In November 2014, health officials began investigating two concerning clusters of Listeria infections that seemed to be related. The two strains of bacteria had already killed at least five people and hospitalized a few dozen others by the time they got on the case, and they wanted to find the source and stop it as quickly as possible.
Her hypothesis: When the sticks (used for holding the caramel apple) punctured the contaminated apples, they spread small amounts of apple juice over surface of the otherwise dry outer skin. Then, once the caramel coating was applied, it locked in the juice and created a micro-environment in which the Listeria on the surface of the apple could grow undisturbed.
Within days, the amount of Listeria present on the caramel-coated room-temperature apples more than doubled, while the caramel-free room-temperature apples saw mild, steady growth of the bacteria over a much longer period of time. The caramel-coated apple in the fridge still grew a significant amount of bacteria, while the growth of bacteria on the caramel-free refrigerated apple was relatively minimal.
FoodWorld 