Validation study for the tree-fruit industry: effective strategies to sanitize harvest bins and picking bags

Reduction of foodborne illnesses associated with produce can be better achieved by controlling potential food safety risk points during harvesting, processing, and distribution. Harvesting tools, bins and containers have been recognized as microbial reservoirs and contamination sources in several outbreaks and recalls. Nevertheless, undefined recommendations for cleaning and sanitation of harvesting bins and picking bags have created challenges for producers and handlers. For all these reasons, Kansas State University/University of Missouri and Washington State University have come together to develop science-based recommendations that will help improving cleaning and sanitation practices for harvesting operations. This project will first evaluate the effectiveness of commercially available sanitizers in controlling Listeria monocytogenes, Salmonella, and Escherichia coli on representative food contact surfaces encountered during harvesting. The results obtained will guide the selection of treatments for the subsequent validation study at commercial facilities, located at primary sites of small-scale (Kansas, Missouri, and Iowa) and large-scale (Washington) apple production areas in the United States. The data generated from this project will improve the competitiveness of tree fruit crops by increasing the number of available strategies that can be implemented by growers and packers of many sizes and scales, while managing food safety risks tied to sanitation of picking bags and harvesting bins.

Tree fruits growers and handlers have learned over the years the importance of careful handling, sanitation, and temperature management to ensure the safety and quality of fruits and vegetables after harvest. When harvesting tools, bins, and containers are not effectively cleaned and sanitized, microorganisms can develop and provide a reservoir for cross-contamination of produce. The use of chemical sanitizers is the most common practice used to prevent and control microbial risk. While the produce industry has recognized the importance of these recommendations, the wide diversity and size of farm management systems and the undefined recommendations for cleaning and sanitation of food contact surfaces encountered during harvesting have created challenges for the industry. Further research is needed to support the development of effective on-farm and/or packinghouse cleaning and sanitation strategies for harvesting bins and picking bags. The two teams involved in this research, Kansas State University/University of Missouri (KSU/MU) and Washington State University (WSU) have a history of successful partnerships and collaborations in research and outreach efforts for the produce industry. The proposed project will focus on 1) validating cleaning and sanitation practices that are already approved and commercialized to minimize contamination risks during harvesting and handling and 2) encouraging growers to implement these routines through data sharing and education. Our overall goal is to validate, through lab and field testing, several strategies for cleaning and sanitizing harvest bins and harvesting bags in collaboration with growers and packinghouse stakeholders. Two main research objectives have been identified: 1) evaluate the effectiveness of commercially available sanitizers (chlorine, chlorine dioxide, peracid, steam, and silver dihydrogen citrate) in reducing Listeria monocytogenes, Salmonella and Escherichia coli on experimentally inoculated coupons (wood, plastic and nylon) representative of food contact surfaces commonly used in the apple industry during harvesting; and 2) validate selected sanitizers against surrogate microorganisms (Enterococcus faecium, Listeria innocua, and generic Escherichia coli) on bins and harvesting bags at commercial operations. Two bacteria life-forms were selected for this study - sessile and mature biofilms - in order to simulate frequent versus sporadic post-harvest sanitation practices, respectively. Both conditions are relevant and practical to understand sanitizers’ effectiveness and microbial susceptibility. This project will have a broad, meaningful economic and public health impact: the data generated will improve the competitiveness of tree fruit crops by increasing the number of available strategies that can be implemented by growers and packers to ensure produce safety linked to sanitation of picking bags and harvest bins. Furthermore, we will be able to reach and engage a wide and diverse audience, since our multidisciplinary team has access to primary sites of small-scale and large-scale apple production in the United States (Midwest and Pacific Northwest).

1. Evaluate the effectiveness of commercially available sanitizers (chlorine, chlorine dioxide, peracid, steam, and silver dihydrogen citrate) in reducing Listeria monocytogenes, Salmonella and Shiga-toxigenic Escherichia coli on experimentally inoculated coupons (wood, polycarbonate (plastic) and nylon) representative of food contact surfaces commonly used in the apple industry during harvesting. 

2. Validate the selected sanitizers against surrogate microorganisms (Enterococcus faecium, Listeria innocua, and generic Escherichia coli) on bins and harvesting bags at commercial operations.

In this research, we evaluated nylon, plastic (high density polyethylene), and wood surfaces with the following sanitizer treatments: chlorine, peracetic acid, silver dihydrogen citrate, chlorine dioxide gas, and steam. Wood proved to be one of the most challenging surfaces among the three evaluated. Surprisingly, nylon was also equally hard to sanitize in many cases. The hard plastic was consistently the easiest surface to sanitize, but it can easily transfer microorganisms if not regularly cleaned and sanitized. In the lab-based experiments, additional exposure time did not significantly improve efficacy within the time evaluated. When pathogens were inoculated on food contact surfaces and treated, they were successfully reduced by at least 99% in multiple treatments for all contact surfaces. Far less inactivation was observed when cells were allowed to grow into a biofilm. This is why cleaning and sanitizing at regular frequency is important to control the formation of biofilms on surfaces. 

Chlorine dioxide was the most effective treatment against biofilms, while steam was the least effective. Chlorine dioxide and silver dihydrogen citrate were selected for the field trials in Washington, Iowa, and Kansas. These strategies are newer to the industry and showed efficacy in the lab setting (objective 1). Chlorine dioxide treatments resulted in 90 to >99.99% inactivation of target organisms, with the greatest reductions on plastic and least on wood. In the Midwest where relative humidity in the treatment room was higher, greater inactivation was observed, highlighting the potential need to increase relative humidity if utilizing gaseous chlorine dioxide in arid regions (e.g., Washington). Silver dihydrogen citrate performed better at inactivating Listeria, with limited efficacy on E. coli. 

When thinking about which sanitation strategies to apply in produce operations, the interactions between material, sanitizer, and application times should be taken into account when implementing the intervention.