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Effect of pH on desiccation survival of Salmonella
Title
Effect of pH on desiccation survival of Salmonella
Creator
Chen, Fangyu
Date
2019
Description
One intrinsic property of foods, pH, can affect Salmonella survival in high-moisture foods, but its effect in low-moisture foods is unknown....
Show moreOne intrinsic property of foods, pH, can affect Salmonella survival in high-moisture foods, but its effect in low-moisture foods is unknown. In this study, the effect of pH on desiccation and persistence of Salmonella was explored using two approaches. First, the pH range that affects survival in low-moisture environments was explored. The effect of acid adaptation in acidic low-moisture environments was also explored. Salmonella Anatum was grown on trypticase soy agar with 0.6% yeast extract (TSAYE). After harvest, cells were divided and one portion treated independently at pH 3, 4, 5, 7, and 8 for 30 min. Both portions were then desiccated on a cellulose filter in a biohazard cabinet (23±2°C) overnight (24±2 h). After desiccation, cells not previously pH treated were resuspended in buffers at pH 3, 4, 5, 7, and 8 for 30 min, and cells previously pH treated were resuspended in buffered peptone water (BPW). All suspensions were plated on TSAYE with ammonium iron citrate and sodium thiosulfate to determine surviving populations. In addition, S. Anatum was grown on TSAYE adjusted to pH 4, 5, 7, and 8. Cells were either harvested with buffer with the same pH of the growth media or with saline then treated at pH 4, 5, 7 and 8. All were desiccated as indicated before. Desiccated cells were stored at 20% RH at 25°C for up to 29 days. To determine the effect of prior acid adaptation on survival in acidic environments, Salmonella Anatum and Salmonella Agona were grown on agar with or without 1% glucose, harvested, then suspended in buffer at pH 4, 5, and 7. Each culture was desiccated on cellulose filters and stored at 30% RH at 25°C for up to 29 days. Harvested cells were also stored in buffers at the same pH held stored at 25oC for the same time periods. In addition, acid-adapted cells were harvested with saline, desiccated and stored as indicated for each type of cells on wheat flakes. Results from pH range finding experiments indicated pH did have an effect on the survival of Salmonella during desiccation. Desiccation prior to treatment will affect survival at different pH levels. However, prior pH adaptation did not result in increased survival under different pH conditions once cells were desiccated. Acid adaptation prior to desiccation at low pH adversely affected survival for S. Agona but not S. Anatum. Survival after desiccation at different pH levels was greater than survival in the same pH buffers. No advantage or differences in survival was observed with a commercial wheat flake product indicating results obtained in a model environment may be reduced or eliminated when food components are present.
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Development of validation guidelines for high pressure processing to inactivate pressure resistant and matrix-adapted Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes in treated juices
Title
Development of validation guidelines for high pressure processing to inactivate pressure resistant and matrix-adapted Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes in treated juices
Creator
Rolfe, Catherine
Date
2020
Description
The fruit and vegetable juice industry has shown a growing trend in minimally processed juices. A frequent technology used in the functional...
Show moreThe fruit and vegetable juice industry has shown a growing trend in minimally processed juices. A frequent technology used in the functional juice division is cold pressure, which refers to the application of high pressure processing (HPP) at low temperatures for a mild treatment to inactivate foodborne pathogens instead of thermal pasteurization. HPP juice manufacturers are required to demonstrate a 5-log reduction of the pertinent microorganism to comply with FDA Juice HACCP. The effectiveness of HPP on pathogen inactivation is determinant on processing parameters, juice composition, packaging application, as well as the bacterial strains included for validation studies. Unlike thermal pasteurization, there is currently no consensus on validation study approaches for bacterial strain selection or preparation and no agreement on which HPP process parameters contribute to overall process efficacy.The purpose of this study was to develop validation guidelines for HPP inactivation and post-HPP recovery of pressure resistant and matrix-adapted Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes in juice systems. Ten strains of each microorganism were prepared in three growth conditions (neutral, cold-adapted, or acid-adapted) and assessed for barotolerance or sensitivity. Pressure resistant and sensitive strains from each were used to evaluate HPP inactivation with increasing pressure levels (200 – 600 MPa) in two juice matrices (apple and orange). A 75-day shelf-life analysis was conducted on HPP-treated juices inoculated with acid-adapted resistant strains for each pathogen and examined for inactivation and recovery. Individual strains of E. coli O157:H7, Salmonella spp., and L. monocytogenes demonstrated significant (p <0.05) differences in reduction levels in response to pressure treatment in high acid environments. E. coli O157:H7 was the most barotolerant of the three microorganism in multiple matrices. Bacterial screening resulted in identification of pressure resistant strains E. coli O157:H7 TW14359, Salmonella Cubana, and L. monocytogenes MAD328, and pressure sensitive strains E. coli O15:H7 SEA13B88, S. Anatum, and L. monocytogenes CDC. HPP inactivation in juice matrices (apple and orange) confirmed acid adaptation as the most advantageous of the growth conditions. Shelf-life analyses reached the required 5-log reduction in HPP-treated juices immediately following pressure treatment, after 24 h in cold storage, and after 4 days of cold storage for L. monocytogenes MAD328, S. Cubana, and E. coli O157:H7 TW14359, respectively. Recovery of L. monocytogenes in orange juice was observed with prolonged cold storage time. These results suggest the preferred inoculum preparation for HPP validation studies is the use of acid-adapted, pressure resistant strains. At 586 – 600 MPa, critical inactivation (5-log reduction) was achieved during post-HPP cold storage, suggesting sufficient HPP lethality is reached at elevated pressure levels with a subsequent cold holding duration.
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