Identification of routes and mechanisms for distribution and establishment of Listeria monocytogenes and Listeria spp. in avocado packing environments

This proposal focuses on studying the potential for LM to contaminate fresh avocados during postharvest processing at packing plants that practice dry packing, because of small amounts of residual water that can support growth cross-contamination. We plan to determine the effect of residual water on the processing equipment, after wet cleaning procedures, on the ability of LM to colonize the plant surfaces, particularly those surfaces of equipment and utensils that have direct contact with the product. This goal will be approached by using modern DNA-based methods to accurately determine whether Listeria is found in the environment of avocado packinghouses. In addition, we plan to apply a survey of avocado packers to inquire about their level of confidence they have on wet vs. dry cleaning. With the data generated from this study, we plan to develop a model to predict, based on the data collected, the likelihood of avocado contamination under specific scenarios. All these findings will help develop effective and easy to apply, practices in sanitation that will really reduce the risk of foodborne diseases, with a tremendous societal benefit.

This proposal focuses on the study of the potential for Listeria monocytogenes (LM) to contaminate fresh avocados during postharvest processing at packing plants that practice dry packing, which consists of completing the avocado packing without any washes or wet treatment. These practices are interesting, since commonly, the cleaning and sanitizing procedures followed are based on wet cleaning and sanitizing, which may introduce some moisture in the environment, favoring the attachment and potential growth of pathogens such as (LM). The project aims at determining how current sanitation practices affect the ability of LM to colonize the packing plant environments. Information on the impact of environmental factors such as air humidity and temperature at different locations of the avocado packing plant on the ability to maintain an adequate level of sanitation is scarce. This project aims at evaluating the impact of current sanitation practices at dry avocado packing plants on the ability of LM to colonize the packing plant environment and the areas of the plant that potentially facilitate harborage at avocado dry packing plants (ADPP) in Mexico (the largest supplier of avocados to the US).

1. Determine harborage sites and spreading mechanisms for Listeria spp. and Listeria monocytogenes (LM) in the environment inside avocado dry packing facilities. 

2. Develop a map of areas of potential Listeria spp. and LM cross-contamination to avocado fruits during packing. 

3. Evaluate the effect of plant layout, avocado contact surface materials, and environmental factors on attachment and biofilm formation on avocado-contact surfaces during packing. 

4. Develop a simulation model to predict the levels of cross-contamination in a fresh avocado processing facility with visualization of the whole processing line.

Findings: 

1. The presence of Listeria spp. is low but constant in the avocado packing environment.Current in-plant cleaning and sanitizing procedures did not seem to be effective in eradicating Listeria from packing equipment. 

2. Visual inspections showed steps that may have negative effects in the effectiveness of the cleaning and sanitizing procedure. 

3. Whole genome sequencing may be a useful tool to track Listeria and identify possible sites of establishment inside avocado packing plants. 

4. LM can attach to the surfaces of equipment and utensils and, if the atmosphere is of high RH, can follow an efficient biofilm formation path. 

5. Dry cleaning did not show advantages over wet cleaning in pilot-plant studies. 

6. The developed model, while highly intricate, demands substantial computational resources to simulate the process accurately. Once fully validated and verified, employing machine learning techniques could streamline simulation time, allowing wider accessibility for avocado producers and packinghouse managers. Based on the preliminary results outlined in this report, meticulous attention is warranted regarding both the quantity and level of contamination, particularly with pathogens like Listeria, which serve as significant sources of cross-contamination within the facility and vice versa. 

Recommendations include: 

1. Intensify cleaning and sanitizing, following an effective procedure with adequate verification of effectiveness. 

2. Although reducing humidity in the plant is highly desirable, there is room for improvement of the procedures for maintaining dry equipment during pre-operative cleaning and sanitizing. 

3. If switching to a fully dry packing process is being considered, keep in mind that pilot studies showed that dry cleaning may not necessarily result in a greater prevention or eradication of Listeria spp. 

4. To achieve the development of a model for predicting the risk of contamination that helps address risk reduction strategies during sanitation, comprehensive data from the facility are essential, encompassing all areas susceptible to contamination, cleaning protocols, and experimental data pertaining to surfaces in contact with avocados. However, the efficacy of the model is contingent upon the availability of scarce contamination data. Targeting Listeria originating from primary contamination sources, such as contaminated incoming avocados, appears to be the most effective approach in reducing prevalence, offering a potentially applicable strategy facility wide. Therefore, focusing on internal corrective measures, such as enhancing surface sanitation frequency and decreasing contamination spread between equipment, designed to the specific conditions of each facility, may prove to be more logical.