An ongoing project funded by the Center for Produce Safety (CPS) is investigating ways to increase the antimicrobial activity of food-grade waxes, which are commonly applied to fruit and vegetables to extend shelf life.

The research is led by Nitin Nitin, Ph.D. of the University of California–Davis and Meijun Zhu, Ph.D., of Washington State University.

According to Dr. Nitin, food-grade waxes currently used by industry provide minimal antimicrobial activity. To potentially improve the food safety benefits of food-grade waxes, Drs. Nitin and Zhu are testing the efficacy of various antimicrobial compounds and are also examining how typical hot air drying of the waxes (ranging from 40-55 °C) may enhance microbial inactivation.

So far, the researchers have found the food-grade waxes used by the produce industry to be complex, with a variety of formulations with different conditions like pH.

For their experiments, the researchers are using apples and oranges, and test strains Listeria innocua (a surrogate for Listeria monocytogenes) and Escherichia coli.

Initial tests involved the addition of inoculated olive pomace extract or propyl gallate to a wax suspension, which was then exposed to a mild 40 °C heat treatment. These conditions resulted in more than 4-log pathogen reductions after 10 minutes, with L. innocua more susceptible than E. coli.

Next, the researchers simulated a packinghouse scenario by inoculating orange skins with E. coli O157:H7 before applying different treatments to the fruit. These treatments included unaltered wax; wax with olive pomace extract or propyl gallate; wax and 55 °C heat; and a combination of wax, antimicrobial treatments of olive pomace extract or propyl gallate, and 55 °C heat. From this experiment, the researchers found that although the wax alone had minimal antimicrobial activity, the combination of wax, olive pomace extract or propyl gallate, and heat resulted in up to 3-log pathogen reductions. Additionally, the researchers observed that the combination treatment was not as effective near the calyx (flower end) as at the sides or stem area of the fruit.

When the same experiment was run using apples instead of oranges, the researchers saw a maximum 0.5- to 1-log pathogen reduction. Dr. Nitin theorized that this difference in efficacy could be caused by the specific wax formulations used by the apple industry. The researchers then applied olive pomace extract or propyl gallate without wax to inoculated apples, followed by hot air drying, and achieved up to 2.5-log reductions—confirming that the wax was interfering with the antimicrobial compounds.

Moving forward with the project, using apples, Dr. Zhu is studying a two-step treatment of antimicrobial compound application with hot air drying, followed by wax application. The fruit will be held in cold storage and sampled periodically to assess microflora populations and fruit quality.