5, temperature 42℃,6%(w/v) sucrose or 1%(w/v) WPC.