Inactivation Mechanisms Of Vegetative Microorganisms And Bacterial Spores By High Pressure Homogenization

High pressure homogenization (HPH) opens up new areas for food pasteurization and sterilization. In this study, HPH was used to inactive Escherichia coli, Staphylococcus aureus and Bacillus amyloliquefaciens spores suspended in PBS buffer, milk and apple juice. The cell structure of E. coli and S. aureus and proteome of E. coli were analysed to study possible inactivation mechanisms. The results and conclusions were summarized as follows.(1) After HPH treatment at 200 MPa with the inlet temperature at 40 ℃ for a residence time of 1.15 s, the count of E. coli suspended in PBS, milk and apple juice was reduced by 3.42,3.67 and 3.19 logioCFU/mL respectively, while the count of S.aureus decreased by 2.21,1.02 and 2.33 log10CFU/mL, which indicated that S. aureus was more resistant to HPH. The inactivation data were well fitted by the polynomial equation. Apple juice could provide a protective effect for E. coli while milk could provide a protective effect for S. aureus against HPH treatments.(2) Both E. coli and S. aureus cells were treated by HPH at 200 MPa with the inlet temperature at 20 ℃ for a residence time of 1.15 s.The surface of E. coli cells after HPH treatments was rough with more wrinkles and vesicles, the membranes showed deformation and materials within the cytoplasm were shrunken and unevenly dispersed, the amount of PI-uptake cells increased. It indicated that the E. coli cell structure was destoryed by HPH. No significant damages were found for S. aureus cells under the same treatments. The activities of intracellular enzymes in E. coli and S. aureus cells were not significantly changed after treatment of 100 MPa with the inlet temperature of 20 ℃ for a residence time of 1.15 s.(3) Proteomic method was used to analyze the total proteins changes caused by HPH. There were 175 differentially expressed proteins between untreated and HPH-treated cells. By analyzing the functions of differentially expressed proteins, it was suggested that HPH could cause a lower activity in the formation of vitamins and coenzymes and the transportation of carbohydrates and thus to get a lower activity in cell metabolism. With the increase of anaerobic respiration and the lipids metabolism, more proteins related to cell wall and cell membrane were expressed, and the cell stress system started to repair the damages caused by HPH.(4) Treatment at 200 MPa with the inlet temperature at 80 ℃ for a residence time of 0.24 s caused no reduction of B. amyloliquefaciens spores. A reduction of 3.5 logio CFU/mL of B. amyloliquefaciens spores was achieved by treatment at 350 MPa with a valve temperature higher than 150 ℃. The modeled thermal inactivation and observed inactivation during HPH treatments suggested that temperature could be the main lethal effect driving inactivation. No sublethal injury was detected after HPH treatments using sodium chloride as selective component in the nutrient agar medium. No spore germination was found after any of these HPH treatments. The inactivation profiles of spores in PBS buffer and milk were compared and fat provided no clear protective effect for spores against treatments.