KEY TAKEAWAYS:
- Food-grade waxes help prolong produce shelf life, but they have limited activity on microbes found on fruit and vegetable surfaces.
- Research is looking at synergistic combinations of food-grade antimicrobials, waxes and hot-air drying.
- Treatments applied to oranges resulted in up to a 3-log reduction in E. coli.
- Same treatments on apples weren’t as effective as the wax may interfere with antimicrobial activity.
eferenced CPS Research:
While food-grade waxes extend the shelf life of produce by reducing moisture loss, they exhibit limited antimicrobial activity on fruit and vegetable surfaces.
To address this shortcoming, Nitin Nitin, Ph.D., at the University of California, Davis, in collaboration with Meijun Zhu, Ph.D., at Washington State University, is leading research to identify a produce treatment that harnesses the synergistic combination of food-grade antimicrobials, wax coatings and hot-air drying.
In addition to reducing microbes on produce surfaces, the combination potentially could lower the risk of cross-contamination during brush-application of wax coatings.
Applying edible food-grade wax to produce is a common practice designed to enhance shelf life and visual appeal. Citing previous research, including projects funded by the Center for Produce Safety, Nitin said the waxes provide minimal antimicrobial activity.“
Research has shown that a wax application on the fruit surface may minimally impact contaminating bacteria,” he said. “In some cases, the wax may protect them to some degree.”
That has led him and Zhu to examine select food-grade antimicrobial compounds that could be added to the waxes. In addition, they’re studying how typical hot-air drying of the waxes, where temperatures reach 40-55 degrees Celsius (104-131 degrees Fahrenheit), may enhance microbial inactivation.
More than halfway through the two-year project, titled “Synergistic antimicrobial activity of food-grade compounds in wax coatings on fruits during wax drying,” Nitin said he and Zhu have made some surprising findings. Some have been positive while others not as much.
“One of the things we discovered initially is that the waxes are very complex; there are many different types with variations in formulation conditions, including pH,” he said.
The researchers are using apples and oranges in their experiments and targeting Listeria innocua, used as a surrogate for Listeria monocytogenes, and E. coli.
In initial laboratory experiments, the researchers added olive pomace extract or propyl gallate to a wax suspension that had been inoculated with the two pathogens. They then exposed the solution to mild heat of 40°C or 104°F.
The treatments reduced the two pathogens by more than 4 log after 10 minutes, with Listeria innocua more susceptible than E. coli.
To simulate a packinghouse scenario, the researchers inoculated orange skins with known levels of E. coli O157:H7. They then applied one of four treatments to the fruit: wax alone; wax with olive pomace extract or propyl gallate; wax and 55°C heat; and wax, olive pomace extract or propyl gallate, and 55°C heat.
While the wax alone had minimal activity against the pathogens, the combination of wax, food-grade antimicrobial and heat reduced microbial populations by up to 3 log, Nitin said. What the researchers also noticed was the three-way combination’s effectiveness wasn’t as strong near the calyx, or flower end, as on the sides or stem area.
But when they applied the four treatments to apples, they saw at most a 0.5- to 1-log microbial load reduction.
“Even our best treatment on oranges didn’t do as well on apples,” Nitin said. “So what we learned on oranges can’t directly translate to apples.”
He theorized that the different wax formulations used by the apple industry could be partly to blame.
Not to be defeated, the researchers applied the antimicrobial treatments alone to inoculated apples, followed by hot-air drying.
“Without the wax, we were able to get 2.5-log reductions, so it was the wax that was interfering with the compounds,” Nitin said.
Zhu currently is running a trial using a two-step treatment that consists of applying the compounds to apples with hot-air drying. She followed with a wax application.
Zhu also will hold the fruit in cold storage, pulling samples periodically to check microflora populations and potential quality.