Project Title Practical validation of surface pasteurization of netted melons

Cantaloupe rind washing achieved a partial surface pasteurization in a recirculating heated thermal-shower system was shown to achieve targeted bacterial reductions, relative to incoming populations on fruit, in model and commercial-scale systems. Over the two year project, several cycles of on-site testing and commercial system design and management were supported by the data developed at the cantaloupe packing operation and in laboratory validation studies. To better characterize the survival of Salmonella and Listeria on cantaloupe, simulating a contaminated irrigation event under field production conditions in California, surrogate strains were spot-inoculated onto the rind of fruit grown at the UC Davis research farm facility. Die-off from inoculated levels was rapid under high air temperature, high solar UV, and low humidity conditions typical of this production region. Surviving, recoverable populations were below the limit of detection within 14 days but at least 30% of fruit had detectable levels of survivors as determined by enrichment. Using desiccation-stress adapted cells in model systems and naturally-occurring environmentally stressed-adapted indicators in the commercial system, we have shown that tolerance to thermal-wash treatment at non-injurious time:temperature settings, alone, may be very high on the netted rind. However, microbiological reduction goals were achieved by a combined final process including a 60C thermal shower followed by a 30-50 ppm peroxyacetic acid spray and surface drying. To maintain these positive effects in microbiological reduction, fruit surface contact cleaning and sanitation downstream of the wash process is essential. Within these defined treatment parameters, no negative impacts on fruit quality were observed under optimal cantaloupe holding and distribution conditions. Extended storage periods or holding treated fruit at elevated and sub-optimal refrigerated storage conditions may result in accelerated water loss as compared to unwashed fruit. In an apparent cultivar associated manner, fruit washed by optimized thermal-shower treatments may result in reduced and delayed development of superficial molds compared to unwashed fruit or fruit exposed to injurious wash water temperatures. Overall, the outcomes of this study provide guidance for optimism if considering thermal-washing for netted melons as well as evidence for precaution and awareness in over-simplifying process design requirements and investment. Background The essential functionality of thermal surface-pasteurization of netted melons has been recognized at the basic and applied research level for over a decade, including vapor heat (steam), hot water immersion, and hot water brush-washing. Despite this evidence, commercial adoption by primary packers of the whole, raw commodity has been limited largely by cost and a limited compelling motivation. Various hot-water brushing and steam pasteurization technologies have been commercially installed at off-shore netted melon shipping points and in a few domestic fresh-cut processor faciities. Following the mutlstate outbreak of Listeria monocytogenes on whole cantaloupes in 2011 attributed to contamination at the packing operation, this prior research attracted attention in hopeful anticipation of solutions for the cantaloupe industry. A widely cited article in the blogmedia, Hot Water Bath Eliminates Pathogens on Cantaloupe. James Andrews. Food Safety News (Feb 13, 2012), promoted potential adoption of the three-minute hot water immersion in 168F (75.5 C). With the interest to convert this solely bench-top study to a high-throughput commercial system across extended seasons, growing districts, varieties, netted rind traits, fruit pulp temperature, and duration of pre-treatment storage, we were invited by committed adopters to participate in the development of validation data to support implementation decisions. The purpose of this proposed research was to provide supportive lab-based data towards answering the following testable hypothesis; Model Validation Criteria: Hot water surface-pasteurization alone or in combination with a sequentially applied labeled disinfectant can achieve at least a 4-log reduction of applied pathogen surrogates in a controlled lab inoculation of netted melons and retain or improve fruit shelf-keeping properties. Commercial Validation and Verification Criteria: Hot water surface-pasteurization alone or in combination with a sequentially applied labeled disinfectant can achieve at least a 2-log reduction of indigenous Total Coliform on netted melon rind and retain or improve fruit shelfkeeping properties. Prevention is the most important tool for addressing food safety goals during melon production. However, if cantaloupes are contaminated with human pathogens in the field, mitigation strategies, including postharvest killing steps, are needed to ensure the safety of this crop during its distribution or industrial processing. Off-loading of fruit to dump tanks for initial washing of cantaloupes in packinghouses, though no longer common in CA, is a common practice in many production areas. However, wash and product handling water, such as in dump tanks and flumes, may be a source of contamination or, more prevalently, a major point of cross-contamination. The re-circulated water systems of dump tanks and flumes often lead to accumulation of organic matter and can potentially become a vehicle of cross-contamination for incoming fresh product. The addition of antimicrobial agents to recycled water can inactivate bacterial cells and fungal conidia or spores, helping minimize cross-contamination. However, several studies have pointed out the ineffectiveness of chemical sanitizers to remove or inactivate foodborne pathogens, especially when they form biofilms on the rind surface. Microstructure of the netted rind gives the cantaloupe, and other netted melons, inherent surface roughness that favor bacterial attachment, acting as protection sites for bacteria as well as surface tension barriers to adequate sanitizer contact. Salmonella can attach itself to cantaloupe rind and may form resistant biofilms after 24 h; thus practical and effective strategies are needed to improve sanitizing treatments in order to penetrate protective rind sites and biofilms on the netted cantaloupe rind. As described above, hot water pasteurization has been proposed as a substantially improved alternative to melon disinfection with chemical sanitizers alone; however the majority of these studies have been developed under ideal laboratory conditions with contact times that, while potentially applicable to fresh-cut processing, have limited relation or reflection of industrial-scale conditions for commodity melons. Simply stated, while fresh cut processing can tolerate some degree of rind scalding and may be treating melons that are pre-chilled, this injury potential is too risky for retail melon marketing. In addition, the high costs associated with the maintenance of pasteurizing water temperature in heated immersion at commercial throughput levels clearly requires carefully assessment before widespread adoption can be recommended. In addition to meeting food safety goals, killing heat shocks may also positively or negatively impact the post-treatment keeping quality of cantaloupes and other more sensitive netted melons. Therefore, during the early phases of adoption of this postharvest disinfection treatment, reaching a balance in which food safety objectives do not compromise the product quality must be considered and addressed to arrive at a set of Best Practice options. Sequential treatments, already in use by the melon industry, may allow the definition of a “safe window’ of heat that is equivalent to the current research reports but lower the requirement for boiler capacity investment and recurring energy costs. The operational window may reduce injury by buffering the impacts of shelf-life reducing variables in fruit traits that always occur over seasonal and regional production that, realistically, have not been addressed. This research was conducted to assess the efficacy of different postharvest mitigation treatments under controlled lab and evolving, industrial commercial systems, against artificially and environmental stress-adapted attenuated S. enterica and Listeria innocua as surrogate of L. monocytogenes, during postharvest handling of whole cantaloupe. With the participation of industry cooperators, we assessed the efficacy of the work-in-progress design and implementation of a high-throughput commercial system. In doing this, developed knowledge and practical experience can be transferred, as one option for rind disinfection, and implemented by interested cantaloupe industry shippers. We anticipate these results would also be extendable to other durable fruit categories. Research Methods and Results