Research targets dry surface risks in packinghouses

September 27, 2022 - Although several CPS-funded projects have examined potential pathogen contamination risks from wet surfaces in packinghouses, few have looked at the risks from dry surfaces. Paul Dawson, Ph.D., with Clemson University, hopes to close that data gap with his project, titled “Survival of Listeria monocytogenes and Salmonella on surfaces found in the dry packinghouse environment and effectiveness of dry cleaning processes on pathogen reduction.”

“Overall, we’re looking to improve produce safety and quality by finding the problem areas in the last parts of the process that may have been overlooked in the plant after washing, waxing and drying,” Dawson said. “Those may be critical areas. By targeting them and determining the survival of the pathogens, we can make the produce safer.”

Joining him as co-principal investigator is Clemson’s Kay Cooksey, Ph.D., who Dawson said brings a wealth of knowledge and experience in packinghouse processes and sanitation.

Dawson said he chose the two pathogens because they’ve been found in packinghouses and have different physiology.

Salmonella is very resistant to drying and can survive for a long time on surfaces with low water activity, meaning a low level of water being available,” he said. “Listeria not as much. Both are in the public eye, and Listeria is one that survives and can grow slowly in cool temperatures. Salmonella can survive (in cool temperatures) but doesn’t grow very well.”
• Project looked at survival of Listeria monocytogenes and Salmonella on dry surfaces of lab-simulated peach packinghouses.
• Researchers measured die-off of dried free-flowing cells and biofilms under different environmental conditions.
• Next step will evaluate food-grade sanitizers’ effectiveness on both free-flowing cells and biofilms in the lab.
• Together with pilot-plant tests, results will help identify management practices to reduce pathogen presence in a dry environment.
During the first objective of the three-part project, the researchers simulated a packing facility within the laboratory using coupons, or disks, cut from dry surfaces within a packinghouse. Working closely with Claudia Ionita, Ph.D., from the Dawson team, George Nikolich Consulting Inc. collected surface swab samples every two weeks from two California Central Valley peach packinghouses so the researchers could use organisms found in naturally occurring biofilms — microbes living in communities, similar to coral reefs.

The researchers inoculated the coupons with dried planktonic — free-flowing — cells or dry surface-formed biofilms. They then measured the die-off rates under different environmental conditions regularly found in packinghouses. 

The results varied widely among the treatments. Planktonic cells on coupons stored at 25 degrees Celsius and 60% or 85% relative humidity only survived a few hours. But at 20% relatively humidity, the free-flowing cells survived much longer.

“So just drying the surface doesn’t affect them,” Dawson said. 

Dried biofilms of each organism, on the other hand, survived several days under the higher humidity regimes but did poorly under low humidity.

The researchers also looked at the effects of three commercial coatings applied to reduce fruit respiration and extend shelf life. Under low relative humidity — 20% — the coatings increased survival for both Listeria and Salmonella planktonic cells. The presence of organic matter, such as that found in peach wash water, had a similar effect.

Dawson said the die-off curves suggest that microorganisms have several mechanisms to adapt to dry conditions. The long survival intervals for dried Listeria and Salmonella biofilms also emphasized the importance of cleaning and sanitizing dry areas to prevent microbial attachment and biofilm formation.

As part of their laboratory work, Dawson said, they are using different methods to analyze the dry biofilms, including plate counts, microscopy, and real-time PCR, a type of genetic fingerprinting. The use of these multiple analyses is to avoid false negatives caused when microorganisms under stress, such as dry conditions, go into a survival mode known as viable but non-culturable cells or VBNC. The organisms can’t be detected using traditional culturing methods, but they remain pathogenic.

The researchers also will look at conditions that cause planktonic cells present on surfaces to form attached biofilm communities.

Their next step is to evaluate a number of approved food-grade sanitizers’ effectiveness on both dried planktonic cells and dried biofilms under lab-simulated dry packinghouse conditions.

They then plan to validate their findings in a packinghouse pilot plant. Together with the lab findings, Dawson said, they hope the results will help identify management practices to reduce pathogen presence in a dry environment.
with Dr. Dawson
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