Abstract: The changes of pH,organic acids,volatile flavor and microbial counts were studied on Tibetan kefir fermentation.Samples were collected at 0,4,8,12,16 and 20h of fermentation respectively.Lactic,acetic and citric acids were analyzed by HPLC.Ethanol,diacetyl,acetaldehyde and acetone were monitored with GC.The results showed that the content of lactic and acetic acids is increasing during fermentation.The production of lactic acid is 20 times more than acetic acid at 4 h,and 9～12 times at the end of fermentation,reaching respectively 8.56g/L(TK1)and 7.73g/L(TK2),while the acetic acid reaches 0.71g/L(TK1)and 0.81g/L(TK2)respectively.The concentration of citric acid gradually decreases with fermentation time.Among volatile flavors,ethanol is produced most,reaching 3134mg/L(TK1)and 4994mg/L(TK2)respectively at the end of fermentation.The production of diacetyl and acetaldehyde is higher than common yoghourt,with the concentration reaching 62.4mg/L(TK1)and 37.8mg/L(TK1)respectively.In addition,microbial counts were evaluated in the paper.At the end of fermentation,the colony counts of lactococcus reach respectively 2.3×109CFU/ml(TK1)and 1.3×108 CFU/ml(TK2),lactococcus 1.3×109CFU/ml(TK1)and 1.1×108CFU/ml(TK2),and yeasts 1.6×106CFU/ml(TK1)and 6.6×106 CFU/ml(TK2).The results show the great differences between TK1 and TK2.

Key words: Tibetan kefir, fermentation, organic acids, volatile flavor, microbial counts