Researchers have pinpointed the likely cause of contamination in an outbreak of listeriosis in 2014 that infected 35 people and killed seven.

The outbreak was linked to prepackaged caramel apples, but since neither caramel nor apples are typical breeding grounds for the bacteria, researchers from the University of Wisconsin-Madison‘s Food Research Institute set out to discover how the listeria outbreak could have occurred.

They found that inserting a stick into the apple causes a small amount of juice to migrate to the surface. That moisture, trapped under a layer of caramel, “creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface,” explained lead study co-author Kathleen Glass, PhD, associate director of the institute.

During the production process, dipping the apples in hot caramel killed off a lot of the surface bacteria, Glass noted. “But those that still survived were the ones that were able to grow. If someone ate those apples fresh, they probably would not get sick. But because caramel-dipped apples are typically set out at room temperature for multiple days, maybe up to two weeks, it is enough time for the bacteria to grow.”

The results, published in mBio, a journal of the American Society of Microbiology, suggest that consumers should look for refrigerated caramel apples or eat them fresh.

Glass also said that caramel apple manufacturers may wish to thoroughly disinfect apples before dipping them in caramel, add growth inhibitors to the caramel coating or apple wax, or use better temperature-time controls to inhibit the growth of L. monocytogenes.