Research examines anti-listerial fruit coatings
Principal Investigator -Kay Cooksey, Ph.D. and Cryovac Endowed Chair, Clemson University
Co-Principal Investigator - Claudia Ionita, Ph.D and post-doctoral researcher, Clemson University
* Anti-microbial coating could offer risk reduction should fruit come in contact with Listeria monocytogenes.
* Edible coating must be compatible with the antimicrobial.
* Coating can't affect fruit storability, appearance or taste.
* Related project examines packinghouse micro-environments where bacteria may hide. Researchers will then test commonly used sanitizers.
* Collaboration with the fresh fruit industry has been essential to both projects.
November 29, 2018 - A team of Clemson University researchers is approaching Listeria risk reduction in stone fruit packinghouses from two angles. The first funded project, "Preservation of stone fruits by spray application of edible coatings with antimicrobial properties," examines coatings that could reduce Listeria monocytogenes populations on fruit without affecting storability, appearance, or taste.
The second and more recent funded project, "Preventive sanitation measures for the elimination of Listeria monocytogenes biofilms in critical postharvest sites," looks at difficult-to-sanitize micro-environments within a packinghouse where bacteria may reside that could serve as reservoirs. In addition to identifying problem areas, the researchers plan to test commonly used sanitizers to determine optimal concentrations and contact times to inactivate biofilms.
Both projects are being led by Kay Cooksey, Ph.D. and
Cryovac Endowed Chair, with co-principal investigator Claudia Ionita, Ph.D. and post-doctoral researcher. The two researchers, both with Clemson University's Food, Nutrition and Packaging Sciences Department, say the arrangement yields "good synergy" because each brings different expertise to the table. Cooksey has a packaging science and food technology background, whereas Ionita is a microbiologist.
Another hurdle technology
Previous CPS-funded research by Mary Anne Amalaradjou, Ph.D., with the University of Connecticut, found Listeria monocytogenes does indeed survive on the surface of peaches and nectarines. Cooksey intends that her project results will offer the fresh fruit industry another hurdle technology that can be used to help reduce those risks. "We're hoping to give them an additional margin of safety," she said.
Having the input from both a local South Carolina packinghouse manager and George Nikolich, Vice President, Technical Operations, for Gerawan Farming Inc. in Reedley, California, has been essential in helping the two researchers understand fruit flow and commercial peach packinghouse operations, they said. This new-found knowledge enables them to simulate as closely as possible in their laboratory tests what happens in a packinghouse. In addition, the researchers seek to avoid creating additional steps for the packer if they aren't necessary.
"The California Fresh Fruit Association has been a key player/advocate in this research through their support of the Center for Product Safety," Nikolich said. "This research would not be possible without the commitment of the CFFA and its dedication to food safety. Listeria species are a natural inhabitant of the orchard environment, and fortunately, the vast majority of those species are not human pathogens. However, Listeria monocytogenes, which can be pathogenic, would be of concern if it were to be found on stone fruit. Past research has shown that stone fruit does not support the growth of Listeria monocytogenes, but it can survive on those surfaces. Therefore, since most stone fruit is already being treated with an edible coating to preserve freshness and minimize food waste, we are very interested in the prospect of a coating that has antimicrobial properties and which could cause some level of die-off of Listeria on fruit surfaces," enthused Nikolich.
The first half of Cooksey and Ionita's project involved looking at commercially available food-grade coatings that could act as a carrier for food-safe anti-listerials. The coating had to not only be compatible with the antimicrobials, but it also couldn't affect the appearance, taste or storability of the fruit. An aloe-based coating, for example, was ruled out because of a possible bitter taste. Whey protein also was ruled out due to whey being be a potential allergen. The two settled on gelatin- and pectin-based coatings as carriers for the antimicrobials.
The researchers also discussed the application method, with Nikolich advising them to use immersion because that method is a suitable lab method to screen for coatings that could be used in a commercial packinghouse compared with sprays.
Using L. monocytogenes cultures, the researchers inoculated peaches with known pathogen levels. They then applied the antimicrobial coating and put the fruit into simulated packinghouse cold storage. For comparison, they do the same treatments using the same coatings but on uninoculated fruit. As they pulled out the inoculated fruit, they washed off the coating and determined the level of L. monocytogenes survival.
The researchers also measured fruit firmness and consumer acceptance of both sets of treatments.
"We want to make sure the coating we've applied doesn't affect the firmness compared to the current industry coating," Cooksey said. "The idea is to not negatively impact (the fruit) but still have a positive impact as an anti-listeria treatment."
Search and destroy mission
The second project, which is still continuing, examines packinghouse critical surface topography and identifies micro-environments (through swabbing) that are difficult to clean and could promote biofilm formation. Akin to microbial condominiums, biofilms are communities of micro-organisms that form in wet environments and use a glue-like substance to adhere to each other and to surfaces. They also are covered with a protective matrix, or film, that helps shield them from antimicrobials.
There are a few questions Ionita intends to answer: Do certain materials used to fabricate packing line brushes provide a more porous surface conducive to biofilm formation than other materials? Because they've become pitted over time, do older non-stainless steel rollers on the conveyor line offer a higher risk of biofilm formation than newer, smoother ones?
"We're discovering that certain materials actually favor biofilm attachment more than others," Ionita said. "That could be very helpful for the brush industry or the peach packers."
Once the critical areas have been identified, the researchers will recreate them as fabricated surfaces in the laboratory. There, they'll inoculate them with Listeria and biofilms allowed to form. The researchers will then treat the biofilm with various concentrations of commercial packinghouse sanitizers using different contact times and temperatures to gauge Listeria die-off.
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