Fermentation Optimization And Characterization Of Exopolysaccharide Isolated From Leuconosto Mesenteroides DRP105

Lactic acid bacteria?LAB?is generally recognized as safe?GRAS?,and widely used in fermented foods.Exoplolysaccharides?EPS?secreted by LAB attract the interests of researchers,due to its specific physiological and biochemical characteristics,such as rheological properties,antioxidant activities,and immunomodulatory.The laboratory previously isolated a high yield EPS-producing strain DRP105 from manchurian sauerkraut.In this paper,the strain DRP105 was identified,its fermentation condition was optimized,and the EPS isolated from broth was analyzed in chain conformation and physicochemical properties.Considering the morphological characteristics,physiological and biochemical tests,and 16S rDNA sequence analysis,the strain was identified as Leuconostoc mesenteroides and named as Leu.mesenteroides DRP105.The phylogenetic tree was constructed based on GenBank database and EzBioCloud database,using Mega software.The strain had a certain tolerance of acid,bile salt and NaCl.Single factor experiments were used to optimize the fermentation condition of strain DRP105,and EPS yield was improved by1.69 times.The test showed that the suitable concentration of Ca²⁺could promote the secretion of EPS,and the optimum fermentation temperature?25°C?was lower than its growth temperature?30°C?.On this basis,response surface methodology was used to further optimize the fermentation condition.Plackett Burman?PB?test showed the content of sucrose and tryptone,and initial pH were the significant factors,and they all had positive effects on EPS yield.Based on the PB test and Path of steepest ascent experiment results,the central composite design was carried out.The optimum condition was sucrose 86.83 g/L,tryptone 15.47 g/L and initial pH 7.18.After test verification,the maximum yield was53.79±0.78 g/L in 36 h fermentation,about 2.29-fold compared with that under non-optimized condition.The fermentation broth was prepared by centrifugation,ethanol precipitation and removal of protein,the precipitant was crude EPS.Crude EPS was further purified by gel-filtration chromatography and lyophilized.The final residual was served as pure EPS.?-elimination result showed the EPS was O-linkage.The chain conformation was characterized by Congo red test and circular dichroism,indicating the EPS exhibited random coil structure in aqueous solution.EPS had a higher degradation temperature of 279.42°C with 32.54%weight loss,suggesting high thermal stability of the EPS,which had the possibility of being used in thermal processing progress.Zeta potential and particle size determination test showed the initial interfacial electrical charge value and stability of EPS aqueous solution were improved with the increasing concentration,within a certain concentration range.EPS showed moderate antioxidant activities in scavenging of DPPH,·OH,O₂·^-,ABTS radicals,H₂O₂ and NO₂^-.EPS had lower reducing power.While its ferrous ion chealting power was higher than Vc.The EPS solution is a non-newtonian fluid with shear thinning.The intrinsic viscosity[?]of EPS at 25?C and 35?C was 220.43 mL/g and 204.43 mL/g respectively.The results of steady-state rheological properties showed consistency coefficient?k?was increased with the rising concentration,then the viscosity was greater.The rheological index“n”was always less than 1,indicating EPS solution was a pseudoplastic fluid.The viscosity of EPS solution was enhanced with the rising concentration,while decreased with the increasing temperature.Low pH and high pH could result in the decline of viscosity.0.1 mol/L Mg²⁺could promote intermolecular association and enhance the visvosity.However,0.1mol/L Na⁺and Ca²⁺could block the expansion of EPS and lead the decrease of viscosity.This research optimized the EPS yield of Leu.mesenteroides DRP105,and studied the properties of EPS,laying a foundation for its industrial application.