Abstract:

According to sensory evaluation and acidity determination, 6 Lactobacilli strains (4-1, 4-7, B-2, E-1, E-5, J-2) were selected from 109 isolated from spontaneously fermented yak yoghurt samples from western Sichuan plateau. The fermentation characteristics of these strains, including growth curve, acid production, fermented yoghurt viscosity, acetaldehyde synthesis were compared. Along with this, the phylogenic analysis based on 16S rDNA sequence was performed. Results showed that maximum growth was observed in the strains cultured for 14 h in MRS medium or 8 h in 10% skim milk at 42 ℃, and the acidity of yoghurt fermented with each of them reached the Chinese national standard of GB2746—1999 for yoghurt, and the viscosity and acetaldehyde amount were in the ranges of 1050－4900 mPa·s and 15.11－26.64μg/mL, respectively. Strains E-1 and E-5 revealed weaker acid-producing ability during cold storage when compared with other four strains. The highest values of viscosity and acetaldehyde amount of yoghurt fermented with each strain appeared on the 5th and 2nd day of cold storage. Furthermore, only slight change was observed in the viscosity of yoghurt fermented with strains E-1 or E-5. The highest acetaldehyde content in yoghurt fermented with strains J-2 or E-5 was observed on the 3rd day of cold storage, while that in yoghurt fermented with other four strains alone on the 2nd day. Homology analysis of 16S rDNA gene sequence indicated that there was a homology above 99% among strains 4-7, E-1 and E-5 and these strains and L.plantarum strains were included in the same branch of phylogenetic tree based on 16S rDNA gene sequence. Strain J-2 belonged to the L.paracasei family, and exhibited a homology of 97.6% to the representative stain of this family, L. paracasei SM63 (AJ441107 ). There was a homology of 99% between strains 4-1 and B-2, which were included in the L. fermentum branch. All these results showed that strains E-1 and E-5 had a good potential as starters for yoghurt production..

Key words: Lactobacilli strains, fermentation characteristics, 16S rDNA gene sequence homology, phylogeny