Enhancing the Microbial Safety of Fresh and Fresh-cut Melon

This research is designed to improve and broaden our understanding of factors contributing to the initial contamination and persistence of human pathogenic Salmonella spp and E. coli O157:H7 on whole and minimally processed cantaloupes, honeydews, and watermelons, in descending order or priority. <P>
A first priority is to conduct melon specific research that will establish the parameters for a uniform and standardized set of methods for inoculation, recovery, and enumeration of target pathogens. A multi-institution and international team of scientists will accomplish the mission-oriented food safety research objectives of this project within the framework of these standardized protocols. </p>
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These protocols will be used to investigate the competitive interactions of Salmonella enterica serotype Ponna with other S. enterica serotypes and representative native microflora of cantaloupes. </p>
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Our further objective is to evaluate and innovate novel methods, including combinations of interventions, of pathogen reduction and disinfection of whole and minimally processed melons. Candidate preventive interventions and disinfection strategies will be evaluated for impacts on textural shelf life and other sensory and nutritive quality attributes. </p>
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Finally, the potential for synergy or interference of postharvest quality retention treatments with disinfection treatments will be determined. </p>
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The overall approach of this project is to develop practical interventions to disinfect fresh melons and prevent contamination and outgrowth of human pathogens in fresh-cut melon products without adversely affecting product quality or shelf life. </p>
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The approach to the progress of research is unique, in this area of microbial food safety research, in that it will take a hierarchical and multidisciplinary approach to developing Recommended Management Practices for near-term adoption. </p>
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In approximate sequence, the following steps will be accomplished; <ol> <li>Develop standardized sampling methods for melons that recover targeted bacteria from subsurface sites or biofilms that will support the development and registration of sanitizing products and processes;
<li>Determine whether bacteria on naturally contaminated melons are in a stress-adapted resistant state, in subsurface locations or within biofilms, and relate such modes of contamination to production practices;
<li>Develop methods of inoculation (artificial contamination) that closely simulate natural contamination to broaden the scope of cost-effective interventions for conventional and organic food handling systems;
<li>Determine and enhance the efficacy of novel treatments in disinfecting fresh and fresh cut melons in which the targeted bacteria are stress-adapted, internalized or in biofilms. We hypothesize that treatment efficacy can be greatly increased by enhancing contact between the antimicrobial agent and the attached bacteria through use of penetrating treatments and by seeking additive or synergistic interactions between treatments. Novel approaches in the use of combinations of heat, vapor phase sanitizers, and irradiation will be evaluated;
<li>Determine whether the novel disinfection treatments are compatible with co-treatments for melon quality or negatively impact visual quality, sensory trait profiles, ripening, or sensory stability in storage. Our hypothesis is that there are novel and enhanced efficacious treatments, capable of inactivating bacteria attached in subsurface locations or within biofilms, that will be differentially suitable for commercial adoption by marketers of whole melons and fresh cut melon processors;
<li>Develop outreach and informational materials and conduct targeted workshops and technology transfer sessions to extend the outcomes of the research to diverse stakeholders.</ol> </p>
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Whether domestically produced or imported, recent reoccurring outbreaks linked to melon consumption have heightened concern for food safety.</p>
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PROGRESS: 2004/01 TO 2004/12<br>
An opportunity for novel research was initiated during this period that explores a biologically-mediated option for the simultaneous control of postharvest decay pathogens and food-borne human bacterial pathogens on whole melons and potentially fresh cut melons. Research conducted during this period demonstrated the importance of minimizing the growth of superficial fungi on cantaloupe rind to reduce the opportunities for internalization of Salmonella in metabiotic interactions. The opportunity became evident with the first reports that characterized the antimicrobial properties of low molecular weight volatiles, produced on sterile substrate, by the recently described endophytic fungus, Muscodor albus. A substantial and attractive aspect of this potential commercially registered biological agent for postharvest applications is the separation of viable organism from the target consumed commodity. In simple words, unlike other biological control agents for postharvest pathogens spatial displacement, nutrient competition, and surface or wound colonization are not required for efficacy. This category of biopesticide has been termed a mycofumigant or biofumigant, and for postharvest applications is not applied directly to the target host. While other issues may ultimately be limiting, reluctance to using this biopesticide by producers or shippers, due to consumer concerns for applied microorganisms, should be minimal. The license for marketing and distribution is held by AgraQuest (Davis, CA) and research during Year 2, supported by an ANR Postharvest IPM Workgroup funded project and this USDA CSREES award, is exploring efficacy in disinfection of cantaloupe, honeydew, and watermelon. The compatibility or synergy of biofumigation with existing optimal and sub-optimal postharvest temperature management practices during storage, transportation and distribution of selected commodities has been the first priority. Longer term interests seek to explore synergy with other postharvest treatments being evaluated by collaborators in the Project, specifically MA/CA shipping of melons. Our further long term interest is to dissect the biological basis for differential inhibition of pathogens and genetic-enhancement or modulation of volatile profile production. Our research progress to date has determined that the mixed volatiles from M. albus have potential for control of wound inoculated Botrytis cinerea, Cladosprium spp. and Geotrichum candidum in vitro and we have initiated studies on melon rind surfaces. We have demonstrated that postharvest storage temperature during biofumigation strongly affects efficacy. We have also shown that M. albus volatiles are biocidal to the bacterial pathogens Salmonella, E. coli O157:H7, Shigella, and Listeria monocytogenes in vitro and in planta. Equally interesting and important for future integrated pathogen control strategies known beneficial microflora are not sensitive to inhibition. This will afford us the opportunity in the future to explore both the efficacy of combined treatments and the mechanisms of sensitivity and tolerance to mixed or chromatographically isolated fractions of the M. albus volatiles spectrum. <p>
IMPACT: 2004/01 TO 2004/12<br>
Multidisciplinary expertise from public institutions, government researchers, and international research collaborators are developing applied science-based options for enhancing the microbial food safety for the melon producer, handler, processor, and consumer. New findings and recommendations are being broadly disseminated in refereed journals, on-line bulletins, video training materials and in industry trade journals. <p>
PUBLICATIONS: 2004/01 TO 2004/12<br>
None (2004)<p>
PROGRESS: 2003/01/01 TO 2003/12/31<br>
Background: The purpose of this research project is to improve and broaden our understanding of factors contributing to the initial contamination and persistence of human pathogenic Salmonella spp and E. coli O157:H7 on whole and minimally processed cantaloupes, honeydews, and watermelons, in descending order or priority. A multi-institution and international team of scientists are developing standardized protocols towards improving interventions and disinfection strategies. Collaborators are working to understand the role of microbial biofilms on the surface of melons as barriers to disinfection and to innovate novel methods, including combinations of interventions, of pathogen reduction and disinfection of whole and minimally processed melons. Examples of sequential disinfection research include vacuum ozonation, gaseous chlorine dioxide, biofumigation with natural volatiles, and electron-beam irradiation. Within the progress of research, regional extension and education programs will bring state of the art information to the production, harvest and postharvest handling industries. Progress to Date: An assessment of the comparative strength of attachment of various serotypes of Salmonella to cantaloupe, honeydew, and watermelon have been largely completed. Intraspecies variability in environmental stress tolerance and competitive competencies have been evaluated using ERIC-PCR to differentiate among serotypes. From these preliminary studies, Salmonella serotypes vary significantly in resistance to desiccation and degree of attachment to the outer rind of watermelon, honeydew, and immature cantaloupe. Relative survival and attachment is essentially uniform on mature netted cantaloupe. Disinfection for whole melons has been evaluated using a five strain mixture of nalidixic acid-resistant Salmonella spp on the outer rind surface of cantaloupe following a four-minute exposure to distilled water (control) and five different sanitizers. Log10 reductions in recoverable populations of presumptive Salmonella (nalr) following immersion in water, chlorine, TsunamiT100, NaHCO3, H2O2, ProSanT and H2O2 + lactic acid at 25 degrees C ranged from 1.68 to 3.78, respectively. Other research by the collaborative team addressed the metabiotic association of molds and foodborne pathogens. Co-infections of these molds and Clostridium botulinum, Listeria monocytogenes, or Salmonella has been shown to result in an enhancement of survival and growth of the pathogens, thus increasing the risk of diseases resulting from their consumption. A study was done to determine if co-infection of cantaloupe rind with Salmonella Poona and five molds isolated from cantaloupes causes changes in rates of death or growth of the pathogen. Results demonstrate that survival and growth of S. Poona are not affected by co-infection with molds commonly responsible for decay of cantaloupes. Growth of S. Poona on intact, wounded, and decaying cantaloupe rind at refrigerated and ambient temperatures is not enhanced by the increase in pH caused by growth and metabolic activities of these molds. <p>
IMPACT: 2003/01/01 TO 2003/12/31<br>
Multidisciplinary expertise from public institutions, government researchers, and international research collaborators are developing applied science-based options for enhancing the microbial food safety for the melon producer, handler, processor, and consumer. New findings and recommendations are being broadly disseminated in refereed journals, on-line bulletins, and in industry trade journals. <p>
PUBLICATIONS: 2003/01/01 TO 2003/12/31<br>
No publications reported this period <p>
PROGRESS: 2002/01/01 TO 2002/12/31<br>
Background: Recent multistate outbreaks linked to cantaloupe consumption, from non-domestic sources, in 1997, 1998, 2000, 2001, and 2002 have brought focus and concern for the microbial food safety of melons, particularly cantaloupes. These reoccurring events underscore the need for integrated prevention and control systems, as with any fruit or vegetable consumed without cooking. This project brings together an international team to address fundamentals of contamination at the source, or during handling and preparation, and limitations in the efficacy of current disinfection treatments of melons contaminated with human pathogens. The technical component focuses on improvements and standardization in methodology for evaluating interventions, the impact of factors that limit the performance of treatments, and the evaluation of novel combinations of treatments to overcome these limitations for both whole and fresh cut melons. In addition, the impacts of these intervention strategies on quality and sensory traits will be determined. Conversely, the impact of current and emerging quality-enhancing treatments on microbial interventions will be evaluated. Progress to Date: An assessment of the comparative strength of attachment of various serotypes of Salmonella to cantaloupe, honeydew, and watermelon are in progress. The influence of time and RH are being studied. The enhancement of an ERIC-PCR system for analysis of the outcome of competitive interactions among serotypes with environmental or postharvest stresses and other pathogens or microbes is in progress. From preliminary studies, Salmonella serotypes vary significantly in resistance to desiccation and degree of attachment to the outer rind of watermelon and immature cantaloupe but relative survival and attachment is essentially uniform on mature netted cantaloupe. In addition, the quantitative potential for transference of external contamination to edible melon flesh is being evaluated using E. coli and Salmonella isolates transformed with a highly stable plasmid expressing the GFP protein as a visual marker. Thus far, detectable transfer during melon cutting has been shown to be cell density dependent, requiring at least 300 to 500 CFU/cm2 to result in viable detection by non-selectoive enrichment. <p>
IMPACT: 2002/01/01 TO 2002/12/31<br>
Multidisciplinary expertise from land grand institutions, government researchers, and international research collaborators will be utilized to direct applied science-based options for enhancing the microbial food safety for the melon producer, handler, and processor. Our broader, overall goal is to validate a set of standard methods for assessing pathogen interventions and mitigation protocols for cantaloupe, honeydew melon, and watermelon and apply these uniform procedures to addressing key knowledge gaps for improved disinfection and optimal quality retention. We anticipate that this approach may serve as a model and template for other researchers to organize commodity or produce category-specific research responses to food safety concerns. New findings and recommendations are being broadly disseminated.