Now, thanks to a new study, scientists think they have cracked the code on how Listeria bacteria can quickly grow on caramel apples even though it rarely grows easily on apples or caramel.
The scientists had been puzzled about why the outbreak was linked to apples, which traditionally are too acidic for Listeria bacteria to grow quickly. Additionally, caramel doesn't often grow the bacteria because of low water content, according to the study.
Because Listeria bacteria can grow even in refrigerated temperatures, researchers found that the apples could potentially have caused infection if they were consumed weeks after being made.
"If someone ate those apples fresh, they probably would not get sick," lead study co-author Kathleen Glass, associate director of the Food Research Institute, said in a statement. "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."
Additionally, in the apples with sticks, researchers found that bacteria concentrations were found around the stick inside the apple. They theorized that the stick pushed the bacteria into the apple where it was protected from hot caramel and could grow.
Dr. William Schaffner, an infectious disease expert at Vanderbilt University in Nashville, Tennessee, said that while cases of Listeria linked to caramel apples remain low, those concerned can trade in the sweet treat for a fresh apple that is carefully washed with soap in the sink.
He explained that Listeria can be an infection difficult to pin down because the incubation period can be weeks.