Screening Of Exopolysaccharide-producing Lactic Acid Bacteria And Structural Characterization And Bioactivities Of Exopolysaccharide

Lactic acid bacteria,as probiotic,are widely found in the natural environment and human intestinal tract and possess a variety of probiotic functions.Furthermore,the exopolysaccharides produced by lactic acid bacteria also exhibit unique physicochemical properties and biological activity,which makes it become a hot spot for researchers.In this study,a high-yield exopolysaccharide lactic acid bacteria was screened and identified from the traditional kimchi in Sichuan,and isolation and purification of exopolysacharide were conducted,meanwhile,structural characterization,antioxidant and hypoglycemic potential in vitro were determined.The main results are as follow:?1?A high-yield exopolysaccharide lactic acid bacteria was isolated and screened with the cosmidogenic colony method and polysaccharide yield,and identified as Lactobacillus plantarum by morphological observation,physiological and biochemical tests and molecular biology and named as Lactobacillus plantarum HY.The liquid fermentation culture was carried out with Lactobacillus plantarum HY as the test strain.The fermentation medium was centrifuged,the protein was removed by trichloroacetic acid,cold ethanol precipitated,dialyzed,freeze-dried to obtain crude exopolysaccharide,and the yield was 2.324 g/L.The crude expolysaccharide was perified by DEAE-Sepharose Fast Flow ion exchange column and Sepharose CL-6B gel column chromatography,dialyzed,freeze-dried,the pure expolysaccharide was obtained and named as HY EPS,the yield was 1.423 g/L with 65%purification rate.?2?Combined with ultraviolet spectroscopy and gel permeation chromatography,HY EPS was free from nucleic acid and protein,and the weight average molecular weight?Mw?was 9.549×10⁴ Da,and the polydispersity index?Mw/Mn?was 1.35.The monosaccharide composition was analyzed by high performance liquid chromatography,the results showed that HY EPS was mainly composed of mannose,galactose,glucuronic acid and glucose with mass percentages of 72.99%,17.27%,6.99%and 2.75%.The characteristic functional groups of HY EPS were detected by fourier transform infrared spectroscopy.The results of nuclear magnetic resonance showed that the HY EPS glycoforms were all pyranose rings,and both?and?configurations existed,and?configuration was main,meanwhile,?-D-mannose and?-D-glucose residues were existed.?3?Scanning electron microscopy and atomic force microscopy were used to observe the microscopic morphology of HY EPS.It was found that the dry HY EPS surface was rough,porous,multi-branched,and had a stable and compact three-dimensional network structure combined with sheet and tube.The HY EPS was dense and uneven network structure after dissolving and drying.?4?HY EPS thermal stability evaluation was carried out by thermogravimetric analysis and differential scanning calorimetry.The results showed that HY EPS began to degrade at around 230°C,and the degradation rate reached the maximum at 319.3°C.When the temperature was raised to 846.1°C,the degradation was complete and the melting temperature was 122.78°C,and the process enthalpy is 94.30 J/g,so HY EPS possessed good thermal stability.?5?HY EPS could scavenge DPPH·free radicals,reduce Fe³⁺and Mo⁶⁺in vitro with dose-dependent,and had certain antioxidation ability in vitro.At the same time,HY EPS could inhibit?-amylase activity and had certain potential for lowering blood glucose.In summary,Lactobacillus plantarum HY had the ability to produce exopolysaccharides,and the purified exopolysaccharide possessed good physical properties and physiological activity in vitro,which provided theoretical basis and research ideas for the in-depth development of lactic acid bacteria exopolysaccharides as natural food additives and pharmaceutical supplements.