Study On The Mechanism Of Postacidification Of The Plain Yogurt During Storage And The Methods To Control It

Postacidification would occur in the storage of yogurt. Consumers are discontent with the heavy acid taste. In order to avoid it, this experiment systematically research the mechanism of postacidification and put forward some methods to control it. The yogurt sample occurred postacidification at the second day when the storage temperature is I 2C .The acidity is more intensive with the storage time. The count of S.t and L.b of yogurt sample decreased with the storage time, besides, the loss count of S.t is more than L.b (99.96%). In the sample, there is no glucose but galactose and lactose. The content of lactose is 1.48% at the fifteenth day and has been used 69.3%. The content of galactose increased from 0.8% in the first day of storage to 1.05% in the fifteenth day. The viscosity of the stirred yogurt decreased more than the set yogurt. In order to determine the main organism that causes postacidification, S.t and L.b were incubated respectively in the storage temperature 25 C. The culture of S.t and L.b is in the same condition(the count of LAB is 7.4X lO7cfu./ml or so . pH value is 4.50).At the eleventh day of storage, the culture pH value of S.t is 4.57.otherwise, L.b is 3.85.The influence of L.b is stronger than that of S.t to the pH value of the yogurt. In order to study the characteristic of P -galactosidase, Supersonic way to destroy the cell wall was compared with the way of lysozyme~ osmotic pressure and acetone diy power. Then I choose the supersonic way as the optimum way to destroy the cell wall. The characteristic of P -galactosidase of S.t is similar with that of L.b. At the temperature of 45-55~C,the activity of P -galactosidase is the bigger than the other temperature. At the same time, the activity of P -galactosidase is the biggest when pH value is 7.0.Before and after sonication, the specific activity of P -galactosidase is different especially for L.b in the low p1-I value. The microbial cell membrane, cell wall or both of L.b may aid in protecting the P -galactosidase from acid denaturation. Streptomycin can kill the starter bacterium but does not affect the activity of P -galactosidase. The pH value difference of adding streptomycin in yogurt and the pltin yogurt is 0.21 at the sixth day of storage. I adopted some methods to control the postacidification based on the mechanism. 3 Firstly, I tried to mutagenesis L.b with ultraviolet in order to make it sensitive to the high acid, but the characteristic of mutant could not inherit steadily. Secondly, I increased the proportion of S.t to L.b in the yogurt culture. The yogurt acidity of the second day is less than 1100 Twhen the proportion of S.t to L.b is less than 35.32% in the yogurt culture by the regression analysis. The flavor of yogurt (S.t t: L.b is 35:65) did not change much as the traditional yogurt (S.t : L.b isl :1) .In this experiment, the changes of milk dry solid has not much influence the yogurt postacidification. The result of adding Nisin to yogurt is good, but adding lysozyme to the yogurt sample isn抰 ideal. At the seventh day of storage, the pH value of yogurt (including bile) only decreased by 0.17 compared to 0.39 of the pH value of the plain yogurt.