The effects of the protective agent on the survival rate of cells and the living cell number of freeze -drying starter were studied during the freeze-drying of S. thermophilus, Bifidobacterium and Lactobacilus planetarium. The highest survival cell contents of the above-mentioned bacteria were 1.2×109/g, 8.4×108/g and1.4×109/g, respectively. The freeze-drying starters with suitable protective agent were fermented in milk medium at 42 ℃ for 10 h and their growth curves, acid-producing curves and the pH value curves with or without freeze-drying had no obvious difference. It was also found that the storage of complex bacteria was better than that of the single bacteria below 4 ℃. The survival percentage of the complex bacteria was higher than that of the single in the artificial gastric juice.
