Development Of Direct-throw Starter Of Lactobacillus Delbrueckii QS306 With ACE Inhibitory Activity And Its Application In Fermented Milk

In this study,Lactobacillus delbrueckii QS306,which has the inhibitory activity of Angiotensin converting enzyme(ACE),is used as the research object.The ACE inhibitory activity was improved by optimizing fermentation conditions and adding growth promoting factors.Vacuum freeze-drying technology was used to prepare direct starter culture,and the freeze-drying survival rate was used as the main index.Single factor and response surface method were used to optimize the formula of freeze-drying protective agent,and the storage stability of direct starter culture was evaluated.The effect of direct cast starter on the quality of fermented milk was studied.Specific research results are as follows:(1)With fermentation time,inoculation amount and fermentation temperature as variables,the optimal fermentation conditions for QS306 to improve ACE inhibitory activity were determined by single factor and orthogonal test:fermentation time 48 h,inoculation amount 3%,fermentation temperature 43℃,under these conditions,ACE inhibitory activity was 69.43±0.21%.(2)In view of the slow growth of strain QS306 in reduced skim milk,the test results of adding growth promoting factors in reduced skim milk showed that the addition of0.05 g/L adenine had certain promoting effects on the acid production,proteolytic ability and ACE inhibition activity of strain QS306.The inhibitory activity of ACE was79.52±1.55%,which was 26.29±0.27%higher than that of ACE without growth promoting factor.(3)The formula of lyophilized protective agent for strain QS306 was optimized by single factor and response surface tests,and the activity of direct starter was measured during storage.The optimal formula of lyophilized protectants was trehalose(9.5%),skim milk(12.0%)and sodium glutamate(16.0%).The survival rate of lyophilized protectants was 78.91±1.47%.Scanning electron microscope was used to observe the morphology of direct starter culture,which was freeze-dried by composite protective agent.The overall structure of starter culture was smooth and the encapsulation effect was complete.The number of viable bacteria was 2.36×10~9CFU/g and the survival rate was 78.77%when the starter culture was stored at-20℃for 60 days.(4)At the end of storage,the ACE inhibition rate and proteolytic capacity of the fermented milk prepared by strain QS306 were 72.21±1.79%and 1.03±0.19μg/mL,which were significantly higher than those of the control group(P<0.05).The number of viable bacteria in fermented milk is high and the water holding capacity and viscosity are high.