Research And Control On Key Enzymes Involved In Producing Lactic Acid Of Streptococcus Thermophilus In Low PH Envionment

Fermented yogurt was very popular among consumers but the continuing production of acidc metabolities during storage called postacidfication may not only result in an undesirable sourness in yogurt but can also adversely affect the viability of lactic acid bacterias. Domestic and foreign researchers studied this phenomenon and put forward a number of ways to inhibited postacidification, but did not achieve the fundamental objective.From the perspective of metabolism of lactic acid baterias, studied the enzymes of metabolic pathway from lactose to lactic acid and H+-ATPase in Streptococcus thermophilus during storage of yogurt to find which enzyme is the most important one that results in postacidification of yogurt and the main factor controls the key enzyme, inhibited the postacidification by adding the key factors or breeding mutations which had weaker key enzymes.In this study, 16 strains of Streptococcus thermophilus were studied on their abilities of acid production by fermentation experiments. 5 strains had difference significance of abilities of post-acidification were selected, that is TM111, SH94.3, 1703la and 495b2-17 and a weaken postacidification strain HAN1 separated from Hansen power.Using HPLC to detected the content of lactose,glucose,galactose and lactic acid,the products of metabolic pathway of 5 strains during the yogurt storage. Find that the key metabolic pathway of acid production of Streptococcus thermophilus is from lactose to glucose and then to lactic acid. The strains with strong abilities of postacidification had metabolismed much more lactose and glucose and produced more lactic acid than the weak ones. The activities of key enzymes of five strains during the storage of yogurt were detected and compared the relationship between the activities of key enzymes and postacidification. The key enzymes of lactose metabolism in Streptococcus thermophilus play important roles in producing lactic acid in low pH isβ-galactosidase, phosphofructokinase and H+-ATPase.Different types of inhibitory factors on the inhibition of key enzymes were studied, the result was that Zn²⁺,Ca²⁺,EDTA,citric acid,histidine and transglutaminase could inhibit the key enzymes significantly. Two methods with sulface neomycin and nitrosoguanidine to breed mutantions of 495b2-17 were used , each method got two acid-sensitive strains and weak key enzymes mutants sepearately.Inhibiting factors and mutants were added to ferment yogurt. The result was that addition of zinc gluconate, calcium gluconate, histidine, and transglutaminase or using mutants by sulface neomycin NS19, NS28 to ferment yogurt could weaken postacidification, improved pH 0.15 units compared with control and also guaranteed yogurt products safety and good flavor.