Molecular Structure And Physicochemical Properties Research Of Exopolysaccharide Produced By Leuconostoc Citreum N21

Lactic acid bacteria(LAB),as a recognized food-grade microorganism,is a type of polysaccharide that is produced and secreted into the extracellular environment during the process of growth and metabolism.It is divided into capsular polysaccharide and mucopolysaccharide,collectively called lactic acid bacteria exopolysaccharide(LAB EPS).As a biological macromolecule,lactic acid bacteria EPS has many excellent properties,such as adding it to food to increase the viscosity and taste,as well as anti-virus,anti-oxidation and other drugs,which is widely used in the fields of medicine and food.Therefore,research and development of LAB EPS have important practical significance and good application prospects.In this paper,the traditional fermentation of food as a resource,which is used to isolate LAB producing EPS.Then the structure and physico-chemical characterizations of the EPS was studied.That's beneficial to develop and utilize the LAB producing EPS.The experimental sample of this paper is a homemade dairy fermented dried milk cake made by Inner Mongolia pastoralists.Based on the drawing of the colony and determination of polysaccharide content,a strain of Lactic acid bacteria N21 with high production of EPS was isolated and screened.Morphological observation,physiological and biochemical tests and 16S rDNA sequence analysis confirmed that the strain was Leuconostoc citreum,and named it Leuconostoc citreum N21.Subsequently,L.citreum N21 was inoculated into MRS-S medium and cultured at 30?for 48 h under shaking to obtain EPS fermentation broth.After centrifugation,removing bacteria and protein,alcohol precipitation and dialysis,the crude polysaccharide solution was obtained.The polysaccharide was purified by Sephadex filtration,and the polysaccharide product was obtained after lyophilization.The yield was 24.5 g/L.The purification,molecular weight and other properties of the purified polysaccharides were studied by UV analysis,agarose gel electrophoresis and high performance size exclusion chromatography.The results showed that the purified polysaccharides contained almost no nucleic acid and protein,the weight average molecular weight was 6.07×10~6 Da..Gas chromatographic analysis showed that the purified polysaccharide contained only one kind of monosaccharide,namely glucose.The structure of polysaccharide was further analyzed by FT-IR and NMR spectra.It was concluded that the polysaccharide produced by L.citreum N21 was a kind of dextran with no side chain structure.The main chain was linked by?-(1?6)glycosidic bonds.Through the study of the physical and chemical properties of polysaccharides,it showed that it had high solubility,reaching to 82.5 g/L.Scanning electron microscopy showed that the microstructure of the EPS was spherical and porous,the surface was smooth and shiny.The emulsifying properties of the polysaccharide showed that the polysaccharide had good emulsifying stability.The thermodynamic analysis showed that the polysaccharide had a high thermal stability with Td of 308.46?.In this paper,L.citreum N21 with high yield of EPS was isolated from dried milk cake.The polysaccharide molecular weight,molecular composition and molecular structure were elucidated.The research on the physicochemical properties of polysaccharides showed that it had a good application prospect,which provides a theoretical basis for further application of polysaccharides in food and other fields.