Isolation And Identification Of Lactic Acid Bacteria From Traditional Fermented Yak Milk In Pastorals Of Gansu Province And The Screening Of Super Strains

In this study, lactic acid bacteria(LAB) which were isolated from 12 samples of traditional fermented yak milk from the herdsmen′s homes in Pastorals of Gansu Province(Gannan, Tianzhu and Sunan), were identified and then screened for the super strains. The results showed that:1. isolation and identification of LAB115 strains of LAB were isolated which included 71strains of lactobacillus and 44 strains of lactococcus. 115 strains were ascribed to 6 genuses after the preliminary physiological and biochemical identification. There were 20 strains of Lactococcus (L.), 10 strains of Leuconostoc (L.), 10 strains of Enterococcus (E.), 71 strains of Lactobacillus (L.), 2 strains of Vagococcus (V.) and 2 strains of Pediococcus (P.). Lactobacillus is the predominant genus (61.7% of the total isolation) during the fermentation process of yak milk. 115 strains were ascribed to 16 species after the further study. There were 23 strains of L. plantarum, 21 strains of L. acidlophilus, 12 strains of L. casei, 7 strains of L. fermentum, 3 strains of L. brevis, 3 strains of L. minor, 2 strains of L. curvatus, 10 strains of L. lactis subsp.lactis, 9 strains of L. lactis subsp.cermoris, 5 strains of E. faecalis, 3 strains of E. durans, 2 strains of E. faecium, 5 strains of L. lactis, 5 strains of L. mesenteroides subsp. dextranicum, 1 strain of P. dextrinicus and1 strain of P. acidilactici, 1 strain of Lactococcus and 2 strains of Vagococcus were not determined in species. In lactobacillus, the percent of L. plantarum was 20.0% and L. acidlophilus was 18.3%; in lactococcus, the percent of L. lactis subsp. lactis was 8.7%. So L. plantarum, L. acidlophilus and L. lactis subsp. lactis are the predominant species.2. screening of super LAB(1) First screening: 16 strains of LAB were obtained with coagulation time of 7~10h, good texture profile and sensory, pH4～5 and high retentionrate among which 11 strains were lactobacillus and 5 strains of lactococcus.(2) Growth capability: The study of the growth graph on these 16 strains in the MRS liquid medium showed that the optimum harvest time for starter culture was for D5, M5 and H9 by 12 hours and for others by 9 hours.(3) Fermentation capability: The results from evaluating the acidity, post acidification, contents of acetaldehyde and diacetyl and viable number of LAB in yoghurt made with the LAB strains showed that strain E2 presented a better acidity than other strains, whose the acidity is 105.6 o T after 24 hours of fermentation, then C7, G12 and K1 took the second place, whose the acidity were 96.6 oT, 97.5 oT and 95.9 o T; during the cooling storage, the lactococcus strains J6 and B7 were good at capability of endure acid, the yoghurt made with two strains have a good stable quality and their acidity only increased just by 5.4 oT and 6.5 oT, and so did lactobacillus E2 and G12 whose acidity only increased by 6.4 oT and 6.6 oT; The study about the aroma-producing capability of LAB exhibited by 11 strains had lower ability of producing diacetyl, the range of which was 0.41~1.21μg/mL, while acetaldehyde production of E2 was 30.53μg/mL which was higher than others, and G12 took the second place whose acetaldehyde production was 29.71μg/mL; the viable number of LAB in all yoghurt were kept more than 106 cfu/mL during the 20 d storage period at 4℃which could measure up to the standard of GB2746-1999, E2 had the maximum viable number of bacteria and kept more than 109cfu/mL all the way, and then were A12, G12, J6(﹥108 cfu/mL).(4) Tolerance property to bad environment: strains E2 and K1 exhibited the highest acid resistance at ultimate pH1.0; Most strains performed better tolerance under pH3.0 and E2 could keep100% survival not only at pH2.0 but also pH3.0, but strains G12, J6, K1 could keep 90% survival; strains B7, E2, G12 and J6 kept more than 70% survival at 0.3% bile salts.(5) Strains K1, B7, E2, G12 and J6 owned the superior quality of LAB after the screening. According to the results of the identification of LAB we came to a conclusion that these five strains were L. plantarum, P. acidilactici, L. acidlophilus, L. casei, L. lactis subsp. cermoris.In summary, there were rich LAB resources in the traditional fermented yak milk in Pastorals of Gansu Province. The species of LAB which were isolated and identified could rich our country's germplasm resources of LAB. And the study on the screening super LAB strains would provide theoretical basis on preparing Direct Vat Set (DVS) and raising economy benefit of enterprises.