Study On Screening And Fermentation Performance Of Super LAB Strains From Fermented Yak Milk In Pastoral Gannan Prefecture

The main purpose of this research is to select fine starter culture in 86 Lactic Acid Bacterial (46 lactobacillus and 40 lactococcus) strains, which were isolated from 10 portion of Fermented yak milk in Gannan Tibetan Autonomous Prefecture of Gansu province(Ladai village of Nawu township in Hezuo city, Zhajiu village and Seladuo village of Bola township in Xiahe county, Xiliningbagajiamu village and Minmalong village of Amuquhu township in Xiahe county), by using turbidimetry, National Standards titrmetry for yoghurt, spetrometry, rotation method, slip method etc. Then the super strains, combination of strains and optimum fermentation condition were determined by first screening, further screening and combined fermentation experiments. The results are following:1. First screening: According to the determination results of genetical stability, coagulation time, fermentation acidity and sensory characteristics, 8 LAB strains were obtained with coagulation time of 3~6h, fermentation acidity of 62~103~oT, good texture profile and sensory, among which 4 were lactobacillus(B-3-7, B-4-1, E-4-3, F-2-1) and 4 were lactococcus(A-2-1, D-7-1, H5, J7).2. Further screening:(1) Growth performance: The study of the growth graph on these 8 strains in the liquid medium and in milk showed that: the log phase ended for lactobacillus by 12 hours and for lactococcus by 10 hours, the optimum harvest time for starter culture was for lactobacillus by 10 huors and for lactococcus by 8 huors.(2) Fermentation performance: The results from evaluating the acidity, post acidification, contents of acetaldehyde and diacetyl, viscosity and viable number of LAB in yoghurt made with 8 LAB strains suggest that: 4 lactobacillus present a better acidity than 4 lactococcus, and the range of acidity is 60~oT~112~oT; during the cooling storage, the lactococcus strains is good at capability of endure acid, thereinto the yoghurt made with D-7-1 strains have a good stable quality and its acidity only increased by 8~oT; the study about the aroma-producing capability of LAB exhibited that 8 strains have lower ability of producing diacetyl, the range of which is 0.61~1.15μg/mL, while acetaldehyde production of B-3-7 is 32.16μg/mL which is higher than the other, and F-2-1 took second place; the Average viscosity of D-7-1 is up to 5.72×103mpa.s, which is good for the texture of yoghurt; the viable number of LAB in all yoghurtwere kept more than 107cfu/mL, during the 20d storage period at 4℃. (3) Tolerance property: All of 8 LAB strains could tolerance a certain density of acid, bile salts and osmolarity. Only F-2-1 can grow in ultimate pH1.5, which exhibited strong acid resistance; when the high level of bile salts reached 0.5%, strains D-7-1, A-2-1, E-4-3 and F-2-1 was inhibited; strains F-2-1, E-4-3 and D-7-1 had high degree tolerance property of 8% NaCl.(4) Antibacterial activity: Strains E-4-3 and B-3-7 have better effect of inhibition from Staphylococcus aureus, with inhibit circle 21mm and 20mm respectively, while F-2-1 and J7 have excellent effect of inhibition from Escherichia coli, with inhibit circle18mm; and F-2-1 have the strongest effect of inhibition from Bbacillus subtilis, with inhibit circle 22mm.3. Combined fermentation: That D4(E-4-3 and D-7-1) was considered as the best combination based on the test of synergistic effect, physicochemical index, sensory evaluation of 8 LAB strains. The best fermentation technological parameters of the combination strains were gained by the orthogonal experiment of L9(34): the proportion of E-4-3:D-7-1 is 1:1, and the inoculum size is 3%, and the cultivation temperature is 42℃, and the concentration of suger in milk is 6%. In comparison with the single strain fermentation, co-inoculation had shorter fermentation time with 3h10min, better acidity with 110.21oT, higher viscosity with 5.68×103mpa.s, more acetaldehyde and diacetyl with 30.36μg/mL and 0.85μg/mL respectively, and had viable bacteria counts more than 109cfu/mL.In summary, 8 super LAB strains that screening from Gannan exhibit a favorable characters in the first screening, further screening and combined fermentation experiments, and each has its merits. It provided not only scienticfic basis to the further utilization, exploitation, and research of LAB, but also positive effects on preparing Direct Vat Set(DVS), reducing production costs, and raising economy benefit of enterprises.