Abstract: Objective:In order to improve the cell viability of Lactobacillus paracasei during freeze-drying,the lyoprotectants and the vacuum freeze-drying operational parameters were optimized. Methods:Using the survival factor as the index,the protective agent was optimized by orthogonal experiment after single factor test. The process parameters of freeze-drying were optimized by single factor tests,and the microscopic morphology of the powder was observed by scanning electron microscope. Results:The formula of lyoprotectant was optimized as follows:Skim milk powder 15%,sucrose 20%,polyvinyl pyrrolidone(PVP K-30)7%,glutathione(GSH)1.3%. The vacuum freeze-drying operational parameters were obtained as follow:Pre-cooled temperature 15 ℃,pre-cooled time 2 h,the protective agent pH6.5,the ratio of bacteria to protective agent 1:1.5 (v/v),the material thickness 0.5 cm,and the pre-freezing temperature-196 ℃,time 15 min. Under these conditions,the survival factor of L. paracasei after freeze-drying could reach(0.998±0.001)and the number of viable cells was(2.35±0.02)×10¹¹ CFU/g. The scanning electron micrograph revealed that the morphology of the cells was integrity. Conductions:The experiment showed that the optimized protective additive and the freeze-drying process would be suitable for the preservation of L. paracasei,and the activity of the bacteria remained well. This freeze-drying technology has great potential in application and development.