The study found that apples with dipping sticks inserted were much more likely to facilitate Listeria growth. The researchers believe that the act of inserting the stick causes juice to leak to the surface, which creates a microscopic environment between the caramel and apple where bacteria can thrive.
When apples were left unrefrigerated, the researchers report, this growth happened even more quickly.
It's important to note that these apples in the study were deliberately covered in Listeria before being prepared. The bacteria have to come from somewhere, and if you're using well-washed apples in a reasonably clean kitchen, there's probably no reason to worry. But it might be worth thinking twice about the origin of a store-bought caramel apple.
And if you decide to store your homemade apples in the fridge just to be safe, remember to let them warm up a bit before biting in -- or you're likely to trade Listeria risk for a chipped tooth.