| Microbial exopolysaccharides(EPS) are one important kind of hydrocolloids. They have the function of gelling, thickening, emulsifying, water-binding and film-forming, and are widely used in food products, such as meat products, dairy products, beverage, emulsion and edible film. Since xanthan gum, the first industrialized EPS, has achieved great commercial success, the development of new EPS attracted much attention from academic and industrial communities. EPS from Klebsiella sp. has recently become a hot research topic. EPS from Klebsiella sp. could be used in vaccine preparation and wastewater treatment due to its functions of immunomodulatory activity, flocculation and adsorption capacity of heavy metals. In this study, a strain with high EPS yield was isolated from activated sludge, and identified and named as Klebsiella sp. PHRC1.001. The medium components and fermentation conditions for PHRC1.001 EPS production were optimized. The physicochemical properties of PHRC1.001 EPS and its application in emulsification were also studied. The main conclusions are as follows:1. One strain with high EPS yield was isolated from activated sludge. According to the 16 S r DNA sequence and phylogenetic analysis, the strain was identified and named as Klebsiella sp. PHRC1.001 and deposited at China Center for Type Culture Collection(CCTCC) under the accession number M 2014427.2. The optimized medium consisted of 40 g/L sucrose, 0.8 g/L Ca Cl2, 2 g/L KNO3, 9 g/L Mg SO4·7H2O and 3.5 g/L Na H2PO4, at p H 7.0. The optimized fermentation conditions were as follows: inoculation time of 10 h, rotation speed of 220 r/min, initial p H of 7.0, inoculation temperature of 28 oC, broth content of 50 m L(in 250 m L flask), inoculation amount of 1.0%(v/v) and incubation time of 48 h. After optimization, the EPS yield reached about 21 g/L, which was 2.1 fold of that with initial medium and fermentation conditions.3. PHRC1.001 EPS was purified by a combination of ethanol precipitation and Sevag reagent treatment. Analysis of Zeta potential and uronic acid content showed that the purified PHRC1.001 EPS was anionic and acidic. Monosaccharide composition analysis indicated that this purified EPS was comprised of arabinose, mannose, glucose and galactose in the molar ratio of 1.20: 5.55: 1.00: 9.55: 1.35. PHRC1.001 EPS in water existed in the form of nanoparticles with a molecular weight of 3.29×106 Da, whereas it was completely dissolved in DMSO with a molecular weight of 8.29×105 Da. TEM and XRD results showed that PHRC1.001 EPS was mainly in amorphous state with a small amount of crystals. PHRC1.001 EPS in water or DMSO, was a typical pseudoplastic fluid, exhibiting shear-thinning behavior. It was stable within the temperature range of-40 oC-120 oC and p H range of 3-11. The viscosity of PHRC1.001 EPS was largely enhanced after the alkine treatment followed by neutralization, but was scarcely affected by K+, Ca2+, Na+ and Mg2+.4. Emulsifying activity of PHRC1.001 EPS and its emulsion storage stability were investigated. Using MCT as the oil phase and EPS as an emulsifier, O/W emulsion was prepared using high-pressure homogenizer. With 1% EPS as emulsifier, the emulsion exhibited uniform particles with average size of 4.99±0.01 μm, indicating a good emulsifying activity. Acceleration test at high temperature(60 oC) showed that the emulsion particles began to slightly aggregate on the 7th day, suggesting a good physical stability during long-time storage. |
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