Abstract: Tibetan kefir is a fermented milk product produced by mixed cultures of lactic acid bacteria, yeasts, acetobacter and moulds. Its quality characteristics are significantly different from those of ordinary yogurt. In order to determine the roles of lactic acid bacteria and yeast in yoghurt fermentation by Tibetan kefir, these microbes were separately inhibited by cycloheximide and penicillin during fermentation and the flavor, taste and texture of normally fermented yogurt and fermented yogurts with inhibition of lactic acid bacteria and yeasts were determined and compared. The results showed that the alcohol content of yoghurt was increased from 1.85 to 2.77 g/kg and the VB2 content was decreased from 168 to 157 μg/100 g during fermentation with yeast inhibition, which showed a significant difference compared with normal fermentation. During fermentation with inhibition of lactic acid bacteria, the acidity of yoghurt was increased from 18 to 28 °T, along with a decrease in amino acid nitrogen from 65 to 53 mg/100 g, acetic acid from 1 to 0.7 mg/g and VB1 from 20 to 18 μg/100 g, while lactic acid content was continuously maintained at 2 mg/g; all these parameters were considerably different from those of normally fermented yogurt. During yogurt fermentation, the lactic acid bacteria in Tibetan kefir made great contributions to the acidity, amino acid nitrogen, texture, lactic acid, acetic acid and VB1 while the yeasts made great contributions to alcohol and VB2.

Key words: Tibetan kefir, Lactobacillus, yeast, fermentation