Screening of excellent lactic acid bacteria from traditional fermented yak milk in Sunan prefecture of Gansu Province

Five traditional fermented yak milk from Sunan prefecture of Gansu Province were used as test materials,45 strains of lactic acid bacteria were isolated in the MRS and Elliker broth including 21 bacilli and 24 cocci strains.The screening of excellent lactobacillis was studied according to the characteristics,acidifying,postacidification capacity,aroma-producing capacity and the change of viable bacteria during refrigeration.The results indicated that 6 strains were obtained with excellent sensory evaluation from primary screening,with which curd time was within 7 and 10 h,pH was lower than 5,and water retention rate was above 60%.According to the further study on fermented ability,strain K1 and M5 were found with the most excellent acidogenicity and aroma-producing capability,the acidity of which reached 95.9°T and 82.1°T after 24h of fermentation,and ethanal yield of two strains reached 25.15 μg·mL-1 and 22.80 μg·mL-1,respectively,together with better postacidification capacity during refrigeration and the capable of keeping viable bacteria number at 107 cfu·mL-1;strain J6,which the acidity increased only 5.4°T was found to be the strongest anti-postacidification capacity and the viable lactobacilli number was kept over 108 cfu·mL-1 during the whole refrigeration;diacetyl yield of the 6 strains were found within 0.6~1.15 μg·mL-1.All the above indicate that strains K1,M5 and J6 have the potential characteristics of the excellent starter cultures.