Screening Of Galactose Mutant Strains Of Streptococcus Thermophilus And Its Fermentation Characteristics

As a common starter in the production of yogurt,Streptococcus thermophilus plays an important role in the dairy industry.In this experiment,a strain of Streptococcus thermophilus with good fermentation characteristics was used as the research object,and2-deoxyglucose(2-DG)was used to mutate the strain,a mutant strain with galactose metabolism ability was screened out.On this basis,the mutant strain was further used for milk fermentation to study the dynamic changes of the number of viable cells,p H,titer acidity,viscosity and syneresis of milk during fermentation and storage.At the same time,the dynamic determination of the metabolites of strains in fermented milk was carried out.The specific conclusions are as follows:(1)Streptococcus thermophilus IMAU80287 with good fermentation characteristics isolated from traditional fermented dairy products was used as the original strain,2-deoxyglucose(2-DG)was used to mutate the strain,a mutant strain with galactose metabolism ability was screened out.After culturing at 42? for 12 hours,comparing the activities of?-galactosidase(?-GAL),hexokinase(HK)and pyruvate kinase(PK)in the mutant strain,it was found that the activities of HK and PK were 26.7 and 62.7m U/10~4cell,which were 26%and 57.14%lower than the original strain.the activity of?-GAL enzyme was 0.33 nmol/h/10~4cell,which was 6.81%higher than that of the original strain,that is,the ability of the mutant strain to metabolize lactose and galactose is higher than that of the original strain.(2)The analysis of the fermentation characteristics of Streptococcus thermophilus IMAU80287 and its mutant strains during milk fermentation and storage shows that the p H,titer acidity,viscosity and number of viable cells in the fermented milk of the mutant strain are not significantly different from those of the original fermented milk,but the water holding capacity is significantly higher than that of the original fermented milk.indicating that the mutant strain can be used as a starter in the production and processing of dairy products.(3)After comparing and analyzing the metabolites in fermented milk of Streptococcus thermophilus IMAU80287 and its mutant strains,it was found that compared with the original strain,the mutant strain's ability to metabolize lactose and galactose increased by 6.2%and 25%compared with the original strain,respectively;the content of glucose in fermented milk was low,which did not reach the detection limit of HPLC;the content of lactic acid and citric acid in the mutant strain's fermented milk was also higher than that of the mutant strain,but the content of acetic acid was much lower than that of the original strain.(4)After sequence comparison of the specific genes of the original strain and the mutant strain,it was found that the mutant strain had a single base deletion near the-10and-35 regions of the gal R promoter;base substitutions occurred in py K,and consecutive base deletions occurred in the initial coding region.