Abstract: To optimize the fermentation conditions of direct-feeding yoghurt starter by space mutagenesis Lactobacillus reuteri SS23-52. Viable count index,optimization of high density fermentation conditions by orthogonal experiment L₉(3⁴),using single factor test to determine the formula of freeze-drying protectant and the inoculation amount of direct starter. The results showed that the best fermentation conditions of the strain SS23-52 were as follows:fermentation temperature 37 ℃,inoculations size 3%,fermentation time 16 h,improved by 1.68 times before optimization. The optimal amounts of cryoprotectants fomula were skimmed milk powder 10％ +maltodextrin 5％ (mass volume percent),the survival rate of the strain is 98.67%. The yoghurt was prepared with the optimized direct inoculation starter of lactobacillus Lactobacillus reuteri SS23-52,the inoculation volume was determined to be 1.0‰,the milk could be solidified at 3.5 h,and the sensory score was 58.65. So the space mutagenesis Lactobacillus reuteri SS23-52 direct inoculation starter had high activity,high survival rate after freeze-drying,and curd time can be as good as ordinary yogurt starter.

Key words: space mutagenesis, lactobacillus reuteri, directed vat set yogurt starters, optimize fermentation conditions