Screening Of A Ropy Streptococcus Thermophilus Strain And Study On The Purification And Rheological Properties Of EPS

This study screened ropy Streptococcus strain with viscosity and fermentation performance from kefir fermented milk by sticky silk-screening method and further confirmed by fermentation performance in milk. The full length sequence of 16s rDNA of strain ST-1 was found 100% homology with Streptococcus thermophilus LMD-9,and the strain was classified as Streptococcus thermophilus ST-1 based on morphology , biochemical characteristics as well as the result of 16s rDNA analysis.The curd was well at 4.5 h,the viscosity arrived at 907 mPa·s(64r/min),the acidity was 82°T,and the evaluation of sense-organs was relatively well when the monoxenie was fermented.at 45℃.Campared with Lactobacillus bulgaricus LB40 to yogurt fermention, objective index and the score of the senses from 1d to 7d were good, which approached to the level of commercial yogurt.Test the texture,rheological,and sensory of different yogurt samples,the result displayed the yogurt femention use ropy stains have the best Integrated quality.The effacts of different inoculum concentration,fermentation time and fermentation temperature on EPS produced by Streptococcus thermophilus ST-1 was studied inorder to get enough EPS for further study.This experimental obtained the optimum conditions by RSM:inoculum concentration 4%,fermentation time 14h, fermentation temperature 42℃.The EPS production is 346.51±1.05mg/L.Exopolysaccharides produced by strain ST-I in skim milk were purificatied by anion exchange chromatography on DEAE Sepharose Fast Flow column, two polysaccharide components EPS-Ⅰand EPS-Ⅱwere obtained by the elution of Tris-HCl buffer with 0～1.5mol/L NaCl.Choose EPS-Ⅰwhich had high contant continue to purify by Gel filtration chromatography of Sepharose CL-6B column until single peak EPS-A appear,then detected by HPLC to make sure it was homogeneous polysaccharides.EPS-A was identified as non-protein polysaccharide detected by ultraviolet and infrared,and it was made of galactose and glucose(peak area ratio1:1.21),with the Molecular Weight 800000Da,from the result of HPLC.There were some conclusions from rheological behavior of EPS-A solution: shear-thinning phenomenon was find at high concentration, no such phenomenon of low concentrations; Solution's viscosity decreased with the temperature increasing; pH had little effect on the viscosity of the solution;salt could make the solution's viscosity decrease, especilly by monovalent salt, and it will be affected by concentration.