Screening Of Super LAB Strains From Natural Fermented Milk In Western Sichuan Plateau

The main purpose of this paper is to select good starter culture of the yoghurt in 109 lactic acid bacteria (LAB) strains isolated from the natural fermented yoghurt which were sampled from the western Sichuan Plateau for improving the quality of yoghurt in our country and shortening the disparity of starter culture of the developed countries .By first screening, further screening and combined fermentation experiments, the super strains , combination of strains and the optimal fermentation conditions were determined .Three steps carried out the experiments: The first step: First screeningAfter 20 times inoculating and incubating in the milk, 6 LAB strains with special flavor, good texture profile and stable inheritable characteristics were selected out from 109 LAB strains.The second step: Further screeningBy evaluating the acidity ,contents of acetaldehyde and biacetyl ,viscosity, the number of the liable LAB and content of free amino acid in the yoghurts made with the 6 LAB strains (2-1, 3-12, 6-15, B, D, K) at the fermentation temperatures of 30℃,37℃ and 42℃, respectively, and determining the resistance of the 6 LAB strains to bile salt, acid and hydroxybenzene, the super strains used to ferment single and the fittest condition of fermentation were confirmed.From the velocity of acidification, we can see the lactobacillus strains 2-1 and 3-12 had higher velocity than the lactococcus strains. The acidifying velocity of the four lactococcus strains was not significantly different with one another. The acidifying velocity increased as the fermentation temperature raised in the temperature range 30℃~42℃. During the cooling storage, the lactococcus strains presented a better post acidification than that of the lactobacillus strains.Lactobacillus strains 2-1 and 3-12 showed higher ability of producing acetaldehyde, while the four lactococcus strains produced more biacetyl than the lactobacillus strains.LAB have lower proteolysis rate. The content of free amino acids in the yoghurt made by lactococcus strain D was 0.0138%, which was the highest content in the 6 LAB strains. The yoghurts made by Lactobacillus strains 2-1 and 3-12 had higher content of the free amino acids, 0.0126% and 0.0079%, respectively, than lactococcus strains 6-15, B and K.The four lactococcus, especially strain 6-15, were capable of producing high level of viscous substances which are good for the texture of yoghurt, when stored in the refrigerator at 5°C, 6-15 always kept high viscosity (>104mpa.s). The fermented temperature affected the viscosity of yoghurt significantly. The viscosity increased as the decrease of the temperature.The numbers of the viable lactic acid bacteria in all yoghurts were kept more than 108cfu/ml, during the 20d storage period at 5 °C.By determining the resistance of LAB to bile salts, acid and hydroxybenzene, the viabilities of strains 6-15 and D were proved to be 46.4 % and 47.0%, respectively, at the level of pH 2.5.Strains 3-12 and 2-1 can't stand 0.5% bile salt, and strains B had lower viability, 37.4%, at the level of 0.5% bile salt, but strains 6-15, D and K had higher viabilities, 92.3%, 86.5% and 60.8%, respectively. All the LAB strains could stand 0.5% hydroxybenzene.By the above experiments and the results obtained, we can see lactococcus strains 6-15 and D had super characteristics: lower velocity of acidification, better post acidification, good aroma, keeping higher viable number during cooling storage and higher viability after dealing with acid, bile salt and hydroxybenzene. So strains 6-15 and D were considered as the best strains, which could be used in yoghurt fermentation.The third step: Combined fermentationThe results of co-inoculations with the tested six strains revealed that lactobacillus strain 2-1 and the lactococcus strain 6-15 showed the best symbiotic properties. The orthogonal experiment of L9 (34) indicated that 37 °C incubation temperature ,1:2 ratio of 2-1 and 6-15, 2% inoculation and 6% sugar concentration as the optimal fermentation conditions. The optimal fermentation conditions were: the fermented temperature is, theproportion of 2-1 to 6-15 is 1:2, the ratio of inoculation is 2% and the concentration of sugar in milk is 6%. hi comparison with the single strain fermentation, the co-inoculation with 2-1 and 6-15 had higher velocity of acidification, better post acidification, better texture, and higher mark by the panelists.