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Michigan State University looks at Honeycrisp, Gala and Granny Smith

Project gauges Listeria survival on apples in storage

Research that examines Listeria survival on whole apples in storage and after waxing could fill data gaps and help the industry target risk-reduction measures in packinghouses.

The project -titled ‘Fate of different Listeria monocytogenes strains on different whole apple varieties during long-term simulated commercial storage’- is led by Michigan State University professor of Food Science and Human Nutrition, Elliot Ryser, PhD, Michigan State University.

In addition to looking at Listeria survival on apples during cold and controlled atmosphere storage, the research also will look at how waxing affects pathogen populations.

"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 t  he 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."

As explained at, 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.

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