CPS held the fifth and final installment of our five-part 2020 Research Symposium on July 21, 2020. The ongoing Coronavirus pandemic meant that the 2020 annual event needed to be conducted virtually over the course of five consecutive weeks.
Session 1 explored the use of computer-based modeling to help address two burning issues for the produce industry: understanding potential Listeria growth and persistence in whole produce commodities and the development of sampling strategies to support the validity of assumptions surrounding microbial testing needs and design of acceptable protocols. Session 2 expanded our knowledge base on Listeria monocytogenes and its persistence and growth on specific commodities and fresh-cut products and examined novel methods to control Listeria growth on food contact surfaces. Session 3 explored projects that took holistic, systems approaches to solving challenges with pest intrusion into leafy greens fields, pathogen transference on co-managed farms and the impact of traits associated with concepts of soil health on pathogen persistence. We also examined Cyclospora presence in the irrigation canal systems in the Yuma, AZ production region. Session 4 featured the use of genomics and metagenomics to address challenges in identifying new or revisited indicators and to index testing-targets of human viral pathogens that may ultimately be used in the produce industry, the distribution and relatedness of Listeria species in the United States, and the use of that information to better understand source-risk related to facilities and product, identification of competitors of Listeria monocytogenes that might control that organism in composts, and build our knowledge base of bacterial pathogen persistence and rates of genetic diversification in the Yuma and Salinas vegetable production regions.
Session 5 featured research describing the “die-off” rates of human pathogens in agricultural water from three locations around the world, the persistence of pathogens in shade-house production environments, pathogen persistence in wash water systems and the potential role of damaged cells to contaminate washed products, the efficacy of irrigation water sanitation and the potential role of sediments in canal systems as reservoirs of human pathogens. These outstanding presentations and discussions generated the following executive summary and more lengthy Key Learnings.
These learnings are meant to inform and provoke thought with an eye towards inspiring readers to examine their own produce safety programs and to use the research to make improvements. They are not meant as a directive on what must be done to produce safe food.
This and other recordings of CPS webinars are available via CPS’s website. The latest information about specific research projects mentioned in this document is available via CPS’s website, including our extensive research database and other produce safety resources.
Key Learnings Executive Summary
- Pathogen die-off in baby spinach and lettuce occurs in two, segmented linear phases. The initial phase involves rapid die-off and the rate is determined by relative humidity, produce type and bacteria strain. The second phase is more prolonged, and detection of remaining bacteria can often only be accomplished by enrichment techniques.
- Set FDA pathogen die-off rates may not be reliable in every instance. The set 0.5 log10 die-off rate per day for 4 days determined by FDA during FSMA regulation development was found to be generally acceptable for E. coli, which is seemingly less hardy than the Salmonella strains used in this study. However, there was a large variation in the bacterial counts at each time point of the die-off across experimental plots and trials. Relative humidity can slow die-off, indicating that the produce industry should be aware that the current time-to-harvest intervention does not completely eliminate the associated food safety risks.
- Pathogens can persist and grow in certain shadehouse production environments. Attenuated Salmonella and avirulent E. coli strains inoculated on field-grown cucumber, jalapeño pepper and Roma tomato fruits die-off quickly but are more persistent and can grow in shadehouse production environments when humid conditions are encountered.
- Persistence and growth can lead to transference. As has been documented in field production previously, once pathogens are present in a shade house, they can be transferred to the produce in the course of crop maintenance and harvest. Every precaution should be taken to prevent entry of pathogens into the production environment.
- Incomplete wash water disinfection can damage pathogens so that they cannot be cultured, but they are not dead. Experimentally, wash water sanitizer and fresh-cut produce combinations that reflect industry practices have been identified where pathogen cells are killed, some are damaged, and some are still alive. These damaged or viable but not culturable cells (VBNC) may represent a previously unaccounted for source of cross contamination in wash systems. Research is ongoing to assess the risk.
- Irrigation water disinfection to reduce pathogens is a risk management tool but does not eliminate the potential hazard. Pathogens can be reduced by irrigation water sanitation, but variability in irrigation delivery system designs, flow rates, water quality, sanitizers employed, and target organisms means that bacterial breakthrough at the last point before the water exits the system can be observed. Research continues to expand trials, fine tune disinfection parameters, and determine impact of sanitizers on soils.
- Irrigation canal sediments may be a reservoir for pathogens. Irrigation canal sediments can harbor pathogens that can be resuspended in irrigation water by weather or other conditions that stir up the canal bottom, representing a contamination hazard for crops irrigated with that water. Ongoing research will provide growers with tools to identify “hotspots” where pathogens accumulate and models to guide water use.
This is the Key Learnings Executive Summary only. See the PDF link for more extensive information.