Growth and control of Escherichia coli O157:H7 in processing environments and fresh beef products and of Listeria monocytogenes on processed meats during home storage and thawing.

摘要

This dissertation delineates a set of studies designed to examine the fate of Escherichia coli O157:H7 on fresh beef and Listeria monocytogenes on processed meats during simulated processing and storage conditions in order to better understand the food safety implications associated with each scenario. One study evaluated: (i) the efficacy of different residual slaughter-floor decontamination runoff fluids against natural contamination and E. coli O157:H7; (ii) the ability of naturally occurring contamination and inoculated E. coli O157:H7 to survive in such decontamination fluid residues during beef carcass chilling; and, (iii) the efficacy of post-chilling decontamination treatments against pathogen cells that survive in decontamination fluids during the chilling process. To do so, water was collected as it ran off of several beef carcasses sprayed with 82°C water, and fresh beef brisket tissue was excised from beef carcasses (n = 27), at a commercial beef packing facility. No decontamination treatment was capable of reducing natural contaminants or inoculated E. coli O157:H7 populations from 5.5 and 4.0 log CFU/ml, respectively, to below the level of detection following enrichment. These data indicate that E. coli O157:H7 entrapped in defects on carcass surfaces can evade slaughter-floor decontamination treatments and survive the carcass chilling process, and that survivors may contaminate other carcasses or the surrounding environment during fabrication. These data also indicated that approved fabrication-level interventions were only moderately effective against E. coli O157:H7, and less effective against acid-stressed cells.;Biofilms in food processing environments typically exhibit increased resistance to otherwise lethal interventions and may act as recurring sources of contamination. The second set of studies examined the effects of fresh meat contact-surface material types, pre-conditioning of their surface, inoculation substrate, presence or absence of fluid during incubation, presence of air at the liquid-surface interface, incubation substrate, and incubation time on the attachment of E. coli O157:H7 cells and subsequent biofilm formation. In general, pre-conditioning surfaces did not appear to influence attachment of E. coli O157:H7, regardless of inoculation substrate. As an exception, greater pathogen attachment was observed on pre-conditioned compared to dry HDPE coupons when a liquid inoculum was used; no difference was observed when fat was used as the inoculation substrate. The presence of air at the liquid-surface interface allowed pathogen populations to grow during incubation while growth was inhibited on fully-submerged coupons. These results indicate that the process of fabricating beef carcasses may be conducive to the attachment of E. coli O157:H7 onto meat-contact surfaces and subsequent biofilm formation. Furthermore, substrates found in beef fabrication settings should be incorporated into future studies designed to investigate E. coli O157:H7 biofilms in such environments.;The third study evaluated the efficacy of approved food contact surface sanitizers and sanitizers which are unapproved for use on food contact surfaces, against E. coli O157:H7 cells in biofilms formed on surface materials used in beef fabrication facilities. The influence of different surface materials and biofilm age on sanitizer treatment efficacy was also examined. Thee data indicate that, while no sanitizer consistently reduced pathogen populations by more than 1 log cycle on soiled surfaces, approved concentrations of ASC and peroxyacetic acid-based sanitizers may be more effective against E. coli O157:H7 on inadequately cleaned surfaces than other sanitizers. Increased resistance in older biofilm cells also emphasizes the importance of thorough cleaning before sanitation and applying sanitizers at the highest allowable concentrations for extended exposure times.;The fourth study examined the effects of antimicrobials, refrigerated storage, freezing, thawing method, and post-thawing storage (7°C) on L. monocytogenes on frankfurters. Pathogen counts on control samples increased by 1.5 log CFU/cm2 at d-7 of aerobic storage, and reached 5.6 log CFU/cm2 at d-14. As indicated by these results, consumers should freeze frankfurters immediately after purchase, and discard frankfurters without inhibitory agents within 3 d of thawing and/or opening the package. (Abstract shortened by UMI.)