Preliminary Study On The Mechanism Of Weak Acidification Of Mixed Lactobacillus Helveticus Starter

The control of post-acidification of fermented dairy products is the key to improving the quality of fermented milk and prolonging the shelf life of dairy products.In the process of screening the weak acidified lactic acid bacteria,it was found that the specific Lactobacillus strains can effectively control the acidification of fermented milk.Based on the research of the combinations of weak post-acidification strains and the preparation condition of the freeze-dried starter,I further studied the mechanism of post acidification of lactic acid bacteria from the aspects of characteristics of related enzymology.The main results are as follows:1.Effects of lactic acid bacteria and combinations on post acidification of fermented milk.After studying the growth rate and post-acidification of lactic acid bacteria,two strains L.helveticus LH-3 and SH with weak post-acidification and the strain L.helveticus LH-1 with high post-acidification as reference were selected.The research on the fermentation and post-acidification of the selected Lactobacillus helveticus and combinations showed that mixed fermentation could significantly reduce the post-acidification of fermented milk(P<0.05).The post acid degree of mixed culture of L.helveticus LH-1,LH-3,SH and Streptococcus thermophilus 427(A,B,C)for 20 d were 16.07 0T,15.35°T and 17.89°T respectively witch were significantly lower than that of other combinations(P<0.05).And the post-acidification of the mixed strain B was relatively weak.Compared with single strain,the time of fermentation of mixed strains could reduced by about 1?5 h,and the acid production rate could be increased by 3?5°T/h.The viscosity,texture and sensory quality of the milk fermented by mixed strains were better than that of single strains.The texture and sensory score of milk fermented by mixed strain B were better than that of others.Considering all the indexes,the combination of B was determined to be the best combination of low post acidification.2.Optimization of preparation technology of freeze drying compound fermentation agent.The single factor and orthogonal experiments were used to optimize the conditions of centrifugal concentration,the composition of protective agent and the condition of pre-incubation with the number of viable bacteria was used as the evaluation index.Then the differences of the textural properties and the post acidification of fermented milk between starter B and commercial starter culture LH91 were studied.The results revealed that 9000 r/min,15min was the best condition of cell enrichment.The best cryoprotectant(L-1)was obtained using orthogonality test:skim milk 100 g,trehalose70 g,glycerol 20g,L-glutamate 20 g,and the survival rate of freeze-dried S.thermophilus 427 reached 74.38%.The best cryoprotectant(L-1)of L.helveticus LH-3 was obtained using orthogonality test:skim milk 100g,glucose 70g,glycerol 20g,L-glutamate 10g,and the survival rate reached 77.36%.The optimum survival rate of S.thermophilus 427 and L.helveticus LH-3 reached 86.49%and 82.69%,with the temperature of 37? and the pre-incubation time of 60 min and 90 min respectively.The comparative study showed that the acid production rate of agent B was higher than LH91.The state and sensory score of milk fermented by B were better than that of LH91.The titration acidity of milk fermented by B was lower than that of LH91 by about 2.02 °T,and the pH was higher by 0.15 after the storage for 20 d.The degree of post acidification of the two kinds of fermented milk was 12.49 °T and 14.94 °T respectively.The post acidification of B is relatively weak witch can be used in industrial production.3.The preliminary study on the mechanism of lactic acid bacteria.In this paper,the factors affecting the acidification of lactic acid bacteria and the preliminary study of the mechanism of lactic acid bacteria were studied from the aspects of the number of viable bacteria,metabolism of sugar and activities of related enzyme of lactic acid bacteria during the storage period.The results showed that acid resistance ability of L.helveticus LH-1 and its combination A with strong ability of post acidification were relatively better than others.The ability of post acidification of single strain had a great influence on combinations.The viable count of lactic acid bacteria was not the most direct cause of post acidification.The acidification of fermented milk was significant positive correlated with the changes of lactose,lactic acid and?-galactosidase activity(P<0.05),was significant negative correlated with glutamate decarboxylase(P<0.01).Both of the activities of ?-galactosidase and glutamate decarboxylase are the key enzyme affecting the ability of acidification.Neither of the lactate dehydrogenase and protease are the key enzymes that affect the production of lactic acid during storage.The metabolism of lactose was the key way to produce acid during the storage period of single strain and mixed strain.The ability of carbohydrate metabolism and the activities of ?-galactosidase and glutamate decarboxylase of the mixed strains were significantly lower than that of of Lactobacillus helveticus.