"I think it goes back to packing facilities and what to watch out for," said Ryser. "If we do see greater survival, then it raises the question about different brushes or those possible sites that could harbor Listeria. And can the pathogen persist longer in those areas where wax is present in the packing line?"
Joining Ryser as co-principal investigators are Randy Beaudry, Ph.D. a Michigan State University horticulture professor, and Sophia Kathariou, Ph.D., a professor of Food Science and Microbiology at North Carolina State University.
As Ryser and his team put together the project, they received input from apple packers and growers. Ryser feels that having that type of interaction is necessary to ensure the results are meaningful.
"It's critical because we need to mimic what happens in the real world," Ryser said. "We need to duplicate conditions in the packing and storage facility in order for the results to be relevant to the industry. That's always the challenge of applied research -- to come as close as possible to real-world conditions."
The project involves Honeycrisp, Gala and Granny Smith apple varieties harvested in three different states: Michigan, Pennsylvania and Washington.
Cooperating packers ship the apples to Ryser directly from the field before they undergo packinghouse handling.
The six Listeria strains used in the study, including two from a caramel apple outbreak, were selected based on their diverse genetic backgrounds. Kathariou genetically sequenced the different strains and then tagged them with unique genetic bar codes before they were mixed together in a multi-strain Listeria cocktail.
After the apples were dipped in the Listeria cocktail and air dried, they were put in cold storage for up to three months or in controlled atmosphere (CA) storage for up to seven months. CA storage involves holding apples in a sealed room with about 2% oxygen and the temperature, humidity, nitrogen, and carbon dioxide are carefully controlled to reduce respiration and slow ripening.
Ryser said they were fortunate to use Michigan State University's pilot cold storage and CA storage facilities that mimic what occurs in commercial facilities.
At set intervals, the researchers pulled samples to assay for
. The bar codes helped them differentiate the strains and their survival ability.
The second part of the research involves examining how waxing influences Listeria survival. Apples are typically waxed after they are pulled from storage and shortly before they are shipped to market.
For one treatment that simulated long-term contamination prior to waxing, the researchers waxed some of the treated fruit that had been in long-term storage. Another treatment mimicked Listeria contamination just prior to waxing, while a third involved fruit that was contaminated during the waxing stage.
The researchers then returned all of the treated waxed fruit to cold storage for up to two more months, removing samples periodically to gauge the
"We'd expect longer survival on the waxed apples, based on what has been reported in the literature to this point," surmised Ryser.
Ryser said they are nearing the end of the first set of experiments and plan to repeat them a second time. Although still very preliminary, he said, a few trends have begun appearing.
Inoculated apples had higher Listeria populations on the calyx and stem than on other parts of the surface. Listeria populations also were higher on Honeycrisp compared to the two other apple varieties, and populations were the highest on fruit from Michigan, followed by Pennsylvania and then Washington.
Learn more about the project and other CPS funded research