Study On Technology Of Bacillus Sterilization By Ultra-High Pressure

Juices, such as strawberry juice, watermelon juice and Chinese goosebeery juice, are sensitive to heat, which flavor could be changed by thermal treatment. But ultra-high pressure provides many advantages with regards to nutrition and flavor of juices. Bacillus is most difficult to sterilize in ultra-high pressure processing. In the paper, Fengxiang strawberry (from Changfeng county, Anhui province) as experimental material, bacteria were identified from fresh strawberry juice. Effects of ultra-high pressure on spores germinating, vegetable cell dying, metabolic enzymes denaturing, and survival restoring and growing of Bacillus cereus were studied. The shelf life of strawberry juice after pressure was investigated based on neural network. Results shows:1. Hygienical microbiology was examined for fresh strawberry juice according to the standards of food hygiene of People Republic of China. Results as follows:â‘ Aerobic bacterial count, Coliform bacteria, Molds and Yeasts were too large to exceed the standards of food hygienical microbiology in strawberry juice;â‘¡17 species bacteria were identified from strawberry juice (belong to 11 genuses, 9 families). They are Actinobacillus ureae, Sphingomonas paucimobilis, Pasteurella haemolytica, Pseudomonas fluorescens, Pseudomonas putida, Pantoea agglomerans, Enterobacter cloacae, Enterobacter asburiae, Citrobacter freundii, Citrobacter braakii, Agrobacterium radiobacter, Agrobacterium tumefaciens, Presumptive acinetobacter lwoffi, Salmonella species, Staphylococcus auricularis, Staptococcus cohnii and Bacillus cereus;â‘¢Bacillus cereus could be regarded as an indicator bacterium in strawberry juice, because it is not only a dominant and pathogenic bacillus, but also difficult to sterilize.2. Gradient pressure and cooperative temperature were designed to sterilize Bacillus cereus. Results as follows:â‘ It was found that ultra-high pressure range from 200 MPa to 300 MPa could stimulate spore germinating of Bacillus cereus. Cooperated with moderate heat, the spores of Bacillus cereus were induced to germinate almost by pressure; â‘¡In ultra-high pressure processing, the relation between lethal rate (Y) of vegetable cells of Bacillus cereus and experimental factors such as pressure (x₁), temperature (x₂) and time (x₃)follows the equation, Y=62.8+27.2x₁, +7.3x₂ +3.2x₃ +1.0x₁² +2.6x₂² +1.0x₃² -2.0x₁x₂-1.0x₁x₃-0.7x₂x₃;(3) The experiment verified that ultra-high pressure could induce spores germinating and die vegetable cells. It is suggested that Bacillus could be sterilized efficiently through the way of making spores germinating first by lower pressure, then kill them by higher pressure.3. Through examination of damaging, restoring and growing for survival Bacillus cereus, it was found:â‘ Uultra-high pressure could affect the utilization ability of C source material in Bacillus cereus.For example, Bacillus cereus requires more of mini-molecular C source materials after different pressure treatment;â‘¡Ultra-high pressure could denature the activities of ATPase and SOD of Bacillus cereus;â‘¢Ultra-high pressure could change sensitivity to drug of Bacillus cereus;â‘£Survival Bacillus cereus could grow slowly through restoring and repairing after pressure. With the pressure increase, the lag phase duration increase and growing rate slow. Fitted data indicated that lag phase duration changed with pressure as model of quadratic (R²=0.9857), and growing rate as model of logistic (R²=0.9791) .4. A back propagation neural network about shelf life of strawberry juice processed by pressure was established applying neuroshell 2 software exploringly. The model is significant, which determination coefficient R-Squared is 0.9988, mean square error is 241.7, relative error less than 4.0%. Therefore, this neural network model could be used to forecast and mimic shelf life of strawberry juice for its high approximation.