Sterilization Effect And Mechanism Of Pressurized CO₂on Pseudomonas Fluorescens In Milk

Pressurized CO2is a non-thermal method which has received considerable attention for potential applications in a number of food processing fields, and can maintain the nutrition,flavor and color of food for heat-sensitive foods. The study of sterilization effect and mechanism of pressurized CO2on microbial is meaningful because of no residual impurity or poisonous materials after treatment of pressurized CO2. Pseudomonas fluorescens which is a psychrophilc bacteria in milk often causes the hydrolysis,clots and bitter of UHT milk and dairy procducts in the shelf-life. The studies at abroad were the effects of dissolved CO2on bacteria growth,and no research on sterilization and mechanism of pressurized CO2on Psedomonas fluorescens at home.Therefore, Psedomonas fluorescens was as test object to study sterilization effect and mechanism of pressurized CO2, and provided a theoretical basis for the application of pressurized CO2in the dairy industry.The effects of pressure,time and temperature on sterilization rate were pre-tested through single factor experiment.Then,orthogonal experiment was used for the optimal sterilizing condition.On the basis,the sterilization effects of pressurized CO2on Pseudomonas fluorescens in different media were studied.At the same time,the delay period and regrowth of survival bacteria after pressurized CO2treatment were studied.The effect of pressurized CO2on integrity of bacterial cell wall and membrane were studied.Meanwhile,the leakages of protein and nucleic acid were detected. And the direct and indirect effects on DNA double-strands of Pseudomonas fluorescens were studied by the agar gel electrophoresis. This study was conducted to examine the effects of pressurized CO2on the ultrastructure of bacteria in order to elucidate the bactericidal mechanism of pressurized CO2.Sterilization effects of pressurized CO2on Pseudomonas flurescens in milk were studied. The effects of pressure,time and temperature on sterilization rate were pre-tested through single factor experiment.Then the optimal sterilization conditions was determined by the orthogonal experiment. The results showed that the sterilizing rate of Pseudomonas fluorescens was99.80%when it was under the optimal sterilization conditions of7MPa,70min and40℃.The bacteria was protected by protein,fat and other componets,so different matrix should use different sterilization conditions. Compared with the control group, the delay period of survival bacteria was significantly prolonged, and the regrowth of survival bacteria was significantly inhibited.Sterilization mechanism of pressurized CO2on Pseudomonas flurescens were studied.The results showed that:The growth of Pseudomonas fluresecens on agar of complete medium was better than on agar of selective medium,indicating the membrane loss of salt tolerant.Compared with untreated Pseudomonas fluresecens,the bacterial membrane permeability changed bigger after treatment of pressurized CO2.The leakage of protein and nucleic acid showed a rising trend after treatment. The bacteria nucleic acid was not caused significant mechanical damage of pressurized CO2in the pressure range used in this experiment.Compared with the contol group, the cell wall and membrane separated after treatment,and cell membrane appeared wrinkle of different degrees.The cell contents was aggregation, and the agglutination phenomenon is more obvious as time longer.