In view of the current backward technical level of high-efficiency direct-injection starter in China and the widespread problem of yoghurt post-acidification in applications,it is imperative to develop a weak post-acidification high-efficiency direct-injection starter with independent intellectual property rights.In this paper,the self-selected weak post-acidification Lactobacillus bulgaricus Lb-s1 rp-1 was used as the starting strain,and the proliferation culture was carried out on an optimized carrot juice complex bacteria culture medium.Firstly,the effects of single factors such as culture temperature,culture method,initial pH value of medium and inoculation amount on the growth of weak post-acidification strains were studied,and then the fermentation conditions of batch culture in shake flasks were optimized by L9(33) orthogonal experiment.The effects of pH adjustment,glucose feed supplementation,both pH adjustment and glucose feed supplementation on the growth of weak post-acidification strains were studied through expansion culture experiment in 50 L fermenter with batch expansion culture as a control.The results showed that the optimal bacterial growth conditions for the weak post-acidification Lactobacillus bulgaricus Lb-s1 rp-1 in laboratory shake flask batch culture were:the culture temperature was 37 ℃,the initial pH of the medium was 6.5,and the amount of inoculation was 1.0%,static culture,under which process condition the strain was cultured,the time to reach the end of logarithmic growth was 6 h,and the number of live bacteria in the stationary phase was 2.28×1010 CFU/mL.The strains were cultured in three different feeding methods in 50 L fermenter.The time to reach the end of logarithmic growth was 8,6 h,and 8 h,respectively and the number of live bacteria in the stationary phase was 2.29×1010,2.39×1010,2.48×1010 CFU/mL,which was not significantly different from the number of live bacteria in batch expansion culture (P>0.05).Therefore,it is feasible to expand the cultivation of weak post-acidification Lactobacillus bulgaricus Lb-s1 rp-1 by batch fermentation in 50 L fermenter.This study provides a low-cost cell proliferation culture technology for industrialized production of weak post-acidification Lactobacillus bulgaricus direct-injection yogurt starter,and also provided a reference for the study of other cell culture techniques of weakly acidified probiotic bacteria.