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Contamination of juices with pathogenic microorganisms has caused numerous illnesses and some fatalities. Several technologies for non-thermal processing have been commercialized including high-hydrostatic pressure. In this study, the impact of heat and pressure treatment on bacterial cell surface structure leading to injury and inactivation was investigated. Escherichia coli O157H:H7 bacteria was inoculated in apple juice to a final 7.8 log10 CFU/ml. Bacteria inoculated apple juice was thermally treated with thermal death Time (TDT) disks at 25, 35, 45, 50, 55 and 60oC for 4 min or pressurized at 350, 400 and 450 MPa at 25, 35, 45, 50, 55 and 60oC for 20 min. Scanning electron microscopy (SEM) was used to characterized differences between treated and untreated bacterial cell surface structures. Injury and leakage of intracellular- bacterial substances including adenosine triphosphate (ATP) and UV-materials as a function of membrane damage were investigated. Thermal treatment at 55oC and above and 350 MPa pressure and above led to injury. SEM observation of thermal and pressure treated cells showed differences in damaged bacterial cell structures. Leakage of intracellular UV-materials and ATP of thermal injured cells were lower than the values determined from pressurized cells. Similarly, recovery of thermal injured cells occurred faster than pressurized cells during storage of treated samples at 22oC. Bacterial cells in pressurized apple juice stored at 5oC did not recover suggesting that immediate storage after pressure treatment at 5oC will enhance the microbial safety of the apple juice.

Abstract

Contamination of juices with pathogenic microorganisms has caused numerous illnesses and some fatalities. Several technologies for non-thermal processing have been commercialized including high-hydrostatic pressure. In this study, the impact of heat and pressure treatment on bacterial cell surface structure leading to injury and inactivation was investigated. Escherichia coli O157H:H7 bacteria was inoculated in apple juice to a final 7.8 log10 CFU/ml. Bacteria inoculated apple juice was thermally treated with thermal death Time (TDT) disks at 25, 35, 45, 50, 55 and 60oC for 4 min or pressurized at 350, 400 and 450 MPa at 25, 35, 45, 50, 55 and 60oC for 20 min. Scanning electron microscopy (SEM) was used to characterized differences between treated and untreated bacterial cell surface structures. Injury and leakage of intracellular- bacterial substances including adenosine triphosphate (ATP) and UV-materials as a function of membrane damage were investigated. Thermal treatment at 55oC and above and 350 MPa pressure and above led to injury. SEM observation of thermal and pressure treated cells showed differences in damaged bacterial cell structures. Leakage of intracellular UV-materials and ATP of thermal injured cells were lower than the values determined from pressurized cells. Similarly, recovery of thermal injured cells occurred faster than pressurized cells during storage of treated samples at 22oC. Bacterial cells in pressurized apple juice stored at 5oC did not recover suggesting that immediate storage after pressure treatment at 5oC will enhance the microbial safety of the apple juice.

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