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Microbial Safety and Quality of Foods

Marks, Bradley; Booren, Alden; Todd, Ewen; Ryser, Elliot
Michigan State University
Start date
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The overall goal of this five-year research plan is to develop strategies to enhance the safety, quality and shelf-life of RTE foods with the major focus being to provide critical quantitative data on the transfer of E. coli O157:H7 during commercial-scale processing of leafy greens. Having the only pilot plant-scale processing line for leafy greens in the country, my laboratory is uniquely positioned to provide answers to these and other urgent questions related to E. coli O157:H7 transfer and reduction during commercial shredding, step-conveying, fluming, washing and drying of leafy greens. This proposed research will further fill a critical data gap in our E. coli O157:H7 risk assessment for fresh-cut leafy greens. In so doing, we will identify potential changes in equipment design that could enhance cleanability and reduce microbial loads in the end product. The quantitative date being collected will then be modeled and used to enhance current risk assessments that will ultimately impact food safety policy decisions.

The specific objectives being proposed are to:

  1. Determine the transfer coefficients for the numbers of E. coli O157:H7 transferred from dip-inoculated wet head lettuce and wet spinach to equipment surfaces during conveying, dewatering and shredding
  2. Determine the extent of E. coli O157:H7 internalization during vacuum cooling of head lettuce and spinach
  3. Inactivation of E. coli O157:H7 during pilot plant-scale sanitizer washing of fresh-cut leafy greens,
  4. Develop a mathematical risk model for E. coli O157:H7 cross-contamination of head lettuce and spinach during processing, and use the model to identify candidate risk mitigation strategies
  5. Assess the efficacy of long-wave X-ray irradiation for inactivation of E. coli O157:H7 on leafy greens.
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NON-TECHNICAL SUMMARY: Contamination of foods with disease-causing microorganisms will forever be of universal concern to the entire food industry with cross-contamination and transfer of bacteria to foods occurring throughout the food production chain. While such transfer has always been known to occur, the numbers of various disease-causing bacteria transferred during these various steps has remained largely unknown until very recently with this quantitative data urgently needed to develop reliable risk assessments that can provide science-based predictions regarding the likelihood of an individual becoming ill from consuming a particular food product. While no food product will ever be 100% safe, such risk assessments are needed to develop reasonable food safety laws and regulations that the government, industry and consuming public can accept. As examples, major concerns have been raised regarding the potential spread of E. coli O157:H7 during the processing of leafy greens with the ready-to-eat meat industry now coming to grips with the risks associated with the transfer of Listeria during the production of deli meats. The overall goal of this project is to quantify the transfer of E. coli O157:H7 from inoculated leafy greens to equipment and vice versa during commercial-scale shredding, conveying,, washing and drying. This research will fill a critical data gap in the current E. coli O157:H7 risk assessment for fresh-cut leafy greens. A second outcome will include the identification of potential changes in equipment design that could enhance cleanability and reduce microbial loads in the end product.

APPROACH: Leafy greens will be inoculated at levels of 106, 104 and 102 CFU/g and then processed using commercia-scale equipment to assess transfer of E. coli O157:H7 from the product to the equipment. In addition, transfer of E. coli O157:H7 from the equipment surfaces to lettuce will be examined by inoculated followed by uninoculated product. lettuce after which 45.4 kg of uninoculated lettuce will be processed. To assess the efficacy of various commercial sanitizers during processing of leafy greens, E. coli O157:H7-inoculated products will be exposed to 80 ppm chlorine, 30 ppm peracetic acid, 30 ppm mixed peracid or water (control) w/o a 10% added lettuce slurry in a commercial-size flume tank with continuous agitation for 1.5 minutes. Thereafter, product and processing water will be quantitatively examined for E. coli O157:H7. Transfer of bacteria during the processing of leafy greens is being modeled as a complex, multi-modal process that involves the following six transfer scenarios: (1) product-to-water, (2) product-to-equipment surface, (3) water-to-equipment surface, (4) water-to-product, (5) equipment surface-to-water, and (6) equipment surface-to-product. Using data generated in our experiments, a simplified probabalistic model will be developed and used to quantify the aggregate rates of transfer for a specific complete process. Multiple simulations of the model will then be performed using @Risk Palisade Software with the end result providing an estimate of E. coli populations in commercially bagged product. Lastly, the relationship between X-ray irradiation dose and inactivation of E. coli O157:H7 will be quantified on leafy greens with the dose distribution within a commercial package of leafy greens also mapped and simulated to validate the predictions of overall. Based on our previous work, the same transfer concepts, ideas and methodologies are now being used to answer critical questions regarding E. coli O157:H7 transfer during processing of leafy greens with these findings again urgently needed to develop reliable risk assessments for the fresh-cut produce industry that will ultimately dictate our food safety laws and regulations. In addition, x-ray irradiation is being explored as a non-thermal microbial reduction strategy that can assure the safety of fresh produce.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Risk Assessment, Management, and Communication
Bacterial Pathogens
Escherichia coli
Meat, Poultry, Game