| Lactobacillus delbrueckii subsp.bulgaricus has become one of the essential and important strains in yogurt fermentation agents due to its superior fermentati on characteristics.However,the viable cell count and vitality of domestic directvat-set fermentation agents are relatively low,so it is of great significance to stu dy the vacuum freeze-drying process for improving high-quality direct-vat-set fer mentation agents with independent intellectual property rights.In this study,Lact obacillus delbrueckii subsp.bulgaricus M-58 was used as the research object,an d vacuum freeze-drying technology was adopted.The survival rate was used as t he main index,and the vacuum freeze-drying process conditions were optimized by combining single-factor and response surface methods.After obtaining the opt imal process conditions,the bacterial activity and damage were analyzed by scan ning electron microscopy,plate counting method,and flow cytometry,and the su rvival rate during the storage period,as well as the changes in β-galactosidase ac tivity and lactate dehydrogenase activity,were observed to evaluate storage stabi lity.The specific research results are as follows:The single-factor experimental results of vacuum freeze-drying preparation o f Lactobacillus delbrueckii subsp.bulgaricus M-58 showed that pre-freezing tim e,loading thickness,and suspension p H had a significant impact on the survival r ate of the dried bacteria powder.After optimization by response surface methodo logy,the optimal process conditions were determined to be a pre-freezing time of3 hours,suspension p H of 6.0,and material thickness of 0.7 cm,with a survival rate of 84.2%.Scanning electron microscopy revealed significant differences in the external morphology of the bacteria powder prepared before and after the optimization of process conditions.After optimization,the prepared bacteria exhibited a rod-sha ped surface with minimal cell surface rupture,leading to leakage of contents.Flo w cytometry analysis demonstrated that the optimized process significantly incre ased the proportion of live cells and reduced cell damage during vacuum drying f or both pre-and post-optimization samples.By analyzing the changes in survival rate and key enzyme activities of the ba cteria powder prepared by vacuum drying process after storage at 4°C and 25°C f or 28 days,it was found that the activity of Lactobacillus delbrueckii subsp.bulg aricus M-58 strain was affected by storage temperature,and the increase in stora ge temperature accelerated the decline in survival rate.At the end of storage,the extracellular β-galactosidase content and intracellular lactate dehydrogenase cont ent of the bacteria strain prepared by the optimized vacuum drying method were significantly higher than those of the control group(P<0.05),indicating that the optimization of the process to a certain extent protected the activity of the bacteri al strain.The survival rate of the bacteria powder prepared by the optimized vacu um drying process after storage at 4°C was 80.1%,significantly higher than the 75.3% survival rate of the control group(P<0.05).When the storage temperature was 25°C,the survival rate of the experimental group was 65.2%,higher than the49.4% of the control group,indicating that the dried bacteria powder prepared af ter optimization had a better survival rate compared to the pre-optimization,and low-temperature storage was more conducive to preserving bacterial strain activit y than room temperature storage. |
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