| Recently, China enjoys a very fast development in dairy production, especially, in production and consumption of yogurts. Thus it becomes an important theoretical and practical problem to stabilizer and increase their quality in our country. So, the lactic acid exopolysaccharides (LAB EPS) bears a grate importance to do with the texture, mouth feeling, and susceptibility on syneresis (STS). Hence, it will be of signality not only in dairy science but also in dairy processing investigating the relationship and reaction mechanism between the LAB EPS and the quality of yogurt.At first, one strain of S. thermophilus SGX03-3 and one strain of L. delbrueckii subsp. bulgaricus SGX03-1 were obtained by using the methods of separation, quantitative filtration, curd measurment and observation by electron microscope from 7 mixed and 4 separated starters respectively. It was shown that the S. thermophilus SGX03-3L delbrueckii subsp. bulgaricus SGX03-1 was a ropy strain and the L. delbrueckii subsp. bulgaricus SGX03-1 was a capsular strain.The result demonstrates that the set yogurt was a kind of thixotropic liquid in which its viscosity and shear stress were related to the shear rate, the temperature and the shear time by using the concentric cylinder viscometer measurement. The mixed orthogonal design was used to investigate the factors influencing the yogurt quality and the relationships between the factors and quality items. The relationships were opened out between the strains, material components, processing technology and quality of yogurt. The statistical results shows that the most important 3 influencing factors on yogurt sensory quality evaluations were strains employed in fermentation, fermented acid degree and the material compositions. The content of EPS was not significant in different yogurts fermented by different composition and starters. There was a no direct relationship between the apparent viscosity and the content of EPS in the yogurt. The eating viscosity had a nearest relationship with the yogurt sensory quality grade in the 6 sensory quality items. Their equation relation was Y=3.38318 + 0.00630X3 (P=0.0002)o The order of influencing effects on eating viscosity in the processing factors was established from high to low: Strains > protein content > fermented acid degree > cream content > substance-non-fat.A quantitative evaluation system for yogurt sensory quality was established by the steps that at first, the subjective and objective shares was calculated by the unionization on the evaluating results of yogurt sensory quality, and the combinative shares were found by the subjective and objective shares, at last obtained the final synthesis grade and sequence of every sample by using the method of mixed syntheses through a statistical treatment of positivization and non-dimensionization on the results of yogurt quality measurement. The quantitative evaluation results between the set yogurts fermented in lab and that bought from market show that the_quality of the set yogurt fermented from SGX03-1+SGX03-3 reached or is superior to the quality of good quality yogurt sold popularly at the market by using the new evaluation system. The system can be used in compositor of quality for yogurt samples and determining the degree of the difference among samples, and also can be used in evaluating and separating of starter.The LAB EPS was first extracted by proteolysis, and then it was centrifugated and precipitating by ethanol. Trichloroacetic acid was used to precipitated proteins and further purification was done by dialysis, ultrafiltration, DEAE-Sepharose- CL-6B ion-exchange chromatography and Sepharose-CL -2B gel permeation chromatography. Finally, a purified sample ESP03-lgm was obtained. The molecule mass and the amount of electric charge of the purified sample EPS03-lgm were demonstrated a nice homogeneity and reached the standard of structure identification by HPLC and gel permeation chromatography.The structure of EPS03-lgm was identified by the technology of HPLC, IR, UV, GC and NMR. The...
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