A research paper has unveiled that if caramel apples are pricked with dipping sticks and not kept in refrigerator for many days, they may become a house of Listeria monocytogenes, a bacterium that can cause serious infections like fever, headaches, gastro issues and in serious cases, death.

The study was carried out due to a listeria outbreak that took place in 2014. In the outbreak, 35 people from 12 states suffered infection and seven lost their lives. Investigation was carried out and blame was on prepackaged caramel apples.

Considering neither caramel nor apples to be not the likely source for listeria, Kathleen Glass, associate director of the Food Research Institute started the study. They conducted an experiment in which they put a stick into apple and noticed that by doing so some amount of juice moves to surface and get trapped under a layer of caramel.

This creates a perfect environment for growth of any L. monocytogenes cells. As per the researchers, the growth of these cells already present on apple’s surface gets enhanced at room temperature. The researchers said that in the study, they themselves have witnessed a 1,000 fold rise of listeria monocytogenes on caramel apples with sticks that were not kept in fridge for three days.

In the case of apples that were present without sticks and stored temperature, growth of listeria slowed. If caramel apples with sticks are kept in fridge then no listeria growth takes place for up to a well, but after three weeks, some growth starts.

On the other hand, apples without sticks and kept in fridge did not show any listeria growth for over four weeks. In the study, the researchers have pricked 144 Granny Smith apples.

The hot caramel killed a lot of the surface bacteria, "but those that still survived were the ones that were able to grow," Glass said. "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."