Screening Of High-producing Exopolysaccharide Lactic Acid Bacteria And Structural Analysis Of Exopolysaccharide

Sourdough is a mixture of flour and water containing microorganisms such as yeast,lactic acid bacteria and mold.As an important component of sourdough,lactic acid bacteria have an important influence on the flavor,structure and nutritional quality of fermented products,and exopolysaccharides(EPS)produced by lactic acid bacteria play a key role in it.In addition to effectively improving the structure of pasta products and extending the shelf life,EPS also has physiological activities such as immune regulation and antioxidant.In view of the importance of lactic acid bacteria and its product EPS,this thesis had conducted a systematic study on lactic acid bacteria isolated from traditional sourdough.The main research contents and results were as follows:Firstly,51 lactic acid bacteria strains deposited in the laboratory were screened,and the high-yield strain was selected as lactic acid bacteria SXU-1001,and the EPS production was 190.32±2.36 mg/L.Secondly,a series of basic characteristics of this strain were studied.The type of this lactic acid bacterium was determined by analyzing the bacterial genomic DNA.Therefore,the strain was renamed as Lactobacillus sanfranciscensis Ls-1001(abbreviated as L.sanfranciscensis Ls-1001).The physiological and biochemical results of L.sanfranciscensis Ls-1001 shown:it was gram-positive,catalase was negative,the bacteria was rod-shaped,the biomass was 24 ± 1 g/L,the reproductive algebra was 0.36,the growth rate constant was 0.045,and the generation time was 22.42.During fermentation in liquid medium,the lowest pH that L.sanfranciscienss Ls-1001 can produce was 4.09 ± 0.01,and the corresponding total acidity was 0.70 ± 0.01 g/L.In addition,the bacterial cell components obtained fatty acids,proteins,polysaccharides and some substance by fourier transform infrared spectroscopy(FT-IR).During the fermentation process,L.sanfranciscensis Ls-1001 can utilize maltose,cannot use substances such as esculin hydrate,D(+)-cellobiose,mannitol,D(-)-salicin,sorbitol,D(+)-sucrose,raffinose,inulin,lactose and sodium hippurate(1%).Secondly,the response surface method was used to optimize the production conditions of EPS produced by L.sanfranciscensis Ls-1001.Under the culture conditions of 25 g/L of maltose,10.24 g/L of yeast peptone and 12.92 mL/L of fresh yeast extract(FYE),the EPS yield of this strain was the highest,which was increased by 31%compared with the initial medium,and the final yield was 249.30 mg/L.Finally,the structure of purified EPS was identified using modem spectral detection techniques.High performance liquid chromatography(HPLC)analysis shown that pure EPS contained only glucose,indicated that the polysaccharide was a glucan.Fourier transform infrared(FT-IR)analysis indicated the presence of hydroxyl and carbonyl groups.Nuclear magnetic resonance(NMR)analysis confirmed that EPS contains a-glycosidic bonds and pyranose residues.Atomic force microscopy(AFM)shown that the pure EPS produced by L.sanfranciscensis Ls-1001 were regular water droplets in distilled water and were evenly distributed.Moreover,the EPS also had a certain antioxidant effect,which can effectively remove ABTS·+ions,and the clearance rate increased with increasing concentration.L.sanfranciscensis is the dominant lactic acid bacteria in Chinese traditional sourdough.This research systematically clarified the basic characteristics of the lactic acid bacteria and the structural composition of its metabolite EPS,which laid the foundation for the development of lactic acid bacteria resources in sourdough and could promote the industrial development of traditional fermented pasta products in China.