| Lactic acid bacteria(LAB)are generally recognized as safe(GRAS)food-grade microorganisms.Some LABs produce exopolysaccharides(EPS),which have biological and physicochemical properties such as regulating immunity,anti-oxidation,thickening and heat resistance,showing a good application prospect in the field of food and biomedicine.In this paper,the fermentation conditions of EPS produced by Leuconostoc citreum were optimized,and the functional characteristics of EPS were studied.The strain used in this study was Leu.citreum B-2.The experimental results of biological characteristics showed that the optimal culture time of the strain was 16 h,the optimal growth temperature was 30?,the optimal initial pH value was 6.0,and the strain had good tolerance to urea(0-10.0%),certain salts(0-4.0%)and bile salts(0-0.1%).Leu.citreum B-2 was a strain with high EPS yield,and its EPS was named B-2EPS.Single factor method and response surface methodology(RSM)were used to optimize the fermentation conditions of EPS produced by Leu.citreum B-2.The results showed that the yield of B-2 EPS increased from 10.01±0.53 g/L to 59.33±1.34 g/L when the key factors were sucrose 99.8 g/L,yeast extract 5.94 g/L and pH=6.12,which was 5.93 times that of the unoptimized condition.On the basis of separation and purification of B-2 EPS,its structure characteristics were analyzed.The results of chemical composition analysis showed that the contents of C,H,N and S were 39.05±0.25%,7.19±0.08%,0.72±0.03%and 0%,respectively,and the content of uronic acid was relatively high.Scanning electron microscopy(SEM)showed that B-2 EPS had a large lamellar,smooth surface and pore structure.The result of?-elimination reaction showed that there were O-linked glycosidic bonds in B-2 EPS.Congo red experiment results showed that B-2 EPS did not contain a triple-helical structure and presented irregular-coiled conformation in aqueous solution.The physicochemical properties of Leu.citreum B-2 EPS were analyzed.The results showed that the average particle size of EPS was 111.8 nm,and Zeta potential analysis showed that it was relatively stable at high concentration.Comprehensive thermal analysis showed that the degradation temperature of B-2 EPS was 313?.The rheological properties showed that B-2 EPS was a pseudoplastic fluid with high viscosity under the conditions of high concentration(55 mg/m L),low temperature(4?)and low pH(pH=3),and could coagulate skimmed milk within a certain concentration range.B-2 EPS had higher water solubility(82±0.64%)and water retention(144±2.83%),and its emulsifying ability to vegetable oil was better than that of hydrocarbon,while its emulsifying stability to hydrocarbon was better than that of vegetable oil.In order to study the biological activity of B-2 EPS,antioxidant,probiotic and antibacterial experiments were carried out in vitro.Six in vitro antioxidant experiments showed that the H2O2 clearance rate was up to 100%when the concentration of B-2EPS was 15 mg/m L.Five common probiotics,Streptococcus thermophilus,Lactobacillus delbrueckii,Lb.casei,Lb.paracasei and Lb.plantarum were used to conduct in vitro probiotics experiments.The results showed that the addition of B-2EPS could promote the growth of probiotics,and the growth promotion of Lb.delbrueckii was particularly obvious(When cultured for 16 h,compared with the control group,the biomass of the strain increased by 19.18%with the addition of B-2EPS.).Five common and harmful strains,Escherichia coli,Staphylococcus aureus,Micrococcus luteus,Salmonella enterica,Bacillus subtilis,were used in the in vitro antibacterial experiments.The results showed that B-2 EPS did not have antibacterial activity.In summary,the yield of B-2 EPS can be greatly increased under optimized fermentation conditions.B-2 EPS has unique structural characteristics,physicochemical properties and biological activities,so it has a good prospect to be used as a stabilizer,thickener and emulsifier in food,medicine,cosmetics and other fields. |
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