Effect of High-Pressure Carbon Dioxide on Desiccated Escherichia coli and Salmonella

  • Author / Creator
    Schultze, Danielle Morgan
  • The safety of low water activity (aW) foods is a concern due to the survival of low-infectious dose pathogens such as Escherichia coli and Salmonella. Desiccation of non-heat resistant E. coli and Salmonella increases their heat resistance; therefore, a non-thermal alternative is necessary to ensure the safety of low aW foods. High-pressure CO2 (HPCD) is a well-established food processing method for high aW foods, but low aW foods present a challenge due to the reliance of the inactivation mechanism on the presence of water. This study aimed to identify conditions which could achieve a 5-log reduction of pathogenic E. coli and Salmonella on dry food. Four strains of Shiga toxin-producing E. coli (STEC) and 1 strain of enteropathogenic E. coli (EPEC) were treated together as a cocktail, and five strains of Salmonella were treated individually. Non-pathogenic surrogate organisms are necessary for validation of treatment processes in the food industry without the risk of introducing pathogens. The suitability of E. coli AW1.7, Pediococcus acidilactici FUA 3072, Enterococcus faecium NRRL B-2354 and Staphylococcus carnosus R6 FUA 2133 as surrogate organisms was evaluated. Treatments were also validated in two low aW foods, beef jerky and almonds. Samples were equilibrated to various aW and treated with heat, HPCD or pressurized N2. Selective agars and selective enrichments were used after HPCD and N2 treatments to differentiate healthy and injured cells of E. coli, Salmonella and E. faecium.Comparisons between isogenic strains of E. coli and Salmonella demonstrated that at a aW of more than 0.80, the locus of heat resistance (LHR), a 14 kbp genomic island conferring resistance to wet heat, becomes detrimental to the survival of S. Typhimurium ATCC 13311 pLHR, but not E. coli AW1.7 pLHR, whereas strains without the LHR remain heat resistant at all aW. Treatment of desiccated E. coli AW1.7 and the STEC cocktail with dry gaseous CO2 (5.7iiiMPa and 65 °C) did not reduce cell counts; however, treatment with gaseous CO2 saturated with water (5.7 MPa and 65 °C) reduced cell counts of all E. coli. Treatment of beef jerky inoculated with E. coli and Salmonella with saturated gaseous CO2 resulted in greater than 5-log reductions for all E. coli and Salmonella. E. coli AW1.7 is an acceptable surrogate for dry STEC cells and STEC on beef jerky treated with HCPD. P. acidilactici FUA 3072 was more sensitive than some strains of Salmonella but E. faecium NRRL B-2354 and S. carnosus R6 are suitable surrogates for dry Salmonella cells and Salmonella on beef jerky or almonds treated with HPCD. Treatment time did not affect treatment efficacy. Increasing the aW of beef jerky samples from 0.75 to 0.9 did not affect treatment efficacy. Treatment of beef jerky with water-saturated gaseous CO2 was more effective than treatment with supercritical CO2 or treatments with N2 at the same temperature and pressure. Overall, the treatment of low aW foods with saturated gaseous HPCD can meet industry standards by achieving a greater than 5-log reduction of E. coli and Salmonella. Additionally, surrogate organisms to represent pathogenic E. coli and Salmonella have been validated in beef jerky and almond food matrices.

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  • Degree
    Master of Science
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