Cross-contamination, modeling, and risk assessment for pathogens on fresh-cut produce

The overall goal of this dissertation was to understand key factors that influence the microbial safety of fresh-cut leafy greens and tomatoes during processing. Laboratory experiments, computer modeling and risk assessment were used to achieve these objectives. In Chapter II, E. aerogenes and avirulent E. coli O157:H7 were found to be suitable surrogates for pathogenic E. coli O157:H7 when comparing mean log reduction when washing with water. These organisms were not suitable surrogates for studying transfer from inoculated to non-inoculated lettuce leaves during water washing. Post-inoculation drying time and water volume influenced transfer of pathogens during washing but the ratio of inoculated to non-inoculated leaves did not have a statistically significant effect. Chapter III demonstrates that while curli producing E. coli O157:H7 attach significantly more to lettuce than the non-producing curli strain, curli production has no significant effect on cross-contamination to non-inoculated lettuce during water washing. In Chapter IV, the growth of Salmonella, E. coli O157:H7 and L. monocytogenes on leafy greens during transportation was predicted using real-world transport truck data using time series analysis to simulate transit temperatures. Most models showed relatively close agreement, but some models predicted less growth due to the use of modified atmosphere growth conditions or the use of limited published data. A Quantitative Microbial Risk Assessment (QMRA) was developed to calculate the probability and the total number of illness caused by Salmonella on cut tomatoes in Chapter V. QMRA results showed that using a mechanical slicer, and slicing at 4°C decreases the transfer of Salmonella on cut tomatoes. These findings from these studies will help improve the safety fresh cut leafy greens and tomatoes.Ph.D.Includes bibliographical referencesby Ann Charles Vegdah