This project evaluates the role of three key produce processing steps - conveying, dewatering, and shredding - in the potential transfer of bacterial pathogens immediately after leafy greens exit the flume tank. The resulting data will help to quantify the extent and likelihood of bacterial transfer during the processing of leafy greens. The project will also develop a set of recommendations on cleanability and sanitary design of processing equipment for leafy greens. The goal is to develop a scientific basis for minimizing the risk of contamination of fresh-cut leafy greens through pathogen dissemination in commercial processing equipment.
The project's specific objectives are to: <ul>
<li>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 conveyance on a conveyor belt, dewatering by shaking/centrifugation, and mechanical shredding using commercial processing equipment.
<li>Determine the extent of E. coli internalization during processing of head lettuce and spinach; and
<li>Develop a mathematical risk model for E. coli O157:H7 cross-contamination of head lettuce and spinach during processing, using the model to identify candidate risk mitigation strategies.</ul>
Fresh-cut, pre-washed baby spinach and intact heads of California head lettuce (Iceberg variety) will be surface-inoculated with an avirulent strain of E. coli O157:H7 containing the green fluorescent protein (gfp) plasmid by dipping so as to contain approximately 105 CFU/g or cm2. After brief draining to simulate product exiting the flume tank, numbers of E. coli transferred to equipment surfaces will be assessed during conveying, dewatering and shredding. Selected lettuce and spinach samples containing the highest numbers of gfp-labeled E. coli O157:H7 will be examined for both presence and numbers of internalized cells. For quantification, samples will be surface-sterilized by immersion in 1% silver nitrate, rinsed, ground, and then plated on an appropriate medium. Selected samples will also be examined for internalized cells of gfp-labeled E. coli O157:H7 using Confocal Scanning Laser Microscopy. The results will be compared to those from Objective 1 to determine percent and extent of internalization. After determining the transfer coefficients for the different transfer scenarios, the data will be subjected to our existing bacterial transfer model. As the E. coli transfer coefficients are collected, the information will be plotted to determine the relationship between type of contact surface and the rate of bacterial transfer. By examining the plotted experimental results, several mathematical models will be proposed. Software dedicated to model development, such as TableCurve - 2D and 3D, will be used to identify alternative mathematical algorithms. In all objectives, transfer of E. coli from an inoculated to an uninoculated surface and vice versa will be assessed using statistically-based models.
A quantitative risk assessment model will be developed for assessing the risk of California-grown leafy greens for consumers, and include mitigation strategies, in accordance with the Codex Alimentarius Commission's scheme: exposure assessment, hazard characterization, risk characterization and sensitivity analysis.
It is anticipated that conveyance on a conveyor belt, dewatering by shaking or centrifugation, and mechanical shredding will prove to be important multi-directional transfer points for E. coli O157:H7 dissemination, due to incoming contamination on the leafy greens, low cleanability of some areas of the equipment surface, and potential for E. coli O157:H7 to penetrate into cut surfaces of leafy greens during shredding. Transfer coefficients generated from these data can be modeled and incorporated into risk assessments to enhance the safety of leafy greens for consumers.