| Streptococcus thermophilus was crucial starter strain on account of giving the food,which was widely used in food industry production such as fermented milk and cheese.The exopolysaccharide(EPS)of secondary metabolite was a natural texture improver.It was recognized as a natural probiotic growth factor due to its safety status and beneficial effects on the human body.The study found that a large number of lactic acid bacteria(LAB)EPSs have been reported to regulate and improve the intestinal flora,lower blood sugar,lower blood lipids,immunological regulation and other physiological activities.In this study,S.thermophilus IMAU20756 was selected,which was a high-yield EPS isolated from traditional fermented dairy products was used as the research object,the EPS produced by this strain was optimized of yield,prepared,isolated and purified.In addition,the physicochemical properties,monosaccharide composition and structural characteristics of polysaccharides were studied.The main results were listed as follows:1.Streptococcus thermophilus IMAU20756 with high yield EPS was selected from traditional fermented dairy products.The EPS yield and the viscosity of the fermentation supernatant were higher than those of the other strains.The polysaccharide yield was optimized to reach 404.663±2.79 mg/L,and compared with the output before optimization,it increased by 2 times.Then,the crude polysaccharide was prepared by the method of ethanol precipitation,and separated,purified and verified of homogeneity by DEAE-Cellulose 52 and Sepharose CL-6B.As a result,two main polysaccharide components were obtained.One was a neutral polysaccharide EPS-1a and the other was an acid polysaccharide EPS-3a.2.The polysaccharide components EPS-1a and EPS-3a showed a single symmetrical peak after detection by the laser detector(LS)and the differential detector(DRI)of the gel chromatography(GPC).The result was consistent with the purification of the gel column chromatography.Their relative molecular weights were 1.572×105 Da and 3.825×105 Da,respectively.The former has a poly dispersity coefficient(PDI)of 2.902,its distribution of molecular weight was wider,and most of them are distributed at 2.6×104-5.0×104 Da and 5.0×104-2.6×105 Da,accounting for 55.0%and 33.2%.The remaining macro molecular polysaccharides account for 11.8%;the PDI of EPS-3a was 1.727,which was closer to 1.It showed that the molecular weight of this component was more uniform and the range was narrower.3.The scanning analysis of infrared spectral(IR)showed that purification component of each EPS contained characteristic functional groups of polysaccharides and both components contained pyranosides.At the same time,it found that EPS-1a contains alpha-glycosidic bonds,and EPS-3a has been identified to contain alpha-D-galactopyranosyl.4.It found that EPS-1a was mainly composed of glucose,mannose and galactose by high performance liquid chromatography(HPLC).The molar ratio ofthree was 3.62:1:2.99 and accounting for more than 90%,but EPS-3a was mainly composed of mannose,galactose and arabinose,which accounts for 80.6%of total,its molar ratio was 1.19:1:1.08.Combined with the results of 1D NMR,it further showed that the sugar ring of EPS-1a and EPS-3 a were all pyranose rings,containing a glycosidic bond. |
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